Genetic diversity and efficacy of natural selection in spiders with pre-copulatory sexual cannibalism

Factors that increase reproductive variance among individuals act to reduce effective population size (Ne), which accelerates loss of genetic diversity and decreases efficacy of purifying selection. These factors include sexual cannibalism, offspring investment, and mating system. Pre-copulatory sexual cannibalism where the female consumes the male prior to mating exacerbates this effect. We performed comparative transcriptomics in two spider species, the cannibalistic Trechaleoides biocellata and the non-cannibalistic T. keyserlingi, to generate genomic evidence to support these predictions. First, we estimated heterozygosity and found that genetic diversity is relatively lower in the cannibalistic species. Second, we calculated dN/dS ratios as a measure of purifying selection, higher dN/dS ratio indicated relaxed purifying selection in the cannibalistic species. These results are consistent with the hypothesis that sexual cannibalism impacts operational sex ratio and demographic processes, which interact with evolutionary forces to shape the genetic structure of populations. However, other factors such as the mating system and life-history traits contribute to shape Ne. Comparative analyses across multiple contrasting species-pairs would be required to disentangle these effects. Our study highlights that extreme behaviours such as pre-copulatory cannibalism may have profound eco-evolutionary effects.