# HP1-Mediated Silencing of the Doublesex1 Gene for Female Determination in the Crustacean Daphnia magna

## TL;DR

This study shows how HP1-1 silences the dsx1 gene in female Daphnia magna to prevent male traits, with environmental cues lifting this silencing to allow male development.

## Contribution

The novel finding is that HP1-1 mediates dsx1 silencing in females, linking epigenetic regulation to environmental sex determination in Daphnia magna.

## Key findings

- Knock-down of HP1-1 in female embryos leads to derepression of dsx1 and male trait development.
- HP1-1 is abundantly expressed during embryogenesis and is crucial for female determination.
- Environmental cues likely unlock HP1-mediated dsx1 silencing to induce male production.

## Abstract

The crustacean Daphnia magna produces genetically identical females and males by parthenogenesis. Males are produced in response to environmental cues including crowding and lack of food. For male development, the DM-domain containing transcription factor Doublesex1 (DSX1) is expressed spatiotemporally in male-specific traits and orchestrates male trait formation in both somatic and gonadal tissues. However, it remains unknown how the dsx1 gene is silenced in females to avoid male trait development. Heterochromatin Protein 1 (HP1) plays a crucial role in epigenetic gene silencing during developmental processes. Here we report the identification of four HP1 orthologs in D. magna. None of these orthologs exhibited sexually dimorphic expression, and among them, HP1-1 was most abundantly expressed during embryogenesis. The knock-down of HP1-1 in female embryos led to the derepression of dsx1 in the male-specific traits, resulting in the development of male characteristics, such as the elongation of the first antennae. These results suggest that HP1-1 silences dsx1 for female development while environmental cues unlock this silencing to induce male production. We infer the HP1-dependent formation of a sex-specific chromatin structure on the dsx1 locus is a key process in the environmental sex determination of D. magna.