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.