Bolletino di zoologia

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Ryology and Vertebrate Phylogeny Alessandro Morescalchi To cite this article: Alessandro Morescalchi (1970) Ryology and Vertebrate Phylogeny, Bolletino di zoologia, 37:1, 1-28, DOI: 10.1080/11250007009440095 To link to this article: http://dx.doi.org/10.1080/11250007009440095

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1. ZOO^., 37: 1-28, 1970

RYOLOGY AND VERTEBRATE PHYLOGENY (*)

ALESSANDRO filORESCALCH1 Istituto di Istologia ed Embriologia dell’Universitd di h’apoli

(in redazione il 2 febbraio 1970) RODUCTION TO THE PROBLEM

The morphophysiological differences among the living organisms are mately dependent upon the nuclcotide sequences of their DNA, ordeinto functional units - the gcnes - which in their turn are linearly .nged into chromosomes. The chromosomes of eukaryotes possess a :ific and complex structure made up of various cornponcnts - such as chromomercs, the centromeres, the telomeres, the nucleolar organizers the heterocromatic regions - which confer upon the chromosomes a i degree of integration and organisation, and a particular behaviour .ng the life cycle of the cell. The grouping of genes into chromosomes nits degrees of interaction and control not possible among a group of :pendent genetics units : thus, the chromosomes represent a higher level lrganisation than t h a t implied in a mere string of genes, because the 5cular fashion in which the DNA of any given species is distributed Ing the chromosomes is under strict selective control. It then follows ; the principle governing chromosome changes and evolution are larindependent of the genotype, whereas they are strictly dependent n the properties of the various structural components of the chromoe phenotype (cf. WHITE, 1954, 1957; SWANSON, 1960; SWAXSON I., 19G7 ; JOHNand LEWIS,,1968). The basic chromosome set of a species, or karyotype, is defined b y number, form and size of the various chromosomes, which, with pear exceptions, are typical of each species; related species may differ he morpliology of the karyotype, and the differences depend on chro(*) Paper read at the N.A.T.O. Advanced Study Institute in Vertebrate Evon, Istanbul : August 4-15, 1969. Research carried out through a contribution from 2.N.R.

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mosome changes that occur according to a gradual scries of phenomena, which can be investigated cytologically. It is mainly these two characteristics - the species-specificity of the karyotype morphology and the particular ways of chromosome evolution - that make karyological studies a useful instrument in problems regarding phylogeny : in fact, the concept of utilisation of karyological data in systematics is chiefly based on the principle of considering the karyotype on a par with every other typical morphological characteristic of a species, in that it is specific and it is capable of evolving according to certain particular ways and directions of its own (there are also other cytogenctics, hybridological and cytochemical aspects of the problem, for which useful references may be made to the following works : DARLINGTON, 1937 ; WHITE,1054, 1057 ; BENAZZI, 1957 ; SWANSON, 1960).

*** A knowledge of the chromosome changes appears essential for reconstructing thc evolution of the karyotype ; only some of the morphological aspects of the problem will be described here and references should be made to the works cited for a thorough discussion on genetic bases. Certain types of chromosome changes cause modifications in the chromosome form. These are deletions, or the breaking of LL chromosome arm at two points, with the loss of the intervening section, and dupZications, or the local insertions of extra-portions along the chromosome arm. I n the classes of Vertebrates that present wide variations in the v ~ l u c sof nuclear DNA and in the chromosome sizc, it is thought that duplications may have played an important role in their karyological evolution (cf. ULLERICH, 1966 ; OHNO,1967 ; OKNOet nl., 1008). Since, in certain groups of Vertebrates (i.e., Teleostei, Aves), the evolution is often accompanied by reductions in DNA, it may be that also deletions have played roles of evolutionary importance ; in fact, within certain limits, reduction in the genetic material seems to be correlated with a more rapid metabolism (cf. COMMONER, 1004; STEBBINS, 1900; GOIN and GOIN, 1008; GOIN el d., 1908). Inversions constitute another type of chromosome rearrangement of great evolutionary importance in the Vertebrates ; these cause changes in the form and sometimes also in the number of the chromosomes. The inversions consist in the breaking of a chromosome a t two points, with the subsequent 180-degree rotation of the intervening fragment, which is thus inverted along the chromosome ;if the fragment contains the centromere,

3

the inversion is pcricentric ;if i t docs not contain it, the inversion is paracentric. Paracentric inversions cause various meiotic anomalies, so that they seem incompatible with the vitality of the individual in organisms with chiasmata in the germinal line of the two scxcs (WHITE,1954), as is the case in general with the Vertebrates; on th e other hand, the pericentric inversions, which often cause changes in th e position of the ccntromere on the chromosome, seem more compatible with life. This latter type of inversion seems very common in thc karyotype evolution of the Vertebrates (cf. MATTIIESand PETTER, 1068 ; THAELER, l9G8; WURSTER and BENIRSCIIKE, 19G8b). Changes in the morphology an d in th e number of tlie chromosomes are caused by translocations, rearrangements t h a t involve the exchange of portions of chromosome betwccn two non-homologous elements (reciprocal translocations) or between two regions of the same chromosome. The more common are the former, which can be appreciated cytologically, especially when they are trneqttal - th at is, when two chromosomes exchange portions widely different in size - since they often lead t o reductions in the number of chromosomes: this occurs when one of thc partners of the exchange yelds most of its genetic material without being suitably compensated for this loss and being reduced to a centromere surrounded by a small amount of gcnctic material, which may be lost without harm. A type of rearrangement of this kind has been postulated by the author (19GSb) to explain the general tendency towards rcduction of the chromosome number in many higher Anura, which seems t o occur with the loss of small chromosomes, the material of which is possibly translocated onto larger elements. I n certain classes of Vertebrates (Rcptilia, Aves), which present n karyotype rich in microchromosomes (very small, dot-like elements), which tend t o diminish in evolution, without large variations in the total nuclear DNA, these unequal translocations might play a l a g c part in their evolution. However, there exists another type of reciprocal translocation, which reduces the number of chromosomes without varying the total DNA : thcsc are the so-called u centric fusions u, or translocations involving two acroccntric chromosomes (each provided with a single chromosome arm) which unite b y the centromere to give a single mctacentric chromosome (with two .arms). Thcsc translocations, known as Q Robertsonian after the name of t he author who first drew attention to their evolutionary importance, are easily differentiated from the other translocations, especially the unequal ones, since they reduce the number of chromosomes, though

