View Program - 63rd Annual Drosophila Research Conference

Sex-ratio meiotic drivers act during spermatogenesis to cause unequal production of X- and Y-bearing sperm. The presence of a driver on one chromosome reduces the fitness of the other and puts selection pressure for a suppressor to emerge counteracting the sex-ratio imbalance. This conflicting dynamic of X- and Y-linked drivers and suppressors is thought to underlie copy number amplifications of meiotic genes on sex chromosomes which has been observed all across animals. The recently formed sex chromosomes of Drosophila miranda is an extreme case; on both the neo-X and neo-Y chromosomes, hundreds of previously autosomal genes with meiotic function have massively amplified within ~1.5 million years. This system provides the opportunity to elucidate how these gene amplicons manipulate spermatogenesis to confer transmission advantage. To this end, we applied single-cell RNA-seq on D. miranda testes to understand their spatiotemporal regulation. 10,095 cells were recovered and placed into 14 clusters corresponding to different testis cell types including sperm cells at varying stages of spermatogenesis. Of the 128 multi-copy gene families, the majority are most highly expressed in either the spermatocytes (n = 74), where meiosis occurs, or the premeiotic spermatogonia (n = 28). Interestingly, the expression of ampliconic X-linked genes, on average, peaks in early spermatocytes, while ampliconic Y-linked genes lags behind, peaking in late spermatocytes and lasting through sperm individualization. Because X-linked drivers can be suppressed via complementary small RNAs, we further generated D. miranda testes small RNA-seq and found abundant production of 21bp small RNAs from the ampliconic genes strongly implicating down regulation via the small interfering RNA silencing pathway. Moreover, genes that are co-amplified on both sex chromosmes and have pre-meiotic expression show disproportionally high siRNA production from the Y-linked ampliconic gametologs. These results suggest that Y-linked ampliconic genes may act to downregulate their X-linked counterparts, and maintenance of equal sex ratio likely depends on the premeiotic dosage balanace of X-linked expression and Y-linked siRNA production.

Single-cell testes expression of ampliconic meiotic drivers on the sex chromsomes of Drosophila miranda