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Evidence for Faster X Chromosome Evolution in Spiders.

Jesper Bechsgaard1, Mads Fristrup Schou1,2, Bram Vanthournout1,3

  • 1Department of Bioscience, Aarhus University, Aarhus C, Denmark.

Molecular Biology and Evolution
|March 27, 2019
PubMed
Summary
This summary is machine-generated.

X chromosomes evolve faster due to selection and mutation rate differences. This study in Stegodyphus spiders reveals faster-X evolution and unique patterns linked to mating systems and sex ratios.

Keywords:
faster-Xfemale biassex chromosomesocial spider

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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.