Experimental evolution under biased sex ratios: phenotypic and genomic responses in the bulb mite, <i>Rhizoglyphus robini</i>

Sexual selection may increase population fitness by favouring high-condition individuals and accelerating the purging of deleterious alleles. However, it can also reduce population fitness through intra- and interlocus sexual conflict by promoting male-benefit traits that harm females and maintain polymorphism at sexually antagonistic loci. The balance between these opposing forces remains unresolved, yet it has major consequences for how sexual selection shapes population fitness and genome-wide variation. To explore the genomic and phenotypic effects of sexual selection and sexual conflict, we evolved replicated bulb mite (Rhizoglyphus robini) lines for 28 generations under male- versus female-biased sex ratios and combined phenotypic assays with whole-genome resequencing. Female fecundity and inbreeding depression did not differ between treatments, and genomic analyses revealed no treatment effect on the loss of rare, putatively deleterious SNPs. Contrary to expectations, males from male-biased lines were less harmful to stock females than males from female-biased lines. Genome-wide nucleotide diversity declined similarly across generations in both treatments, although synonymous exonic diversity declined more slowly in male-biased lines. While only a few SNPs diverged consistently between treatments, we identified large treatment-specific haplotype blocks indicating that multiple genomic regions were involved in response to sex-ratio manipulation. Overall, our results indicate that sex ratio manipulation drives evolution of male harm to females and widespread haplotype frequency changes without clear evidence for enhanced purging or maintenance of genetic diversity. The response thus appears to reflect adaptation to altered level of reproductive competition, but without measurable consequences for population fitness and genetic diversity.