Cytoplasmic incompatibility factor proteins from <i>Wolbachia</i> prophage are costly to sperm development in <i>Drosophila melanogaster</i>

The symbiosis between arthropods and <i>Wolbachia</i> bacteria is globally widespread, largely due to selfish-drive systems that favour the fitness of symbiont-transmitting females. The most common drive, cytoplasmic incompatibility (CI), is central to arboviral control efforts. In <i>Drosophila melanogaster</i> carrying <i>w</i>Mel <i>Wolbachia</i> deployed in mosquito control, two prophage genes in <i>Wolbachia, cifA</i> and <i>cifB</i>, cause CI that results in a paternal-effect lethality of embryos in crosses between <i>Wolbachia</i>-bearing males and aposymbiotic females. While the CI mechanism by which Cif proteins alter sperm development has recently been elucidated in <i>D. melanogaster</i> and <i>Aedes aegypti</i> mosquitoes, the Cifs' extended impact on male reproductive fitness such as sperm morphology and quantity remains unclear. Here, using cytochemical, microscopic and transgenic assays in <i>D. melanogaster,</i> we demonstrate that both CifA and CifB cause a significant portion of defects in elongating spermatids, culminating in malformed mature sperm nuclei. Males expressing Cifs have reduced spermatid bundles and sperm counts, and transgenic expression of Cifs can occasionally result in no mature sperm formation. We reflect on Cifs' varied functional impacts on the Host Modification model of CI as well as host evolution, behaviour and vector control strategies.