Evolution and Karyology of the Amphibians

Abstract

- The karyological characters differentiating the Amphibia from most other Vertebrates are thetendency toward genome hypertrophy, the occurrence of wide interspecific differences in nuclearDNA amounts and - in contrast - the relative conservativeness in karyotype morphology exhibitedby 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 selectivepressures tending on the one hand to favour theaccumulation and speedy divergence of new DNAfractions, specifically the middle repetitive ones, and on the other to maintain or reduce thenumber of adaptive linkage-groups acquiredthrough evolution. The primitive Urodela (Cry-ptobranchoidea) display only the first two of theabove three characters. Their karyotypes are infact widely variable in chromosome number andshape at the intergeneric and occasionally interspecific levels; the most prominent trend, fromthe karyological viewpoint, seems to be towardsa progressive reduction in number of acrocentricsand microchromosomes. Evidence so far islacking as to the presence of peculiar fractions ofgenomic DNA in the microchromosomes fromthese Urodeles. Karyological data on the Anura would favour recent hypotheses - put forward on the basis ofstudies of larval forms-which assign the roleof ‘ primitive ’ to the Ascaphid-Discoglossidgroup alone. Conversely, other families of thisorder, sometimes included within the so-calledArchaeobatrachia, have the same karyologicalcharacteristics as the higher Anura. Pipids, however, exhibit a complex situation in that theneotropical forms (Pipa) may have reduced thenumber of chromosome arms in parallel to otherrelict groups; on the other hand, the Africanforms • (Hymenochirus and Xenopus) possess‘higher’ karyotypes, and the latter genus seemsto have largely adopted, for its own differentiation, polyploidization processes. © 1980 Taylor & Francis Group, LLC.