4

without altering that of the chromosome arms (called a nombre fondamental o from MATTHEY’S definition). Robertsonian traslocations perhaps constitute the most common phenomenon in the karyological evolution of Teleostei, Reptiles and many Mammals, and are frequently encountered also in the other Vertebrates (cf. MATTHEY, 1949), and even throughout the animal kingdom, whereas they are curiously rare in plants. About this type of translocations, there are essentially two theories : one mainly upheld by \ 1 7 1 n ~(1954 ~ ; 1957) and the other by MATTHEY (1965), this latter theory returning to the concepts postulated by Robertson. For brevity, I shall not go into the details of these two different theories ; I will merely say that, according to the first, the Robertsonian traslocations are practically in one direction only : that is, the chromosome evolution would occur essentially through centric fusions and rarely through the inverse phenomenon known as a centric fission o (or fragmentation, according to WHITE),by which a metacentric chromosome can give rise t o two acrocentric chromosomes; this is on account of genetic difficulties implicit in the definition given for these translocations and for certain properties of the centromere. According to MATTHEY, on the other hand, the Robertsonian translocations involve phenomena both of centric fusion and fission, either through the simple fusion of two ccntromeres of acrocentric elements or through the transverse fracture of the centromere of a metacentric chromosome. Both these theories find a following among other authors (cf. JOHNand LEWIS,1968). The fact that numerous acroccntric chromosomes are often found also in highly evolved species suggcsts that fissions constitute a fairly common mechanism in karyological evolution ; in certain genera there seems t o exist an evolutionary tendency towards generaliscd acroccntrism (MATTHEY and PETTER, 19G8). WHITE himself, faced with the problem of the presence of karyotypes rich in acrocentric elements a t various taxonomic levels, is compelled to admit that there may periodically be aepidemics of fragmentation B such as to renew the evolutionary potentialities of the karyotype, otherwise it would be necessary to postulate that the progenitor species of the present-day organisms had enormous numbers of acrocentric chromosomes (cf. WHITE,1957). I n every case centric fusions seem more widespread than centric fissions ; together with the localisation of the chiasmata, the fusions lower the genetic variability of an organism (because they link genes previously spread over different chromosomes), proving so useful to the species that are conquering new environments (~YALLACE,1959 ; STEBBINS, 1966).

6

The findings of the inverse phenomenon of centric fissions in some highly evolved taxa (many placental IIammals, some genera of Anurs) may perhaps be explained in this way: it is possible that those species, now predominant in their environment, are favoured by the higher level of genetic variability consequent to the centric fissions, which increase their chromosome numbers (and can sometimes modify their crossingover) ; this is probably the case of those species derived from others, in which the previous phenomena of centric fusions resulted in a too low chromosome number, with a consequent, perhaps excessively, low rate of genetic variability (on the problem, see: TODDN. B, 1967 il-lamm. Chrom. Newsl., 8 :268). It remains t o mention polyploidy, which according to certain modern authors has played an essential evolutionary role in the Vertebrate phylogeny (011x0,1967). At present, cases of polyploidy have so far been found in certain Teleostei, Urodela, Anura and Sauria (for a bibliography, see the same 011x0,1967). Regarding the Teleostei, little is yet known on the extension of these phenomena ; the polyploid Urodela and Sauria (one or two species for each group) are parthenogenetic forms and therefore exceptional among the Vertebrates; only in the Anura of the family Ceratophryilidae (or Leptodactylidne), which show amphimixis, does polyploidy seem to have played an evolutionary role at a level higher than thc.spccific or generic (cf. SAEZand BRIM,1959, 1962; BEGAKet al., 1966, 1967 ; BOGART, 1967). For these reasons, the importance of polyploidy among Vertebrates appears to be limited to certain highly localised forms and, until fresh proofs are forthcoming, there seems no indication that it has played any extensive evolutionary role in the phylogeny of the subphylum.

*** The chromosome changes that take place in evolution are essentially of few types, and the karyotype evolution in a given phyletic line usually occurs by means of the same types of rearrangements (principle of homologous change, after WHITE). This is due to a two-fold selective action on the karyotype: one inside and the other outside the cell. I n the former ease, it seems probable that e the whole architecture of the cells imposes a certain degree of limitation and canalization on the types of structural chromosomal changes which can be expected t o survive and establish themselves I) (WHITE, 1954). I n the latter case, it is probable that adaptation to the environment favours certain rearrangements rather than others.

6

For example, the reduction in the number of chromosomes through unequal translocations (perhaps the most widespread among the evolutionary mechanisms in the karyotype of animals) would lead to a reduction in the genetic variability, since - as I said - it binds to a single chromosome genes spread over several chromosomes ; this reduction mould seem favourable or indispensable for the afirniation of the species that are colonizing new environments. We shall see later that also the variations in the absolutc chromosome size, intimately bound up with specific variations in the values of the nuclear DNA, may have an adaptative .meaning I n conclusion, chromosome evolution follows tracks that are often fixed and ends in chromosome constitutions that are similar to one another. This fact offers certain positive and negative aspects for the utilization of karyological data in taxonomy. The positive aspects lie in the o predictability u with which it e m be deduced that a given karyotype will evolve, which makes it possible to reconstruct sequences of karyological events of which the direction is known and which are useful for outlining the phylogeny of groups of organisms. With organisms closely adapted to the environment, the taxonomist is often compelled to attribute a systematic value to morphological characteristics that may be the result of an evolution convcrging towards the same type of specialisation ; in these cases, a study of the karyological evolution may give useful indications regarding the sequence and direction of the events that have characterised the phylogeny of thc organisms in question. Literature offers numerous examples of this utilization of the karyotype; one of the most instructive, in my opinion, is that resulting from the studies carried out by BAKER (19G7), BAKERand PATTON (1967), Hsu et d. (1968) on the karyology of bats of certain families that are systematicaly somewhat problematic, since it is well known that these mammals show high degrees of specialisation and adaptation to environment. The negative aspects are constituted by the fact that also karyological evolution, like that of other morphological characteristics, is subject to parallelisms and convergences ; indeed, since there are rclativcly .few types of chromosome rearrangements that seem to assert themselves, these phenomena may be more frequent actually in karyological evolution. Moreover, it is well known that the rate of karyological evolution may be different a t the various taxonomic levels of one and the same group of organisms or between different groups, so that it frequently happens that forms are found having a slow karyological evolution and a rapid morpho-

7

logical evolution, and vice-versa. These facts seriously limit the use of karyological data in taxonomy, since it is clear that, by themselves, they cannot be capable of giving absolute indications regarding the phylctic relations of organisms, especially if they are fairly far removed from one another (on the problem, cf. B I m T m Y , 195G ; BENAZZI, 1957). But the existence of phenomena of parallelism and convergence in karyological evolution must not be considered a serious handicap for the use of karyology in the taxonomic field, since even the normal morphological characteristics of the organisms are subject to the same limitations in varying degrees, and the data relative t o them must therefore be integrated with one another and with other types of data in order to give sufficiently valid phylogenetie indications.

