Distinct spermiogenic phenotypes underlie sperm elimination in the Segregation Distorter meiotic drive system

Segregation Distorter ( SD ) is a male meiotic drive system in Drosophila melanogaster. Males heterozygous for a selfish SD chromosome rarely transmit the homologous SD + chromosome. It is well established that distortion results from an interaction between Sd , the primary distorting locus on the SD chromosome and its target, a satellite DNA called Rsp, on the SD + chromosome. However, the molecular and cellular mechanisms leading to post-meiotic SD + sperm elimination remain unclear. Here we show that SD/SD + males of different genotypes but with similarly strong degrees of distortion have distinct spermiogenic phenotypes. In some genotypes, SD + spermatids fail to fully incorporate protamines after the removal of histones, and degenerate during the individualization stage of spermiogenesis. In contrast, in other SD/SD + genotypes, protamine incorporation appears less disturbed, yet spermatid nuclei are abnormally compacted, and mature sperm nuclei are eventually released in the seminal vesicle. Our analyses of different SD + chromosomes suggest that the severity of the spermiogenic defects associates with the copy number of the Rsp satellite. We propose that when Rsp copy number is very high (> 2000), spermatid nuclear compaction defects reach a threshold that triggers a checkpoint controlling sperm chromatin quality to eliminate abnormal spermatids during individualization.