# Experimental evidence for sexual selection against inbred males when it truly counts

## TL;DR

Inbred male mosquitofish have lower reproductive success due to inbreeding's negative impact on sexually selected traits.

## Contribution

This study experimentally demonstrates that inbreeding reduces male reproductive success through sexually selected traits.

## Key findings

- Inbred males had 23% less genome-wide heterozygosity compared to outbred males.
- Outbred males sired nearly twice as many offspring as inbred males.
- Males with longer gonopodium had higher reproductive success.

## Abstract

Although there are many correlational studies, unbiased estimates of inbreeding depression only come from experimental studies that create inbred and outbred individuals. Few such studies determine the extent to which inbreeding depression in males is due to natural or sexual selection. Importantly, traits that are closely related to fitness are predicted to be most strongly affected by inbreeding depression, so measuring fitness or key fitness components, rather than phenotypic traits, is necessary to estimate inbreeding depression accurately. Here, we experimentally created inbred and outbred male mosquitofish (Gambusia holbrooki) by mating full-sibs (f=0.25). We show this led to a 23% reduction in genome-wide heterozygosity. Males were then raised on different diets early in life. We then allowed adult males to compete freely for females to test if inbreeding, early diet, and their interaction affect a male’s share of paternity. Early diet had no effect on paternity, but outbred males sired almost twice as many offspring as inbred males. We also found that males with a relatively long gonopodium (intromittent organ) had greater reproductive success. We demonstrate that inbreeding has important consequences because it negatively affects a key component of male fitness. Given there was no difference in adult mortality this finding can only be due to inbreeding negatively affecting sexually selected traits.