A comparison of the karyotype, constitutive heterochromatin, and nucleolar organizer regions of the new tetraploid species <i>Xenopus epitropicalis </i>Fischberg and Picard with those of <i>Xenopus tropicalis </i>Gray (Anura, Pipidae)

Abstract

A cytological analysis of the recently discovered tetraploid species Xenopus epitropicalis was carried out, using, in addition to the classical orcein method, silver staining and alkaline Giemsa banding techniques. The chromosome number of X. epitropicalis was found to be 40. The chromosomes can be grouped in to sets of four similar chromosomes (quartets), resembling the karyotype of X. tropicalis (2n = 20). However, C-band patterns revealed heterogeneity within the quartets, dividing each of them into two pairs of homologous chromosomes (“duets”). Moreover, there are differences in the position and distribution of constitutive heterochromatin between the karyotypes of X. epitropicalis and X. tropicalis. The secondary constrictions stained by silver and representing the nucleolar organizer regions (NOR’s) appear in both species on chromosome pair 5. During meiosis, usually only bivalents appear in X. epitropicalis. The question of whether this species is of autopolyploid or allopolyploid origin cannot be answered with certainty; however, it seems to have a common ancestor with X. tropicalis.

Evolution of globin expression in the genus Xenopus (Anura: Pipidae).

“…Cytogenetic similarities indicate that these two species are more closely related to each other than to any of the other species in this study (Tymowska and Fischberg 1982). The occurrence of the same number of globin bands in both species suggests that diploidization of the globin genes in X. epitropicalis has been achieved and that in both species four globin genes are expressed.…”

“…However, it is interesting to note that X. fraseri and X. tropicalis differ considerably in their morphological characters. According to the supposition that polyploid Xenopus species arose by allopolyploidy (see Tymowska and Fischberg 1982), we suggest that X. amieti emerged by hybridization of X. fraseri-like ancestors. Much more recently, hybridization of tetraploid and octoploid ancestors might have given rise to the dodecaploid X. ruwenzoriensis.…”

“…Karyological studies have shown that the oldest extant species of the genus, X. tropicalis, has conserved a diploid chromosome number (2n) of 20 (Tymowska 1973), whereas all other species possess diploid chromosome numbers that represent either four, eight, or 12 haploid sets of nine or 10 (Tymowska and Fischberg 1982). Because the DNA content also increases with the increase in the chromosome number (Thiebaud and Fischberg 1977), it is safe to assume that these species developed by polyploidization.…”

Xenopus tropicalis: An Ideal Experimental Animal in Amphibia

“…When using X. tropicalis, it is necessary to confirm the ploidy level, before starting the experiment, because the sister species of X. tropicalis, X. epitropicalis, is tetraploid like X. laevis [40]. Second, X. tropicalis has a shorter generation time.…”

Linkage arrangement in the vitellogenin gene family ofXenopus laevisas revealed by gene segregation analysis

“…This could have arisen by alloploidization or alternatively, by autoploidization. The fact that only bivalents, rather than multivalents, are observed during meiosis could indicate that the first of these two possible events took place (26). However, this observation does not exclude autopolyploidization since, at least in plants, genes have been described that control bivalent pairing in autopolyploids (27,28).…”