EVOLUTIONARY TRENDS IN XENOPUS (ANURA PIPIDAE)

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Abstract
The 11 presently recognized species of the genus Xenopus (Anura Pipidae) represent, with respect to chromosome number and DNA content, a polyploid series in the proportions of 1: 2: 4: 6; all showing disomic inheritance with a bivalent meiosis. Expect for the single species with the lowest DNA content [X. tropicalis (Gray)], karyotypes are multiples of 18. Xenopus species tend to have a parapatric distribution with zones of sympatric overlap, where two species may be found in the same small pond. Between all species, except X. tropicalis, hybrids are viable. Male hybrids are sterile due to univalent meiosis. Female hybrids do produce functional gametes. Meiosis proceeds mainly with univalents; eggs are thus aneuploid and undergo lethal embryogenesis. In some species combinations, females produce polyploidized eggs through endoreduplication. Meiosis in these oocytes shows a somatic number of bivalents and meiotic segregation leads to diploid, isogenic eggs giving rise to triploid female offspring. Polyploid species, due to this nearly automatic polyploidization, are hence likely to be of allopolyploid origin. In spite of their obvious polyploid constitution, species are functionally quasi-diploid (NOR, MHC, hemiploid development). Diploidization does however not apply to serum albumin genes in higher polyploids. X. ruwenzoriensis Fischberg & Kobel is incompletely diploidized (MHC, albumin, sporadic survival of hemiploids).