Insights into the Development of Multiple Sperm Types and Evolving Sex Chromosomes in Drosophila Species
Sexual selection drives numerous animal species to independently evolve infertile parasperm that facilitate, provision or compete with the fertile eusperm. Such sperm heteromorphism provides a paradigm for understanding the evolution of cell-level novelty and altruism, but the underlying genetic and developmental programs remain uncharacterized. Here we use Drosophila pseudoobscura (Dpse) as a model and resolve its two lineages of parasperm that segregate by their spatially-resolved single-cell transcriptomes from the eusperm class and sperm of D. melanogaster (Dmel). Transcribed Dpse-specific new genes, including de novo genes originated from non-coding sequences are underrepresented in early spermatocytes and on the X chromosome due to meiotic X-inactivation; but are enriched in late meiotic cells and known transcription regulators in spermatogonia, as well as those that are differentially expressed between eusperm and parasperm. We identify many candidate upstream transcription factors in spermatogonia that probably orchestrate an expression gradient between the three sperm lineages in many downstream genes functioning in mitochondrial morphogenesis, sperm individualization and tail elongation etc. Our results together support a disproportionately important role of new genes in evolution of new sperm morphs. These low-cost parasperm probably shield the eusperm from the hostile female reproductive tract until fertilization.