Massive Expansion of Testis-Specific Transcripts in Equid Sex Chromosomes

Hemizygosity of mammalian males for the X and the Y chromosomes has exposed these genomic regions to evolutionary forces favoring the accumulation of male benefit genes, i.e., genes that improve male fertility. This is evidenced by the discovery of testis- and species-specific ampliconic genes in the Y chromosomes of several species and more recently, in the X chromosomes of men and mice. We sequenced, assembled de novo and annotated the male specific region of the horse Y chromosome (eMSY). We identified 8 testis-specific ampliconic genes corresponding to at least 63 transcription units. Of particular interest was Equine Testis Specific Transcript in Y 7 (ETSTY7). The transcript has 3 exons and 15 copies, all of which map to a single BAC clone on the boundary of eMSY and the Y heterochromatin (Y-HC). Using short PCR products of ETSTY7 as probes for FISH we showed that the entire Y-HC and the Xq-HC are comprised of ETSTY7 sequences. Thus, the sex chromosome regions previously thought be heterochromatic, are comprised of ampliconic testis-expressed sequences. A different pattern of ETSTY7 sequence distribution was observed in other equids: it is exclusive to Xq-HC in the donkey, but located in Y-HC, Xq-HC and certain autosomal telomeres in 2 zebra species. BLAST analysis of ETSTY7 revealed homology with Parascaris spp. Studies of ETSTY7 in parasite tissues and developmental stages are in progress to determine whether the observed sequence homology is due to parasite genome sequence contamination with the host DNA, or the result of true horizontal transmission.