LINES O F KARYOLOGICAL EVOLUTION I N THE VERTEBRATES

As I said, tlie specific characteristics of the karyotype are defined by the number, form and size of the chromosomes : only the first two characteristics, however, seem to have an importance a t a taxonomic level higher than the species or genus, a t least in some classes of Vertebrates ; the size of the chromosomes, which is generally dependent upon the nuclear DNA value in the various organisms (cf. ULLERICII, 1066 ; RoTriFELs and IIEIMBURGER, 1968 ; Fox, 1969), varies, like the DNA, in a manner that is not correlated with the phylogenetic position (cf. MIRSKY and RIS, 1051 ; ENDRE RELY, 1055 ; STEBBISS, 1906). I shall therefore examine first tlie evolution of the nuclear DNA of the Vertebrates, and then the general lines of their karyological evolution. The evohtioiz of the tauclear DNA The present known facts regarding the values of the nuclear DNA in the Vertebrates (table 1) are synthetized in a diagram (fig. l), representing the range of these values in the various classes of the subphylum: it is reconstructed through the data collected by many authors (VENDRELY, 1955, VIALLI, 1957a ; OHXO,1067 ; MANFREDI-ROMANINI, 1967 ; GOIN and GOIN, 1968). Since these authors have often used different techniques and different units of measurement, the conversion of their results to a single unit (picogrammes per nucleus) is sometimes arbitrary, but it is considered necessary for an understanding of the results.

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TABLE1

- Minimum and masimum amounts of nuclear DANA i n the Vertebrates. Tax3

DC DNAlnucleus

Cyclostomata

2.43-5.0

Elasmobranchii Holocephali Selachii

2.76 5.46-6.67

Actinopterygii Chondrostei Holostei Teleostei

8.2 2.3-2.6 0.94-6.1

Choanichtyes Crossopterygii (Latimeria) Dipnoi

100-200

5.6

Amphibia Apoda Urodela Anura

7.4 20-200 2.1-30

Reptilia Cheloni Squamata Crocodilia

4.94-5.54 2.85-5.02 4.95-5.12

Aves Neognathae

1.7-8.43

Mammalia Marsupialia Placentalia

4.5 6-8.9 6

6.48-9.9

The widest effort to have a single picture of the quantitative DNA evolution in the Vertebrates is that by OHNOand his School, who have collected a large number of data on the subject and have correlated them with other cytochemical and cytogenetics reports (OHNO, 1967 ; OHNOet al., 1968). These authors suggest that the evolution of the genomes of the Vertebrates has occurred through successive a polyploidisations 10 (by gene duplications along the chromosomes, and not by a multiplication of the chromosome number) starting from a so-called ancestral genome, considered a diploid 10, with DNA values equal to about 20% of that of the pla-

Cycl.

Elatm.

Act.

Choan.

Amph.

Rapt.

Avos

Mamm.

Fig. 1

m

I5

DNA PUN

thc nuclear DNA values (in picogrammcs pcr nucleus, along tlic abscissa) in thc various classes of Vcrtcbratcs (along thc ordinatc). The names of many taxa arc abbrcviatcd (SCCtablc 1 ) ; furtlicr explanations arc to bc found in thc text.

- Thc rnngc of

5

1

10

cental 3Iammals (considered stable as regards their nuclcar DNA, equal to about 7 pg. per nucleus). Following this hypothesis, certain Teleostci are the present holders of this diploid genome ; other Teleostci, some Anura, the Squamata and the Birds have a a pentaploid a genome (equal t o 50% of the Nammalian DNA) ; yet other Tcleostei and Anura, the Chelonia and Crocodilia have an e octoploid or nonaploid D genome, close to the a decaploid B genome of tlie 3Iammals ; Urodela and Dipnoi, with their enormous DNA amounts, arc following different evolutionary lines from the other Vertebrates (cf. OIINO, 1067, chapter 3). As a result of this hypothesis, each class of Vcrtcbrates could be subdivided into several groups of organisms with genomes having various degrees of ploidy ; vice-versa, organisms belonging to different classes could be assembled phyletically in various lineages having the same DNA content ; in this way, a a polyphyletic evolution o of tlie vertebrates, and a a polyphyletic origin of terrestrial Vertebrates D is postulated. The hypothesis of a genome evolution through successive polyploidisations conflicts with the reports that, a t least in some classes (Actinopterygii, Aves) the genome evolution seems to have occurred through a reduction in the amount of the nuclear DNA (cf. MIINKY and RIS, 1051 ; GOIN and GOIN, 1908). Then, the ranges of variability of the DNA values in many classes, as reported by other authors, seem larger than that postulated by 011x0and coll., and there are various overlappings between the amounts of DNA of contiguous classes or orders ivhich, in the described hypothesis, should present different degree of ploidy (see fig. l).. The most general conclusions on the nuclear DNA values in plants and animals are that the DNA quantity may vary with different rates at various taxonomic levels, and that in many cases (and cxpccially in the Vertebrates) there is a lack of correspondence between phylogenetic position and DNA content (~IIRSKI and RIS, 1051). I n fact, the differences in the amount of DNA often appear correlated with metabolic and biological factors typical of the various species (STEBBIM,19GG) ; for example, in the Amphibia, wliich have amounts of DNA that are among the most variable to be found in the Vertebrates, the species that have the least amounts of nuclear DNA pass through the larval stages more rapidly, and vice-versa (GOINet al., 1908). Therefore, the amount of DNA appears to be a characteristic useful at a microsystematic level, but of little use a t higher taxonomic levels, at least in some classes, since in the final analysis its variations may liave an adaptative significance. A hypothesis that fits in better with the taxonomic and cytologic known data is that done by GOIN and GOIN (19G8). These authors have

