Sexual selection does not purge deleterious mutations in experimentally evolved Drosophila

Most new mutations are neutral or deleterious. Sexual selection is hypothesised to contribute to purging of deleterious mutations from the genome, as condition-dependent secondary sexual characteristics can lead to greater mating success of males with a lower genetic load, while individuals with a higher proportion of deleterious mutations fall out of the mating pool. Here we examine the contribution of sexual selection to purging the genome of deleterious mutations using experimentally evolved D. pseudoobscura fruit fly populations The extent of sexual selection was manipulated by keeping replicate lines either under enhanced polyandry (6 males and 1 female) or strict monogamy (1 male and 1 female) for 200 generations. We inferred the distribution of fitness effects in each treatment at generations 85 and 200 and found that, contrary to most predictions, monogamous populations contain a significantly smaller proportion of more deleterious mutations than polyandrous populations. This difference between treatments increases over the course of the experiment and on the 3rd and X chromosomes, which have been previously identified as containing hotspots of genes under sexual selection. We conclude that increasing sexual selection, rather than purge the genome of highly deleterious mutations, may cause these genes to be maintained or increase in frequency, probably due to correlated effects on mating success and sexual antagonism or trade-offs with male viability. However, the total genetic load is unchanged.Significance StatementTheory suggests that sexual selection should increase a populations' fitness by purging deleterious mutations from the gene pool. However, experimental evidence has produced both support and opposition for this idea. Here we analysed sequence data from D. pseudoobscura fruit fly males that have evolved in high and low sexual selection lines for 200 generations. Taking a Distribution of Fitness Effects (DFE) approach, which allows us to estimate the relative fitness of derived alleles in each line, we found that high sexual selection lines had a higher proportion of highly deleterious alleles, possibly due to sexual antagonism. This rejects the idea that sexual selection will purge the gene pool of deleterious mutations and could even increase extinction risk.