Evidence for Faster X Chromosome Evolution in Spiders

Area of Science:

• Evolutionary biology
• Genomics
• Population genetics

Background:

• X chromosomes are predicted to evolve faster than autosomes in species with chromosomal sex determination.
• This faster evolution is attributed to factors like positive selection on recessive alleles or relaxed purifying selection.
• Understanding X chromosome evolution provides insights into speciation and adaptation.

Purpose of the Study:

• To investigate X chromosome evolution in Stegodyphus spiders, which exhibit diverse mating systems, sex ratios, and population dynamics.
• To compare X chromosome evolution between a subsocial outcrossing species (S. africanus) and a social, inbreeding, female-biased sister species (S. mimosarum).
• To assess the impact of mating systems, sex ratios, and population dynamics on X chromosome evolution.

Main Methods:

• Novel method combining flow cytometry of sperm cells and reduced representation sequencing to assign scaffolds to X chromosomes and autosomes.
• Estimation of coding substitution patterns (dN/dS) to detect selection.
• Calculation of X chromosome-to-autosome diversity (piX/piA) ratios across multiple populations.
• Simulations to model the effects of population dynamics on diversity ratios.

Main Results:

• Evidence for faster-X evolution was found in both Stegodyphus africanus and Stegodyphus mimosarum.
• S. africanus showed a lower piX/piA ratio (0.57) than neutral expectation (0.75), suggesting hitchhiking and/or a lower X chromosome mutation rate.
• S. mimosarum exhibited a higher piX/piA ratio (0.72), consistent with its female-biased sex ratio, with population-specific variations possibly due to recurrent founder events.
• Lower synonymous divergence on X chromosomes and a male-to-female substitution ratio greater than 1 indicated a higher mutation rate in males.

Conclusions:

• The study provides evidence supporting faster-X chromosome evolution in Stegodyphus spiders.
• X chromosome evolution is influenced by mating systems and sex ratios, with female-biased sex ratios potentially increasing diversity on the X chromosome.
• Population dynamics, such as recurrent founder events, can complicate the interpretation of diversity patterns and potentially underestimate the effects of sex ratios.