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).