11

correlated the amounts of DNA per nuclcus of the various classes of Verteand brates with patterns of the evolutionary changes proposed b y SIJIPSOS RENDEL; according to GOIX and GOIX, in the phylogeny of the subphylum there were two major increase in tlie lcvel of the nuclear DNA, the first a t t he origin of Vertebrates (in fact, the lower Chordata have very small DNA amounts), and th e second at the moment when the Crossoptcrygii were to become terrestrial ; the karyological diffcrentiation of the various classes has occurrcd through reductions in the DNA, starting from each of the two above mentioned increases. This is a very stimulating hypothesis, also from a systematic point of view ; liomcvcr, there seems t o me little proof of this hypothesis where it postulates a large increase in DNA a t the lcvel of the Choanichtyes, a n increase which, according to the authors, could latcr be found also in the first tctrapods, the Aniphibia. I n fact, even if this holds for the living Dipnoi, i t does not seem t o hold for Lnliiiierin (following VIALLI, 1057b, this crossopterygian fish has about 5.0 pg. pcr nuclcus), a single studied caccilian (GOIN et nl., 1008) and many Anura, in which the amounts of tlie nuclcar DNA are not higher than th at found in many Actinopterygii and Amniota (fig. 1). For this reason I consider i t possible th a t the large increase in DNA typical of present-day Dipnoi and Urodela may constitute nothing but a particolar method of karyological specialisation, possibly of adaptativc value, a nd gained through a parallel evolution of spceies of the two orders (see also SZABSKI, 1008). I n conclusion I suggest that the genomes of most of the actual Vertebrates may have evolved from a single line of organisms with a certain amount of nuclear DNA (from 4 to 5 or little more picogrammcs pcr nucleus) ; in fact, this amount is present in spec.ies of a11 classes ; within the various classcs n subsequent karyological specialisation might have occurrcd, with some decrease (Telcostei, Aves) or increase (Dipnoi, many Ampliibia) of the nuclear DNA. Both those kinds of quantitative DNA differentiation (from the suggested basic amount) seem possibIc a t genetic level, given that in all eukaryotcs between 1/3 to 2/3 of the total nuclear DNA is made u p of sequences hundreds of nucleotides long mliicli recur a thousand to a million times (BRITTEN nnd KOIIXE, 10GDa,b). This hypothesis (here schematized in a diagram, fig. 8 ) is in agreement with the conclusions on the karyotype evolution of Vertcbratcs (next chapter), in which I suggest the existence of a single form of karyotype, t h a t was inherited b y the early representatives of each class of tetrapods with minor chromosome (and DNA) changes, and then evolved with different methods within the various classes.

Fig. 2

largc vertical black arrow rcprcscnts the liypothetic central stock, with 8 4 mean I) DNA amount, from which the gcnomes of all actual tetrapod classcs may have diffcrentisted. Further explanations in the tcxt.

- The hypothesis of the quantitative DNA evolution in tlie Vertebrates : the

The evolution of the karyolype To describe the karyotype evolution of the Vertebrates is not a simple task : about 20 years ago MATTHEYhas written a now classic and extensive book on this subject (1989). Here I shall merely dcscribc some of the essential points regarding the general karyotypc evolution of Vertebrates, with the omission of other points, such as the evolution of the sex chromosomes, chromosornc polymorphism etc., for which the following general 1957 ; MATTIIEY, paper may be consulted : WHITE,1954, 1957 ; BENAZZI, 1933; 1\IITTlYOCIr, 1967; OIIXO, 1967; LEWISand JOHN,1968. Karyological knowledge on the lower Vertebrates is still very scanty and fragmentary, though it is now becoming wider. The Cyclostomata have average (48) or high (9G) numbers of chromosomes, mostly acrocentric ; among the Elasmobranchii, the rntfish (Holocephali) shows over 50 chromosomes, nearly all of them acrocentric, including many microchromosomes (fig. 3); the Selachii show very high chromosome nunibers (over 100). Also in certain lower Actinopterygii (Chondrostei, IIolostei) thcrc arc w e rage or high numbers of chromosomes, and often microchromosomes (figg. 4-5) ; the latter are generally absent in the Teleostei (figg. 6-7), the most highly evolved of the class, which show karyotypes that are numerically very variable, but usually with 48 or more chromosomes ; the Robertsonian mechanisms seem to have played extensive evolutionary roles in these organisms (I~ATTIIEY, 1049 ; TAYLOR,1967 ; O m o et al., 1968, 1969).

General conclusions on the karyological evolution of these fishes seem premature as yet. It is interesting perhaps to note that the most primitive living fishes (some Elasmobranchii and the lou-cr Actinopterygii) show the same form of karyotypcs with fairly high chromosome numbers and many microchromosomes ; this fact may mean that this form of karyotype is primitive in the jawed Vertebrates. Given the strict phylctic relationships between the early Actinopterygii and Choanichtyes, it seems possible to admit that even this last class, a t least in the primitive forms, have possessed a similar kind of karyotype ; certain observations on the lower tetrapods lead to the samc conclusions. The Choanichtycs very early subdivided into two divergent lines, the Crossopterygii, progenitors of tetrapods, and the aberrant Dipnoi (noMER, 196’7). The only living crossopterygian, Latimerin, is karyologically still unknown ; as I said, i t seems to have the same nuclear DNA amounts possessed by many Actinopterygii and Amniota. On the other hand, the three genera of living Dipnoi show enormous DNA amounts (over 100 pg.1

14

/nucleus), and a karyotype showing relativcly low numbers (from 34 t o 38) of chromosomes, all very large and generally mctaccntric, that is

quite different from that of the other fishes and of the most primitive living Amphibia, while i t is similar t o the karyotype of the evolved (a higher B) Urodela (fig. 8) (cf. WICKBO~I, 1945 ; OHNOand ATPIN,1966; OHNO, 1967). These observations could seem in contrast with the above-mentioned hypothesis on the possible karyotype morphology of the early Choanichtyes. However, i t must be borne in mind that karyologically Dipnoi appear as specialized organisms (owing to their high DNA values and the reduction in their chromosome numbers) ; also from a anatomical point of view the living Dipnoi are different from Crossopterygii, which retain the generalized characteristics of the class, and their divergence from the crossopterygian stock seems to be quite remote in time ; the differentiation of Dipnoi was precociously achieved in a very short period of extremely fast anatomical changes, followed by a very long period of extremely slow evolution (SIMPSON,1053). Then, it seems to me possible to do the hypothesis that this initial period of tachytclic evolution had been coupled by extensive chromosome rearrangements that differentiated the dipnoan from the crossopterygian karyotypes, in the same manner as the karyotypes of most of the higher Amphibia seem to have differentiated from karyotypes similar to t h a t possessed by the primitive living members of the class (cf. below in thc text). Therefore, the karyological resemblances between the Dipnoi and the higher Urodela, which various authors consider to be proof of the origin of this order of Amphibia from a dipnoan stock (WICKBOM, 1945; 011x0,1967), could perhaps be explained better as a simple case of parallelism in the karyological evolution of the two orders ; this theory seems to me to fit in better with the numerous anatomical and paleontological data collected on those organisms (cf. SZARSKI, 1962). Nany authors consider the Amphibia to be karyologically stable ; most of the studied species, in fact, show diploid chromosome numbers ranging between 22 and 28, with nearly all metacentric chromosomes and without microchromosomcs ; thc karyological variations seem small, at times even at interfamilial level (with some exceptions). However, the observations on the karyotypes of species of the most primitive families of Anura and Urodela, and the few collected data on the Apoda in literature, can lead to different conclusions. I n fact, the studied Apoda, the Urodela of the families Hynobiidac and Cryptobranchidae, the Anura of the families Ascaphidac and (in part) Discoglossidae, possess karyotypes that can vary from specie to species, but with relativeIy high chromo-

