# Cytoplasmic incompatibility factor proteins from Wolbachia prophage are costly to sperm development in Drosophila melanogaster

## TL;DR

Wolbachia proteins CifA and CifB harm sperm development in fruit flies, reducing sperm count and quality, which affects male reproductive fitness.

## Contribution

This study reveals the direct negative impact of CifA and CifB on sperm morphology and quantity in Drosophila melanogaster.

## Key findings

- CifA and CifB cause defects in elongating spermatids, leading to malformed sperm nuclei.
- Males expressing Cifs have fewer spermatid bundles and lower sperm counts.
- Transgenic Cif expression occasionally prevents mature sperm formation entirely.

## Abstract

The symbiosis between arthropods and Wolbachia 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 Drosophila melanogaster carrying wMel Wolbachia deployed in mosquito control, two prophage genes in Wolbachia, cifA and cifB, cause CI that results in a paternal-effect lethality of embryos in crosses between Wolbachia-bearing males and aposymbiotic females. While the CI mechanism by which Cif proteins alter sperm development has recently been elucidated in D. melanogaster and Aedes aegypti 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 D. melanogaster, 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.