THE EVOLUTION OF XENOPUS

Modern amphibians are believed to have originated in the early Triassic ~200 million years (my) ago. Most continental land masses were united into a single block, Panagaea. The region containing Africa and South America (Gondwanaland) separated ~140my and the continents drifted apart. The presence of putative fossil Xenopus in Brazil (Estes, 1975) and Argentina (Baez, 1987) has supported a Gondwanan origin (before the separation of South America and Africa) of the pipoids. Fossil Xenopus are extensively represented in collections from Africa representing the late Cretaceous (100my) to the early Pleistocene (10 my). The occurrence of Xenopus fossils in Yemen (Arabian Peninsula; Henrici and Baez, 2000) confirms the existence of the taxon in the late Oligocene (~25my) and reinforces the view (Tinsley et al., 1996) that its distribution was once more widespread geographically than at present.

Figure 10 Fossil Xenopus from Yemen

Today pipoids (Family Pipidae) are thought to comprise two sub-families: the Pipinae (the genera Pipa, Hymenochirus and Pseudohymenochirus) and the Xenopodinae (genera Silurana and Xenopus). See Cannatella and Trueb, 1988a,b; deSa and Hillis, 1990; http://www.zo.utexas.edu/research/salientia/pipids/pipids.html. In Silurana chromosome numbers are multiples of 20; in Xenopus chromosome numbers are multiples of 18. The genus Silurana currently includes two species: tropicalis and epitropicalis. Recognition of the genus Silurana is not required to render Xenopus monophyletic and there is disagreement about whether to lump the tropicalis group with other Xenopus (resulting in X. tropicalis and X. epitropicalis) or maintain them in a separate genus (Silurana; S. tropicalis and S. epitropicalis). The evolutionary biologists tend to prefer Silurana and the developmental biologists tend to prefer Xenopus. The chromosome number of tropicalis is 20 and this species is thought to represent the ancestral diploid condition of the sub-family. The species in the genus Xenopus have chromosome numbers that vary from tetraploid (36 chromosomes; many species) to dodecaploid (X. ruwenzoriensis, longipes). Based on laboratory-generated hybrids of various species, the chromosomal echelons of the genus are believed due to hybridization-induced endoreduplication (multiplication of chromosome numbers; Muller, 1977; Kobel et al., 1996).

In terms of external morphology, size and skin coloration varies (both between species and within populations of a single species); the presence or absence of a sub-ocular tentacle and a metatarsal tubercle (a protrusion on the inner surface of the foot which can also be clawed) are often used as distinguishing features.Far and away the most distinctive characteristic of each species is the male advertisement call; species can be distinguished on this basis alone. Given the morphological similarities of the Xenopodinae, it is likely that many species menain to be discovered. We have recently discovered an example, a new species called Xenopus itombwensis, in Cameroon (Evans et al., 2008).

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