15

Some numbers (from 3G to G i ) , with various acrocentric and microchromoSomes (figg. 9-12) (SESIIACIIAR, 1947 ; W I c m o > I , 1915, 1949 ; ~ I A K I N O , 1951 ; MORESCALCIII, 1960, 1937, lDGSa,b). This last group of Aniphibia, which comprises the most primitive species of t he three living orders, shows karyological characteristics t h a t appear t o be primitive if compared with the above-mentioned characteristies of most of the other Aniphibia, in which th e numerical reduction of the chromosomes and the formation of metacentric clcments appear as derived conditions (cf. '\VALLACE,1059 ; STEBnISs, 19GG). Thercfore, among the living Amphibia, there esist some species or families th a t.are morpliologically a nd karyologically primitive and other - comprising most of the Anura and Urodela - that are morphologically and karyologically specialized. Interestingly enough, the differences in tlie morphology of the male meiosis between the higher Anura and most of the Urodela, th a t had been emphasized b y some authors to suggest a separate origin of the two orders (cf. WICKBOX,1045), disappear if one considers tlie male meiosis of the most primitive Anura, which show the same type of sperniatocyte chromosomes of many Urodela (and Apoda) (cf. GALGANO,1933; SATO, 1038 ; ~IORESCALCIII, 19GG, l9GSb ; ~ ~ O R E S C A L C I I Iand GALGAXO, 1968): in conclusion, the fact th at the most primitive living Amphibia have chromosome complements th at are generally similar to one another may support tlic theories regarding th,e monopliyletic origin of the three living orders ; possibly, some Apoda and the primitive members of Anura and Urodela conserve the karyological characteristics of the Amphibin that gave rise t o the living ordcrs, just a s they conserve certain of their morphological characteristics. In a karyological sense, i t therefore seems justified t o group together the living Ampliibia in the infra-class Lissamphibia sensu Gadow, in agreement with various systematic findings (PARSONS a nd WILLIAMS, 19G3). Thc karyological evolution of the Lissamphibia must have been rapid and extensive, since most of the actual Anura and Urodcla are karyologically highly diffcrcntiated forms ; at this stage of rapid morphological evolution of the karyotypc there then occurred a stage of stasis, because the karyotype of the higher Lissamphibia, with some exceptions in some families of frogs (IIylidae, Leptodactylidae), appears today t o be variable to a n extremely small extent. The Reptiles have smaller chromosomes than most of the Amphibia, compared with which generally they possess smaller amounts of nuclear DNA.

The Chelonia, the last remaining representatives of the nnapsid branch wich became differentiated from the reptilian stock at an early stage, are the most primitive existing Reptiles. With the exceptions of some species of Pleurodira (HUANGand CLARK, 1969), they possess a relatively high number of chromosomes (from 50 to 06) and many microchromomes (fig. 13) (MATTHEY, 1949 ; OHNO, 1967); in many respects, this kind of karyotype resembles that of the primitive Lissamphibia, except for the chromosome size. The Squamata and the Rhyncocephalia are the present representatives of a lepidosaurian branch that became differentiated from the reptilian main stock later than the Anapsida. The Squamata (Snuria and Ofidia) show chromosome numbers that are generally lower than those of most of the Chelonia (from 26 t o 5 0 ) ; the acrocentric chromosomes, which are numerous in the karyotypes hnving n higher number of chromosomes, tend t o fuse into metacentric elements in the evolution of the orders, while the microchromosomes decrease in number and disappear completely in certain species (figg. 14-15) (cf. MATTIIEY, 1940; BEFAK and BEFAR, 1960). Sphenodon, the only existing member of the order Rhyncocephalia, has 36 chromosomes and is karyologically similar to many Squamata. On the whole, though possessing many characteristics of the Chelonia, the living Lepidosauria appear to be karyologically more highly evolved than the former. Unique among the present reptiles so far studied, the Squamata show sex chromosomes, with male digamety in some Sauria and female digamety in many Ophidia (see B E ~ A K and BEFAK, 1969 ; PENNOCK et al., 1969). Of the Archosauria, which perhaps differentiated from the other reptiles at the same time or later than the Lepidosauria (cf. REIG, 1907), only the Crocodilia survive today ;it is also from the Archosauria that the class Aves is derived. The chromosome numbers of the Crocodilia are relatively low if compared with that of many Chelonis, reaching a maximum of 42 in the species rich in acrocentric elements (see COHEN and CLARK, 1967) ; some of those elements, in my opinion, might be considered as microchromosomes (fig. 10). The Aves show karyotypes that appear typically a chelonian o, since they possess high chromosome numbers (over 60) with many acrocentric and micro-chromosomes (fig. 17) (see MATTHEY,1949 ; RAY-CHAUDURI et al., 1969). The DNA is this class reaches the lowest values among the tetrapods ; the Birds show female digamety cytologically detectable. On those bases, many authors believe that there is a close karyological kinship

- The karyotypes of a holocephalian (fig. a),

a chondrostean (fig. 4) and n liolostcan fish (fig. 5) (modified from OUNO et d t . , 1009). Rgg. 0 7 AIetaphase plates of two species of Teleostei: Carassius auratus (fig. G) and Salrrio irideus (fig. 7) (from : 011x0S., 19G7: Sex Chroniosonies and Sexlinked linked Genes. In : Nonographs on Endocrinolo,ny, vol. 1, Berlin-Heidelberg-New York, Springer).

Fig& 3-5

-

A.

nIORESCALCIII

- Karpfogy and verkbrafe phyrogeny

Fig. 8

- The karptype of the dipnoan Lepidosiren paradoxa

(from 011x0and ATEIN,

1900).

Figg. 9-10

- 3IetapIiase plates of a primitive urodelan (ZZytzobiiis nebulosus, fig.9) and a primitive anuran (Ascaphus iruei, fig. 10).

Figg. 11-12

- ?rIctaphase plates of a higher urodelan ( A t n b p f o m a mexicanuni, fig. 11, from CALLAS, 1000 - J. Cell Sci., 1 : 85-lOS), and of a higlier nnuran (Crinia signifera, fig. 12).

