# Evolution of sex‐biased genes in Drosophila species with neo‐sex chromosomes: Potential contribution to reducing the sexual conflict

## TL;DR

This study shows that neo-sex chromosomes in fruit flies may help reduce sexual conflict by causing more genes to become sex-biased, especially during early development.

## Contribution

The study reveals that neo-sex chromosomes contribute to reducing sexual conflict by increasing sex-biased gene expression in preadult stages.

## Key findings

- Species with neo-sex chromosomes have more sex-biased genes in larvae, pupae, and adult somatic tissues.
- Many unbiased genes become sex-biased after linkage to neo-sex chromosomes, particularly in metabolism-related genes.
- Sex-biased expression on neo-sex chromosomes may resolve cryptic sexual conflict during larval stages.

## Abstract

An advantage of sex chromosomes may be the potential to reduce sexual conflict because they provide a basis for selection to operate separately on females and males. However, evaluating the relationship between sex chromosomes and sexual conflict is challenging owing to the difficulty in measuring sexual conflict and substantial divergence between species with and without sex chromosomes. We therefore examined sex‐biased gene expression as a proxy for sexual conflict in three sets of Drosophila species with and without young sex chromosomes, the so‐called neo‐sex chromosomes. In all sets, we detected more sex‐biased genes in the species with neo‐sex chromosomes than in the species without neo‐sex chromosomes in larvae, pupae, and adult somatic tissues but not in gonads. In particular, many unbiased genes became either female‐ or male‐biased after linkage to the neo‐sex chromosomes in larvae, despite the low sexual dimorphism. For example, genes involved in metabolism, a key determinant for the rate of development in many animals, were enriched in the genes that acquired sex‐biased expression on the neo‐sex chromosomes at the larval stage. These genes may be targets of sexually antagonistic selection (i.e., large size and rapid development are selected for in females but selected against in males). These results indicate that acquiring neo‐sex chromosomes may have contributed to a reduction in sexual conflict, particularly at the larval stage, in Drosophila..

In theory, having sex chromosomes is advantageous to reduce sexual conflict. Our analyses revealed that many genes have acquired the sex‐biased expression on the neo‐sex chromosomes particularly at the larval stage. Our study indicates that cryptic sexual conflict at the preadult stages cannot be ignored, and some of these conflicts may have been resolved by acquiring sex‐biased expression after the emergence of neo‐sex chromosomes in Drosophila.