# Wolbachia Feminises a Spider Host With Assistance From Co‐Infecting Symbionts

## TL;DR

A Wolbachia strain causes male spiders to develop as females, and other bacteria help increase this effect.

## Contribution

First evidence of Wolbachia-induced feminization in spiders and synergistic effects of co-infecting symbionts.

## Key findings

- Wolbachia strain W1 is essential for feminization in Mermessus fradeorum spiders.
- Co-infection with W3 increases feminization rates by ~10%.
- Lower abundance of Rickettsiella and Tisiphia correlates with stronger feminization.

## Abstract

Arthropods commonly harbour maternally‐transmitted bacterial symbionts that manipulate host biology. Multiple heritable symbionts can co‐infect the same individual, allowing these host‐restricted bacteria to engage in cooperation or conflict, which can ultimately affect host phenotype. The spider
Mermessus fradeorum
is infected with up to five heritable symbionts: Rickettsiella (R), Tisiphia (T), and three strains of Wolbachia (W1‐3). Quintuply infected spiders are feminised, causing genetic males to develop as phenotypic females and produce almost exclusively female offspring. By comparing feminisation across nine infection combinations, we identified a Wolbachia strain, W1, that is required for feminisation. We also observed that spiders infected with both W1 and W3 produced ~10% more females than those lacking W3. This increase in feminisation rate does not seem to be due to direct changes in W1 titre, nor does W1 titre correlate with feminisation rate. Instead, we observed subtle titre interactions among symbionts, with lower relative abundance of R and T symbionts in strongly feminised infections. This synergistic effect of co‐infection on Wolbachia feminisation may promote the spread of all five symbionts in spider populations. These results confirm the first instance of Wolbachia‐induced feminisation in spiders and demonstrate that co‐infecting symbionts can improve the efficacy of symbiont‐induced feminisation.

Terrestrial arthropods often host maternally‐transmitted bacterial symbionts that modify host biology to increase production of infected females, with some symbionts feminising genetic males to develop as phenotypic females. We demonstrate that a strain of the symbiont Wolbachia is required for feminisation in the spider,
Mermessus fradeorum
. Co‐infection with four other symbionts increases feminisation penetrance, possibly via subtle shifts in the relative abundance of each symbiont within the host. Our findings suggest that co‐infecting symbionts can have synergistic effects on symbiont‐induced feminisation.