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Fig. 13 - Mctnpliasc platc of tlic chclonian Goplterus ugassizi (from : Ortso s., 1907 Sex Cl~rot~iosotties mid Sex-liiil;cd Genes. In : Monographs on Endocrinology, vol. 1, Uerlin-IIcidelberg-Sc~~-York, Springer). Fig. 14 Metaphasc platc of tlic lizard U f usfaizsburioim(from :I'ESSOCK el nlt., 1908). Fig. 15 ;\Ietapliasc plate of the snake Dolhrops jararaca (from : BE~AI;,BEYAK and NAZARETII, 1962 - Cytogenctics, 1 : 303-313). Fig. 16 The karyotqpes of three crocodilian species: from high, Caitnun selerops. Alligator mississippiensis and Crocodgliis porosus (Modified from COUEN and CLARK,1907). Fig. 17 AIetaphase platc of thc bird Piiasianus coldiicus ( 9 ) (from: KIUSUANand SLIOFFNER, 1900 Cytogenctics. 5 : 53-03).

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-

Fig. 18

- Jletapliase plate of tlie monotrenie TacIi~~gIossusaculeafus (from : Ihcrr and Jd\cIisos, 10G7).

- Jletapliase plate of tlic marsupial Xarmosn

rolinsoni (from : REIG,10G8 Espcricntia, 2.'.: 185-180). Figg. 20-21 - AIetapliase plates of two placental JIammals, Chitichilln Inniger (fig. 2 0 ) and dIicro!us oregoni (fig. 21) (from : Ouso s., 10G7 Sex Clttottiosomes UJld Sex-linked Genes. In : Jlonograplis on Endocrinology-, vol. 1, Berlin-Heidelberg-New York, Springer).

Fig. 10

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17

betmecn the Birds and the Ophidia (BEGAPet a!., 1064; OIINO, 1967); but the snakes have generally lower chromosome numbers, moreover, the recent discovery of the presence of male digamety in some Sauria, which are certainly closer to the Ophidia than to the Aves, can hardly be in agreement with this hypothesis. For the fact that thc Chelonia, the Crocodilia, most of the Sauria and the primitive Ophidia do not show sex-chromosomes, it seems then probable that the genesis of those elements in the class Rcptilia may have been relatively recent (on the problem, cf. WITscm, 1050) ; about the Aves, it seems possible that their sex-chromosomes developed indipendently from that of the relatively recent snakes, and after the detachment of their progenitors from the progenitors of the crocodiles, which have any detectable sex-chromosomes. The Aves, on account of their high number of chromosomes and microehromosomes, seem to present a karyotype of relativcly primitive shape, whose similarity with that of many Chelonia may mean that the Birds have conserved a primitive form of karyotype, possibly more similar to that of the carly Archosauria than is the karyotype of the living crocodiles. The Monotremes, the last living protothcrian Mammals, have karyological characteristics typically reptilian, in the fact that they possess high chromosome numbers (53 and 63 in the d),acrocentric and microchromosomes (fig. 18) ; it is interesting to note that Monotremes shorn male digamety of the type S O , which is rare among the Vertebrates (MATTHEY, 1040 ; BICKand JACKSON, 1967). It is known that the Protothcria probably represent a branch that became isolatcd a t an early stage from the stock of the synapsidian Reptiles, from which the Tlieria are also derived ; it seems possible that these lower Mammals, like the Aves, conserve some of the karyological characteristics of the reptilian stock, whereas, as we shall see, the Theria have generally lost them. Then, the four great reptilian stocks of which the descendants still survive today - that is, the subclasses of the Anapsida, Archosauria, Lepidosauria and Synapsida - may possibly linve possessed karyotypes very similar to one another in the karyologically less evolved forms of/ or derived from the various subclasses. This fact can be in agreement with the suggestion that all these reptilian stocks have originated from a single initial stem, which bccame differentiated from the Amphibia a t an early period (YOUKG, l0GG ; ROSIER, 1967). Moreovcr, the fact that the characteristics of the karyotype of this reptilian stem seem not very different from that of the karyotype possessed by the most primitive Lissamphibia, may support the theories of a single origin of the early tetrapods, which would

18

then be derived from the same stock of Amphibia, the latter having divided into two groups at an early stage: one comprising the Lissamphibia 1962; PARSONS and WILLIABIS, and the other the Amniota (cf. SZARSKI, 1968). I n fact, the Reptilia and the Lissamphibia have similar karyotypes in the primitive representatives or in those that have remained karyologically primitive, though their subsequent chromosome evolution secms in general to have taken place by two somewhat differcnt methods. I have already said that, in an early stage of their phylogeny, the Lissamphibia seem to have attained karyotype forms that are highly differentiated with regard to the chromosome number and form; the Reptilia, on the contrary, appear as organisms that are karyologically more conservative, since they transmit the same kind of karyotype through to highly evolved spccies, with some exceptions which however generally do not attain the high levels of chromosome differentiation reached by. most of the Lissamphibia. The karyological evolution of the Reptilia seems therefore essentially genic (gene mutations are not morphologically detectable), as is also shown by the very limited variability in their nuclear DNA values, whereas that of the Lissamphibia secms to bc essentially chromosomal, since it has involved extensive chromosome rearrangements tending towards a great simplification of the karyotype and wide variations in the DNA values in these organisms. Regarding the Ilammalia Theria, they are karyologically well dimerentiated from the living Prototheria and have generally lost the karyological characteristics typical of the Reptilia, such as the presence of microchromosomes (with the possible exceptions of Tarsius, which significantly has one of the highest chromosome numbers among the Placcntalin) ; only the Y sex chromosomes, typical of the Theria, which have all male digamety, are morphologically similar to microchromosomes. Marsupialia and Placentalia are karyologically distinct from each other ; among the Placentalia, only certain Insectivora and Rodentia show certain karyological affinities with the Marsupialia. I n the Ilarsupialia, the chromosome number is very low, and the chromosomes are generally larger than in the Placentalia, in keeping with the fact that this order has the highest DNA content among the Mammalia (fig. 19) ; there is a characteristic bimodal distribution of the chromosome numbers in the Mnrsupialia around the values of 14 and 22 ; certain families possess both these numbers, while the species with intermediate numbers are relatively rare, which means that these two chromosome formulae are particularly favoured by natural selection (a phenomenon similar to that which seems to occur in the higher Amphibia, with

