Allopolyploid genomes are divided into compartments called subgenomesthat are derived from lower ploidy ancestors. In African clawed frogsof the subgenus Xenopus (genus Xenopus), allotetraploid species havetwo subgenomes (L and S) with morphologically distinct homoeol-ogous chromosomes. In allotetraploid species of the sister subgenusSilurana, independently evolved subgenomes also exist, but their cytogenetics have not been investigated in detail. We used an allotetraploid species in Silurana—Xenopus calcaratus—to explore evolutionary dynamics of chromosome morphology and rearrangements. We find that the subgenomes of X. calcaratus have distinctive characteristics, with a more conserved a-subgenome resembling the closelyrelated genome of the diploid species X. tropicalis, and a more rapidlyevolving b-subgenome having more pronounced changes in chromosome structure, including diverged heterochromatic blocks, repetitivesequences, and deletion of a nucleolar secondary constriction. Basedon these cytogenetic differences, we propose a chromosome nomenclature for X. calcaratus that may apply to other allotetraploids insubgenus Silurana, depending on as yet unresolved details of theirevolutionary origins. These findings highlight the potential for large-scale asymmetry in subgenome evolution following allopolyploidization.

No abstract

Repetitive elements have been identified in several amphibian genomes using whole genome sequencing, but few studies have used cytogenetic mapping to visualize these elements in this vertebrate group. Here, we used fluorescence in situ hybridization and genomic data to map the U1 and U2 small nuclear RNAs and histone H3 in six species of African clawed frog (genus Xenopus), including, from subgenus Silurana, the diploid Xenopus tropicalis and its close allotetraploid relative X. calcaratus and, from subgenus Xenopus, the allotetraploid species X. pygmaeus, X. allofraseri, X. laevis, and X. muelleri. Results allowed us to qualitatively evaluate the relative roles of polyploidization and divergence in the evolution of repetitive elements because our focal species include allotetraploid species derived from two independent polyploidization events — one that is relatively young that gave rise to X. calcaratus and another that is older that gave rise to the other (older) allotetraploids. Our results demonstrated conserved loci number and position of signals in the species from subgenus Silurana; allotetraploid X. calcaratus has twice as many signals as diploid X. tropicalis. However, the content of repeats varied among the other allotetraploid species. We detected almost same number of signals in X. muelleri as in X. calcaratus and same number of signals in X. pygmaeus, X. allofraseri, X. laevis as in the diploid X. tropicalis. Overall, these results are consistent with the proposal that allopolyploidization duplicated these tandem repeats and that variation in their copy number was accumulated over time through reduction and expansion in a subset of the older allopolyploids.

Allopolyploid genomes are divided into compartments called subgenomes that are derived from lower ploidy ancestors. In African clawed frogs of the subgenus Xenopus (genus Xenopus), allotetraploid species have two subgenomes (L and S) with morphologically distinct homoeologous chromosomes. In allotetraploid species of the sister subgenus Silurana, independently evolved subgenomes also exist, but their cytogenetics have not been investigated in detail. We used an allotetraploid species in Silurana—Xenopus calcaratus—to explore evolutionary dynamics of chromosome morphology and rearrangements. We find that the subgenomes of X. calcaratus have distinctive characteristics, with a more conserved a-subgenome resembling the closely related genome of the diploid species X. tropicalis, and a more rapidly evolving b-subgenome having more pronounced changes in chromosome structure, including diverged heterochromatic blocks, repetitive sequences, and deletion of a nucleolar secondary constriction. Basedon these cytogenetic differences, we propose a chromosome nomenclature for X. calcaratus that may apply to other allotetraploids in subgenus Silurana, depending on as yet unresolved details of theire volutionary origins. These findings highlight the potential for large-scale asymmetry in subgenome evolution following allopolyploidization.

Repetitive elements have multiple copies in a genome and have been identified in several amphibian genomes using whole genome sequencing, but few studies have used cytogenetic mapping to visualize these elements in this vertebrate group. Here we compared the numbers and positions of repetitive elements in diploid and tetraploid species of the genus Xenopus, specifically between the diploid Xenopus tropicalis and its close allotetraploid relative X. calcaratus (both from Silurana subgenus), and also in more phylogenetically distant allotetraploids from the amieti, muelleri, and laevis species groups (all from subgenus Xenopus). Results allowed us to qualitatively evaluate the relative roles of polyploidization and divergence in the evolution of repetitive elements because our focal species include allotetraploid species derived from two independent polyploidization events – one closely related to the diploid species and one that is more distant. We used fluorescence in situ hybridization (FISH) to map the U1 and U2 small nuclear RNA (snRNA), histone H3, and retroelement Rex3 repeat loci. Our results demonstrated that U1 and U2 snRNA loci were conserved in copy number and position in the Silurana genomes, with one copy of U1 and U2 snRNA in diploid genome and one copy in each subgenome of allotetraploid species in subgenus Silurana and the muelleri species group. However, snRNA content varied among allotetraploid species in the amieti and laevis species groups, with only one U1 and U2 snRNA copy present in the entire allotetraploid genome of three species. Probes interrogating repetitive H3 and Rex3 loci were present on all chromosomes in diploid and allotetraploid species in subgenus Silurana, and in the muelleri species group. In the amieti and laevis species groups, H3 and Rex3 repeats were present on about half of all chromosomes. Overall, the results indicated that variation in the position and copy number of repetitive elements increases with phylogenetic distance, with high conservation in diploid and tetraploid Silurana species but more variation between Xenopus subgenera and among species groups in the subgenus Xenopus. In general and as expected, the effect of allotetraploidization was to increase the copy number of repetitive elements relative to diploid species, with the exception of the amieti and laevis species groups.