Chromosome divergence during evolution of the tetraploid clawed frogs, Xenopus mellotropicalis and Xenopus epitropicalis as revealed by Zoo-FISH

Abstract

Whole genome duplication (WGD) generates new species and genomic redundancy. In African clawed frogs of the genus Xenopus, this phenomenon has been especially important in that (i) all but one extant species are polyploid and (ii) whole genome sequences of some species provide an evidence for genomic rearrangements prior to or after WGD. Within Xenopus in the subgenus Silurana, at least one allotetraploidization event gave rise to three extant tetraploid (2n = 4x = 40) species–Xenopus mellotropicalis, X. epitropicalis, and X. calcaratus–but it is not yet clear the degree to which these tetraploid genomes experienced rearrangements prior to or after allotetraploidization. To explore genome evolution during diversification of these species, we performed cytogenetic analyses of X. mellotropicalis, including assessment of the localization of nucleolar organizer region, chromosome banding, and determination of the p/q arm ratios for each chromosome pair. We compared these data to a previously characterized karyotype of X. epitropicalis. Morphometric, C-banding and Zoo-FISH data support a previously hypothesized common allotetraploid predecessor of these species. Zoo-FISH with whole chromosome painting (WCP) probes derived from the closely related diploid species X. tropicalis confirmed the existence of ten chromosomal quartets in X. mellotropicalis somatic cells, as expected by its ploidy level and tetraploid ancestry. The p/q arm ratio of chromosome 2a was found to be substantially different between X. mellotropicalis (0.81) and X. epitropicalis (0.67), but no substantial difference between these two species was detected in this ratio for the homoeologous chromosome pair 2b, or for other chromosome pairs. Additionally, we identified variation between these two species in the locations of a heterochromatic block on chromosome pair 2a. These results are consistent with a dynamic history of genomic rearrangements before and/or after genome duplication, a surprising finding given the otherwise relatively conserved genomic structure of most frogs.

<b><i>Silurana</i></b> Chromosomal Evolution: A New Piece to the Puzzle

“…Our cytogenetic analysis brought a new piece to the puzzle of X. mellotropicalis chromosomal evolution confirming nonreciprocal interchromosomal translocation and favoring 1 of the 2 previously proposed scenarios [Knytl et al, 2017].…”

“…This unusual pattern of locus localization on 2 different chromosomes (2, 9) and the findings that X. mellotropicalis is a relatively young species [Evans et al, 2015] with morphologically very similar chromosomes within the quartets [Knytl et al, 2017] can be associated with the idea of a still ongoing diploidization process [Tymowska, 1991].…”

“…Previous studies on X. mellotropicalis showed cytogenetic characteristics and chromosomal rearrangements based on chromosomal banding patterns and Zoo-FISH [Schmid and Steinlein, 2015;Knytl et al, 2017]. In this study, we have described the mechanism of complete translocation t(9; 2) of a heterochromatic block in a diploid predecessor using the FISH-TSA method.…”

“…Chromosome Preparation and FISH-TSA X. mellotropicalis and X. tropicalis primary cell cultures were established as described by Sinzelle et al [2012] and Knytl et al [2017]. Chromosome metaphase spreads obtained from cell cultures were prepared according to Courtet et al [2001].…”

“…tropicalis chromosomes 2a (XME 2a, XEP 2a) and X. tropicalis chromosome 9 (XTR 9) bear strong heterochromatic regions [Tymowska and Fischberg, 1982] that are vulnerable to chromosomal rearrangements, especially translocations, transpositions, or insertions [Knytl et al, 2017]. The remaining species X. calcaratus , whose genome reorganization after/before tetraploidization is still unclear, has probably originated from the same allotetraploidization event [Evans et al, 2015].…”