Abstract
Prdm9 (Meisetz) is the first speciation gene discovered in vertebrates conferring reproductive isolation. This locus encodes a meiosis-specific histone H3 methyltransferase that specifies meiotic recombination hotspots during gametogenesis. Allelic differences in Prdm9, characterized for a variable number of zinc finger (ZF) domains, have been associated with hybrid sterility in male house mice via spermatogenic failure at the pachytene stage. The mule, a classic example of hybrid sterility in mammals also exhibits a similar spermatogenesis breakdown, making Prdm9 an interesting candidate to evaluate in equine hybrids. In this study, we characterized the Prdm9 gene in all species of equids by analyzing sequence variation of the ZF domains and estimating positive selection. We also evaluated the role of Prdm9 in hybrid sterility by assessing allelic differences of ZF domains in equine hybrids. We found remarkable variation in the sequence and number of ZF domains among equid species, ranging from five domains in the Tibetan kiang and Asiatic wild ass, to 14 in the Grevy’s zebra. Positive selection was detected in all species at amino acid sites known to be associated with DNA-binding specificity of ZF domains in mice and humans. Equine hybrids, in particular a quartet pedigree composed of a fertile mule showed a mosaic of sequences and number of ZF domains suggesting that Prdm9 variation does not seem by itself to contribute to equine hybrid sterility.
Highlights
A fascinating unresolved question in evolutionary biology focuses on mechanisms of formation of new species and the processes that preserve species as separate entities
Equid Prdm9 Sequences and Positive Selection We sequenced the final exon of Prdm9, which contains the zinc finger (ZF)
This result suggests that equids may have at least two Prdm genes, one shorter copy annotated as Prdm7 and a second copy not yet annotated corresponding to Prdm9, that differ in sequence and number of ZF domains
Summary
A fascinating unresolved question in evolutionary biology focuses on mechanisms of formation of new species and the processes that preserve species as separate entities. Species maintain their integrity through a set of premating and postmating reproductive barriers. Prdm (Meisetz) has been identified as the first speciation gene conferring reproductive isolation in vertebrates [9]. This gene encodes a meiosis-specific histone H3 methyltransferase that is only expressed in germ cells entering the meiotic prophase [10,11]. This gene is absent (birds, lizards, snakes) or has acquired disrupting mutations, such as a pseudogene in dogs [16]
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