Evolutionary dynamics of meiotic recombination hotspots regulator PRDM9 in bovids

Hybrid sterility or reproductive isolation in mammals has been attributed to allelic incompatibilities in a DNA-binding protein PRDM9. Not only is PRDM9 exceptional in being the only known `speciation gene' in vertebrates, but it is also considered to be the fastest evolving gene in the genome. The terminal zinc finger (ZF) domain of PRDM9 specifies genome-wide meiotic recombination hotspot locations in mammals. Intriguingly, PRDM9 ZF domain is highly variable between as well as within species, possibly activating different recombination hotspots. The present study characterized the full-length coding sequence of PRDM9 in cattle and buffalo and explored the diversity of the ZF array in 514 samples from different bovids (cattle, yak, mithun, and buffalo). Substantial numerical and sequence variability were observed in the ZFs, with the number of repeats ranging from 6 to 9 in different bovines. Sequence analysis revealed the presence of 37 different ZFs in cattle, 3 in mithun, 4 in yak, and 13 in buffaloes producing 41 unique PRDM9 alleles in these species. The posterior mean of dN/dS or omega values calculated using Codeml tool of PAMLX identified sites -5, -1, +2, +3, +4, +5, and +6 in the ZF domain to be evolving positively in the studied species. Concerted evolution which typifies the evolution of this gene was consistently evident in all bovines. Our results demonstrate the extraordinary diversity of PRDM9 ZF array across bovines, reinforcing similar observations in other metazoans. The high variability is suggestive of unique repertoire of meiotic recombination hotspots in each species.