19

formulae of between 22 and 28 chromosomes) ; i t is not very clear which of the two formulae may be considcrcd to bc the more primitive ; certain JIarsupialia possess complex systems of sex chromosomes (cf. SRARMAN, 1961 ; &rARTIN and IXAYhIAN, 1966). The Placentalia presents an astounding variety of karyotypes and of chromosome numbers, which cannot be referred to one karyotype formula - or a few formulae - typical of these organisms (figg. 20-21). Ilost of the Placcntalia (from 50 to 70 % or so, according t o calculations performed by MATTHEY,1958) have numbers ranging from 40 to 56, though the actual range of variability extends from the 17 chromosomes of certain iiIicrotinae to the 84 of Rhinoceros. Thcrc is also variety in the chromosome morphology of these mammals, though the complements lvitli high numbers usually possess many acroccntric chromosomes. Complex systems of sex chromosomes, some of which are similar to those Seen in certain Marsupialia, are relatively frequent in Insectivora and Rodentia, and in somc cases they are also found in Carnivora and Chiroptera (OIINO, 1967 ; HSU et d.,1968). Ncverthclcss, thc X-chromosome of the Marsupialia seems different in origin from that of the Placcntalia, for which reason certain authors maintain that the evolution of the sex chromosomes in the two groups of Theria has followed different courses, though these sometimes run parallel. However, it is curious to observe that it is, in fact, among the Insectivora, the most primitive of the present-day Placentalia, that phenomena of intraspccific polymorphism are still to be found, which, according to some authors, may be a prelude to speciation mechanisms (on the problem, cf. ~ ' ~ A T T I I E Yand ?dEYLAN, 1961 ; ~IATTHEY, 1968 ; TIIAELER,1968) ; this would mean that, a t least in some cases, these Mammalin arc still in an active stage of differentiation. Similar phenomena are encountered, more abundantly, among the Rodentia, though they seem theoretically to be more easily explained here, in view of the plasticity and predominance of these forms in the actual fauna. An interspccific variability is common among Inscctivora, Rodcntia, Chiroptcra and Primates (in addition to the above-mentioned works; see also CIIIARELLI, 1966, 1968) ; on the other hand, other groups of Placcntalia are karyologically very stable (e.g. the Camelidae, cf. TAYLOR et al., 1968) ; in other groups again, karyological evolution seems to be confined to centric fusions (the Bovoidca, according to WURSTERand BENIRSCHKE, 1968b), pericentric inversions (Petomyscus, cf. Hsu and ARRIGHI, 1966), or various evolutionary mechanisms differently intermingled 19GSa). with one another (the Carnivora, cf. WURSTERand BENIRSCIIKE,

20

Karyological heterogeneity among the Placentalia may be explained by an ancient diversification of the various orders, which, with the exception of those derived from the Condylarthra, dates from the late Cretaceous period ; the different orders have then followed various lines of karyological evolution, a t times running parallel. Whereas among the Marsupialis a karyological stability has been reached, varioux taxa of Placentalia are still in proccss of karyological evolution and it is presumed that several of the chromosome formulae revealed by the living Placentalia are in a stage of transition towards other formulae, genetically better adapted, so that, in a more or less distanct future, also the Placentalia may be found to present more homogeneous standardized chromosome formulae (see ~IATTIIEY, 1040, 1954). I n any case, it seems amazing that a group of organisms derived from others relatively conservative from a karyological point of view as the present Reptilia seem to be should reveal such a wide range of different karyological situations ; this may demonstrate the genesis of new and complex evolutionary potentialities that developed at a certain moment perhaps suddenly - in the stock of Synapsida that gave rise to the Theria.

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-

CONCLUSIONS

I n the karyological evolution of tetrapods, there seems to exist a single form of karyotype, which, after passing from the primitive Amphibia to the Rcptilia with few modifications. reaches the Aves and Prototheria. Its presence in the Crossopterygii is not proved, but the fact that the same form of karyotype is present in the lower Actynopterygii suggests the possibility that it may have been present in a common stock of primitive bony fishes, from which it might have been transmitted to the Crossopterygii. This form of karyotype is characterised by high chromosome numbers and the presence of various acrocentric chromosomes and many microchromosomes ; its low variability through the Vertebrates may mean t h a t the karyotype evolution at interclass level has taken place essentially through gene or point mutations, wich give any visible altrations in chromosome structure ; the fact that a certain amount of nuclear DNA (from 4 to 5 or little more pglnucleus) is present in species of all classes of Vertebrates seems to support this theory. The various classes have become karyologically differentiated by departing in different ways from this karyotype model, and the differentiation had been sometimes accompanied by wide variations in the nuclear DNA values.

21

The karyological diffcrcntiation of the fishes has still t o be investigated in its broad outlines ; the Teleostei, the most highly differentiated among the Actinopterygii, have generally lost their microchromosomes, and their DNA seems to decrease with evolution, just as occurs in the Avcs. I n the Lissamphibia - and in a parallel manner, perhaps, in the Dipnoi - t h e karyotype evolution has been accompanied b y extensive of the chromosomes, tending towards their numerical reduction u-ith the precocious lost of th e microchromosomes and with large variations in the nuclear DNA. The karyotype of the Reptilia, possibly inherited from the early Amphibia, ha s generally undergone smaller morphological differentiation (with some exceptions) and lower variations in the nuclear DNA ; this a conservative karyotype has been trasmitted, with relatively few variations, also to the Aves and t o the Prototheria. Regarding the Theria, the chromosomes of the Marsupialia seem to have undergone extensive rearrangements, which have led to a great reduction in their number and the disappcarancc of the microchromosomes, witli certain analogies with the highcr Lissamphibia ; in the Placentalia, Some of these rearrangements seem to be still in progress and various orders are perhaps exploring new lines of karyological evolution. An important difference between the karyological evolution of certain Theria a nd t hat of the higher Lissamphibia is that the DNA of most of the Thcria is almost invariable quantitatively, which demostrates a stability in t he total genetic content of these Mammalia, a condition that had probably already been reached in th e original reptilian stock. The general conclusions of this work have been translated into a diagram (fig.22) in which the ordinates rcfcr to th e degree of karyological evolution of the Vertebrates (measured arbitrarily on the basis of the chromosome rearrangements alone) and the abscissae refer broadly t o the level of general evolution of the Vertebrates and to the relationships among the various main taxa. The graph (which is considered to be valid essentially for the tetrapods) emphasizes the different methods in the general karyological evolution followed by the various taxa and achieved starting from a karyotype constitution th at seems t o be essentially the same in the more primitive representatives of many classes ; this last fact is considered as suggesting a monophyletic evolution of the genome of the various classes of tetrapods,

Acknowkdgment. I thank P r o f s 31. GALGANOand G. GRIARAfor reading the manuscript and for their constructive criticism.

