Evolution and karyology of the amphibians

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Abstract
The karyological characters differentiating the Amphibia from most other Vertebrates are the tendency toward genome hypertrophy, the occurrence of wide interspecific differences in nuclear DNA amounts and - in contrast - the relative conservativeness in karyotype morphology exhibited by the ‘higher’ families from the three orders. On examination of the more advanced Urodeles (Plethodontids and Salamandrids) these characters seem to represent the outcome of selective pressures tending on the one hand to favour the accumulation and speedy divergence of new DNA fractions, specifically the middle repetitive ones, and on the other to maintain or reduce the number of adaptive linkage-groups acquired through evolution. The primitive Urodela (Cryptobranchoidea) display only the first two of the above three characters. Their karyotypes are in fact widely variable in chromosome number and shape at the intergeneric and occasionally interspecific levels; the most prominent trend, from the karyological viewpoint, seems to be towards a progressive reduction in number of acrocentrics and microchromosomes. Evidence so far is lacking as to the presence of peculiar fractions of genomic DNA in the microchromosomes from these Urodeles. Karyological data on the Anura would favour recent hypotheses - put forward on the basis of studies of larval forms - which assign the role of ‘primitive’ to the Ascaphid-Discoglossid group alone. Conversely, other families of this order, sometimes included within the so-called Archaeobatrachia, have the same karyological characteristics as the higher Anura. Pipids, however, exhibit a complex situation in that the neotropical forms (Pipa) may have reduced the number of chromosome arms in parallel to other relict groups; on the other hand, the African forms (Hymenochirus and Xenopus) possess ‘higher’ karyotypes, and the latter genus seems to have largely adopted, for its own differentiation, polyploidization processes.