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.

This file has two tabs. The first one contains the fecundity data (number of eggs laid in 10 days) for females after 13 generations of artificial selection applied on the fighter legs of fighter males of the mite Rhizoglyphus echinopus. Each row contains information for one female mated to two scrambler males, and the columns contain information about (respectively from left to right): replicate selection line; selection direction (up for thicker legs and down for thinner legs); the ID of the line (replicate and direction combined); family ID (the family from which each female was derived in the previous generation); and the number of eggs laid by this female. The other tab has information about sex and morph ratio through the first 9 generations of selection. Each row has information on the offspring of selected sires in each generation, and the columns contain information about (respectively from left to right): generation (1 to 9); replicate selection line; selection direction (as described above); the ID of the line (as described above); the ID of the particular sire; number of fighter males in that sire's offspring, number of females in that sire's offspring, number of scrambler males in that sire's offspring,  number of 'intermorph' males (rare males with one scrambler and one fighter leg — they are counted as males for 'sex ratio' but ignored in the information for 'morph ratio') in that sire's offspring; and then descriptive stats based on these numbers (total of adults, total of males, sex ratio and morph ratio).