23

The species-specificity of t h e karyotype morphology and the peculiar ways of evolution of t h e chromosome phenotype are the main characteristics which make karyolodeal studies a n useful instrument in the problems of phylogeny. Through the knowl.dge of the commonest chromosome rearrangements it is possible t o reconstruct some lines of karyotype evolution in the Vertebrates. About tlie quantitative evolution of the nuclear DNA, after the csamination of some hypotheses on the problem, the author suggests t h a t the genomes of the Vertcbrates may havc cvolved from a single line of organisms with a certain amount of DNA (4-5 pglnucleus ; this quantity is present i n species of all classes), and that a subsequent increase (Dipnoi, Amphibia) or decrcasc (Actinopterygii, Aves) from this amount h a d been characteristic of the karyological differentiation of thc various classes or orders. Some of the general evolutionary trends in Vertebrate karyology are thenbriefly reviewed; tlie low aquatic vertebrates are littlc known from this point of view, while the cllromosomes of tetrapods are largely studied. The reduction in the chromosome number and t h e apparent loss of the microchromosomes sccm t o constitute the main evolutionary trend i n Vertebrate karyology ; both nre achieved i n a somcwliat different way a n d speed in Amphibia and Sauropsida ; the prototherian mammals are similar t o Sauropsida from this point of view, while Marsupialia and Placcntalia show new evolutionary trends in their karyotypc differentiation. From t h e data assembled on the karyology of thc Ampliibia and from a review of the literature on the chromosomes of t h e other Vertebrates, certain working hypotheses are hcre proposed :1 ) a monopliyletic origin of the three orders of actual Arnphibia seems karyologically supported ; 2) Dipnoi seem to have paralleled the Arnpliibia in their karyotypc evolution ; 3) it seems possible t o suppose t h a t the Crossopterygii and the early tetrapods had tlie same kind of karyotype, similar t o that of the actual primitive tetrapods b u t dirkrent from t h a t of the actual Dipnoi ;4) the living primitive Amphibia and t h e Rcptilia shorn many karyological similarities, which is in favour of the monophyletic origin of the two classes from early tetrapods ; 5) the divergent evolutionary trends i n tlie karyolo,~ of Amphibia and Heptilia are in favour of tlie hypothesis of n precocious split of tlie early tetrapods into two major lines, which gave rise respectiveIy t o the actual Amphibia (Lmnrnphibia) and to the Amniota.

RIASSUNTO I1 cariotipo possiede generalmente carattcristiche morfologiche che sono peculinri per ogni specie ;le sue modalith cvolutive dipendono i n p a n parte dalle proprietB dei singoli costitucnti strutturali dei cromosomi. elie eostituiscono il cosidetto * fenotiPo * cromosomico. La specie-specilicia e le peeuliari modaliti evolutivc fanno del an'otipo una caratteristica il cui studio, a1 para di quell0 di nltre caratteristiche morfologiche degli organismi, pub dare utili indicazioni per stabilire i rapporti filetici fra le specie. Attraverso una conoscenza delle pih comuni mutazioni che interessano il fenotiPo cromosomico, B possibile tentare di ricostruire alcune linee generali di evoluzione cariologh dei Vertebrati.

L’evoluzione della grandezza cromosomim, carattcristica clie B proporzionata alla quantith di DNA nucleare presentata dai vari organismi, pub essere indagata attraverso uno studio delle variazioni di questa quantiti. Dopo l’csame di alcune teorie esistenti intorno a questo problema. l’autore fa l’ipotesi chc i gcnomi dei Vertebrati attunli si siano evoluti a partire da quelli posscduti da un unico ceppo di organismi, che possedeva all’incirca 4-5 pglnucleo di DNA (quantith che B presente in alcune specie di tuttc le clsssi attuali), e clie il diffcrcnziamento quantitativo in sen0 a ciascuna classe sia a w e nuto tramitc diminuzioni (come negli Attinotterigi c negli Uccclli) o aumenti (come nei Dipnoi e in molti Anfibi) di quests qunntith di DNA. Circa l’evoluzione del cariotipo, poco si sa sui bassi Vertebrati, mcntre sono numerosi gli s t u d cariologici sui tetrapodi. I n generale, s i nota una riduzione nel numero dei cromosomi ed una apparcntc scomparsa dei microcromosomi, clie si accompagnano all’evoluzione del subphylum; questi due fenomcni awengono perb con modalith c vclocith diverse fra Anfibi e Snuropsidi ; i AIonotrcrni sono i n questo simili ai Sauropsidi, mentre gli altri AIammiferi scmbrano presentare nuove e varic potenzialiti evolutive nel loro differenziamcnto cariologico. In base a d un esame dei dati cariologici acquisiti sugli Anfibi ed R quclli rcperibili in letteratura sulle altre classi, l’autore propone alcune ipotesi di lavoro intorno ali’cvoluzione cariologica dei Vertebrati : 1) i reperti cariologici sembrnno suffragarc l’ipotcsi di iina origine monofiletica dei t r e ordini di Anfibi nttuali, per questo raggruppahili nella sottoclasse dei Lissamphibia sensu Gadow ; 2 ) l’evoluzione cariologica dei Dipnoi sembra aver seguito Knee parallele a quella degli Anfibi ;8 ) sembra possibile che i Crossottcrigi e i primi tetrapodi abbiano posseduto formc di cariotipo simili fra loro, e somiglianti pih a quelle possedute dai tetrapodi attuali pih primitivi che a quellc dei Dipnoi attuali ; 4) gli Anfibi attuali pih primitivi c i Rettili presentano molte amnith wriologiche, cib che pub costituire una prova in favore di unn loro originc rnonofileticn dai primi tetrapodi ; 5 ) le divergenze riscontrabili nell’cvoluzione cariologicn succcssiva in Anfibi e Rettili possono costituire una prova i n favore dell’ipotesi di una precoce divisione dei primi tetrapodi in due grandi ragpuppamenti, che hanno dato origine rispettivamente ai Lissamphibia e agli Amnioti.

REFERENCES BAKER R. J., 1067 - Karyofypes of bais of fhefamily Phyllostomidae and their taxonomic implicafions. J . southwest. Natural., 12 : 407-428. BAKER R. J. and PA~TON J. L., 1007 Kargotypes and karyotypic uariafion of North American cesperfilionid bafs. J . Mamm., 48 : 270-286. BE~AK M. L., BEq4K W. and RABELLO M. N., 1906 - Cgtologic evidence of constant tefraploidy i n fhe biscxual South American frog, Odonthophrynus americanus. Chromosoma (Berl.), 19 : 188-193. BEWKM.L., BEFAR\V. a n d RABELLOM. N., 1967 - Further sfudies on polyploid AmphiEarn (Ceralophrydidae). I. Milofic and rneiofic aspects. Chromosoma (Berl.), 22: 182-201. B E ~ A W., K REFK iW. L., NAZARETH H. R. and OHNOS., 1904 Close Isatgological kinship befmeenfhe repfilian suborder Serpenfes and fhe class Aoes. Chromosoma (Berl.), 1E: 600617.

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