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Frogs are an ecologically diverse and phylogenetically ancient group of living amphibians that include important vertebrate cell and developmental model systems, notably the genus Xenopus. Here we report a high-quality reference genome sequence for the western clawed frog, Xenopus tropicalis, along with draft chromosome-scale sequences of three distantly related emerging model frog species, Eleutherodactylus coqui, Engystomops pustulosus and Hymenochirus boettgeri. Frog chromosomes have remained remarkably stable since the Mesozoic Era, with limited Robertsonian (i.e., centric) translocations and end-to-end fusions found among the smaller chromosomes. Conservation of synteny includes conservation of centromere locations, marked by centromeric tandem repeats associated with Cenp-a binding, surrounded by pericentromeric LINE/L1 elements. We explored chromosome structure across frogs, using a dense meiotic linkage map for X. tropicalis and chromatin conformation capture (HiC) data for all species. Abundant satellite repeats occupy the unusually long (∼20 megabase) terminal regions of each chromosome that coincide with high rates of recombination. Both embryonic and differentiated cells show reproducible association of centromeric chromatin, and of telomeres, reflecting a Rabl configuration similar to the “bouquet” structure of meiotic cells. Our comparative analyses reveal 13 conserved ancestral anuran chromosomes from which contemporary frog genomes were constructed.

Hybrid zones that result from secondary contact between diverged populations offer unparalleled insight into the genetic architecture of emerging reproductive barriers and so shed light on the process of speciation. Natural selection and recombination jointly determine their dynamics, leading to a range of outcomes from finely fragmented mixtures of the parental genomes that facilitate introgression to a situation where strong selection against recombinants retains large unrecombined genomic blocks that act as strong barriers to gene flow. In the hybrid zone between the fire-bellied toadsBombina bombinaandB. variegata(Anura: Bombinatoridae), two anciently diverged and ecologically distinct taxa meet and produce abundant, fertile hybrids. The dense linkage map presented here enables genomic analysis of the selection-recombination balance that keeps the two gene pools from merging into one. We mapped 4,775 newly developed marker loci from bait-enriched genomic libraries in F2 crosses. The enrichment targets were selected from a draft assembly of theB. variegatagenome, after filtering highly repetitive sequences. We developed a novel approach to infer the most likely diplotype per sample and locus from the raw read mapping data, which is robust to over-merging and obviates arbitrary filtering thresholds. Large-scale synteny betweenBombinaandXenopus tropicalissupports the resulting linkage map. By assessing the sex of late-stage F2 tadpoles from histological sections, we also identified the sex-determining region in theBombinagenome to 7 cM on LG5, which is homologous toX. tropicalischromosome 5, and inferred male heterogamety, suggestive of an XY sex determination mechanism. Interestingly, chromosome 5 has been repeatedly recruited as a sex chromosome in anurans with XY sex determination.

Despite their recent divergence, muntjac deer show striking karyotype differences. Here we describe new chromosome-scale genome assemblies for the Chinese and Indian muntjacs, Muntiacus reevesi (2n=46) and Muntiacus muntjak (2n=6/7), and analyze their evolution and architecture. We identified six fusion events shared by both species relative to the cervid ancestor and therefore present in the muntjac common ancestor, six fusion events unique to the M. reevesi lineage, and twenty-six fusion events unique to the M. muntjak lineage. One of these M. muntjak fusions reverses an earlier fission in the cervid lineage. Although comparative Hi-C analysis revealed differences in long-range genome contacts and A/B compartment structures, we discovered widespread conservation of local chromatin contacts between the muntjacs, even near the fusion sites. A small number of genes involved in chromosome maintenance show evidence for rapid evolution, possibly associated with the dramatic changes in karyotype. Analysis of muntjac genomes reveals new insights into this unique case of rapid karyotype evolution and the resulting biological variation.