Evidence for Faster X Chromosome Evolution in Spiders

• In species with chromosomal sex determination, X chromosomes are predicted to evolve faster than autosomes because of positive selection on recessive alleles or weak purifying selection. We investigated X chromosome evolution in Stegodyphus spiders that differ in mating system, sex ratio, and population dynamics. We assigned scaffolds to X chromosomes and autosomes using a novel method based on flow cytometry of sperm cells and reduced representation sequencing. We estimated coding substitution patterns (dN/dS) in a subsocial outcrossing species (S. africanus) and its social inbreeding and female-biased sister species (S. mimosarum), and found evidence for faster-X evolution in both species. X chromosome-to-autosome diversity (piX/piA) ratios were estimated in multiple populations. The average piX/piA estimates of S. africanus (0.57 [95% CI: 0.55–0.60]) was lower than the neutral expectation of 0.75, consistent with more hitchhiking events on X-linked loci and/or a lower X chromosome mutation rate, and we provide evidence in support of both. The social species S. mimosarum has a significantly higher piX/piA ratio (0.72 [95% CI: 0.65–0.79]) in agreement with its female-biased sex ratio. Stegodyphus mimosarum also have different piX/piA estimates among populations, which we interpret as evidence for recurrent founder events. Simulations show that recurrent founder events are expected to decrease the piX/piA estimates in S. mimosarum, thus underestimating the true effect of female-biased sex ratios. Finally, we found lower synonymous divergence on X chromosomes in both species, and the male-to-female substitution ratio to be higher than 1, indicating a higher mutation rate in males.

• In species with chromosomal sex determination, X chromosomes are predicted to evolve faster than autosomes becauseof positive selection on recessive alleles or weak purifying selection. We investigated X chromosome evolution inStegodyphus spiders that differ in mating system, sex ratio, and population dynamics. We assigned scaffolds to Xchromosomes and autosomes using a novel method based on flow cytometry of sperm cells and reduced representationsequencing. We estimated coding substitution patterns (dN/dS) in a subsocial outcrossing species (S. africanus) and itssocial inbreeding and female-biased sister species (S. mimosarum), and found evidence for faster-X evolution in bothspecies. X chromosome-to-autosome diversity (piX/piA) ratios were estimated in multiple populations. The average piX/piA estimates of S. africanus (0.57 [95% CI: 0.55–0.60]) was lower than the neutral expectation of 0.75, consistent withmore hitchhiking events on X-linked loci and/or a lower X chromosome mutation rate, and we provide evidence insupport of both. The social species S. mimosarum has a significantly higher piX/piA ratio (0.72 [95% CI: 0.65–0.79]) inagreement with its female-biased sex ratio. Stegodyphus mimosarum also have different piX/piA estimates amongpopulations, which we interpret as evidence for recurrent founder events. Simulations show that recurrent founderevents are expected to decrease the piX/piA estimates in S. mimosarum, thus underestimating the true effect of femalebiasedsex ratios. Finally, we found lower synonymous divergence on X chromosomes in both species, and the male-tofemalesubstitution ratio to be higher than 1, indicating a higher mutation rate in males.

Ämnesord

NATURVETENSKAP  -- Biologi -- Evolutionsbiologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Evolutionary Biology (hsv//eng)

Nyckelord

faster-x,female bias,sex chromosome,social spider

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