Abstract
The evolution of sociality in spiders is associated with female bias, reproductive skew and an inbreeding mating system, factors that cause a reduction in effective population size and increase effects of genetic drift. These factors act to decrease the effectiveness of selection, thereby increasing the fixation probability of deleterious mutations. Comparative studies of closely related species with contrasting social traits and mating systems provide the opportunity to test consequences of low effective population size on the effectiveness of selection empirically. We used phylogenetic analyses of three inbred social spider species and seven outcrossing subsocial species of the genus Stegodyphus, and compared dN/dS ratios and codon usage bias between social Inbreeding and subsocial outcrossing mating systems to assess the effectiveness of selection. The overall results do not differ significantly between the social inbreeding and outcrossing species, but suggest a tendency for lower codon usage bias and higher dN/dS ratios in the social inbreeding species compared with their outcrossing congeners. The differences in dN/dS ratio and codon usage bias between social and subsocial species are modest but consistent with theoretical expectations of reduced effectiveness of selection in species with relatively low effective population size. The modest differences are consistent with relatively recent evolution of social mating systems. Additionally, the short terminal branches and lack of speciation of the social lineages, together with low genetic diversity lend support for the transient state of permanent sociality in spiders.
Highlights
The effective population size (Ne) affects the balance between effectiveness of selection and genetic drift, factors that are important in shaping the population dynamics of genes, and thereby adaptation
The evolution of sociality in spiders is associated with female bias, reproductive skew and an inbreeding mating system, factors that cause a reduction in effective population size and increase effects of genetic drift
A third model with dN/dS ratio estimated separately for internal, external social, and external subsocial branches was used to test for the effect of social mating system
Summary
The effective population size (Ne) affects the balance between effectiveness of selection and genetic drift, factors that are important in shaping the population dynamics of genes, and thereby adaptation. Inbreeding populations may further experience demographic processes that affects effective population size such as recurrent bottlenecks (Schoen and Brown 1991), frequent cycles of extinction-recolonization events (Charlesworth and Wright 2001), and low effective rate of recombination (Gordo and Charlesworth 2001). These processes may lead to a higher fixation probability of weakly deleterious mutations and loss of weakly advantageous mutations due to the stronger effect of drift (Charlesworth and Charlesworth 1987; Charlesworth 2003; Glemin and Galtier 2012). This results in accumulation of deleterious substitutions (“drift load”) over evolutionary time (Charlesworth and Wright 2001), which increases probability of extinction (Lynch et al 1995)
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