A genus of aquatic anurans found in sub-Saharan Africa. They are commonly called South African clawed frogs. The 16 species in the genus have genome sizes that range from 3.5 × 109 bp to 1.6 × 1010 bp. These size differences are probably the result of a series of chromosome doublings that occurred within the past 40 million years. The ancestral chromosome number for the genus appears to have been 18, but there are now species with 36, 72, and 108 chromosomes. X. laevis and X. borealis are favorites for research in molecular genetics. Studies on the nucleolar mutants of X. laevis have shown that the nucleolus contains about 450 rRNA genes. This frog has one class of 5S rRNA genes transcribed in the oocyte and another in somatic cells. In Xenopus lampbrush chromosomes there are about 20,000 copies of the oocyte 5S rRNA genes per haploid genome, and these are distributed among large chromomeres that terminate the long arms of 15 of the 18 bivalents. There are also 1,300 copies of the somatic 5S rRNA genes, and these are distributed at nonterminal sites along the chromosomes. Since this species has a diploid chromosome number of 36, it can be designated a tetraploid from an evolutionary standpoint. See Classification, Chordata, Amphibia, Anura; Chronology, 1966, Wallace and Birnsteil; 1967, Birnsteil; 1968, Davidson, Crippa, and Mirsky; 1973, Ford and Southern; Species Web Site (www.xenbase.org); Cajal body, concerted evolution, polyploidy, ribosomal RNA genes.
Subjects: Genetics and Genomics.