Selection for Male Weapons Boosts Female Fecundity, Eliminating Sexual Conflict in the Bulb Mite

Bruno A Buzatto,Huon L Clark

doi:10.1038/s41598-020-59254-2

Abstract

Extreme differences between the sexes are usually explained by intense sexual selection on male weapons or ornaments. Sexually antagonistic genes, with a positive effect on male traits but a negative effect on female fitness, create a negative inter-sexual correlation for fitness (sexual conflict). However, such antagonism might not be apparent if sexually selected male traits are condition-dependent, and condition elevates female fitness. Here we reveal a surprising positive genetic correlation between male weaponry and female fecundity. Using mite lines that had previously been through 13 generations of selection on male weapons (fighting legs), we investigated correlated evolution in female fecundity. Females from lines under positive selection for weapons (up lines) evolved higher fecundity, despite evolving costly, thicker legs. This is likely because male mites have condition-dependent weaponry that increases our ability to indirectly select on male condition. Alleles with positive effects on condition in both sexes could have generated this correlation because: the up lines evolved a higher proportion of fighters and there were positive correlations between weapon size and the male morph and sex ratios of the offspring. This positive inter-sexual genetic correlation should boost the evolution of male weapons and extreme sex differences.

Highlights

Sexual dimorphism intrigued Darwin, and led to the establishment of the field of sexual selection that continues to fascinate scientists
We found that lines selected for thicker legs in fighter males evolved higher female fecundity than down lines selected for thinner legs in fighter males
It is possible that selection on relative leg width in males resulted in correlated evolution of aggression, or any other male or female behaviour that might underlie the effects we observed on female fecundity

Summary

Introduction

Sexual dimorphism intrigued Darwin, and led to the establishment of the field of sexual selection that continues to fascinate scientists. We previously applied artificial selection to the modified legs of fighters, and as a consequence we observed the coevolution of relative leg width in both scrambler males and females (Fig. 1) in less than ten generations (results reported in[12]). We hypothesise that the evolution of greater fecundity is due to a genetically correlated response: genes that increase relative leg width in fighter males somehow improve female fecundity as a side effect, perhaps via epistasis.

Results

Conclusion