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Disentangling the Causes for Faster-X Evolution in Aphids.

Julie Jaquiéry^1,2, Jean Peccoud³, Tiphaine Ouisse²

¹INRA UMR IGEPP Domaine de la Motte, Le Rheu, France.

Genome Biology and Evolution

|January 24, 2018

Summary

Faster X chromosome evolution in aphids is driven by relaxed selection, not positive selection. This occurs because many X-linked genes are expressed at low rates, escaping scrutiny. This finding aids understanding of genome evolution.

Keywords:

drift evolutionary rates hemizygosity selection sex chromosome sex-biased expression

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Area of Science:

Genomics
Evolutionary Biology
Population Genetics

Background:

The X chromosome often evolves faster than autosomes due to selection efficiency and population size.
Aphids, with unique sex chromosome inheritance, offer a model to test evolutionary forces on the X chromosome.

Purpose of the Study:

To investigate the evolutionary rates of X-linked genes in the pea aphid.
To determine the primary drivers (selection vs. drift) of faster X chromosome evolution in this species.

Main Methods:

Genome-wide assignment of genes to X or autosomes using sequencing depth ratios in male (X0) vs. female (XX) aphids.
Calculation of nonsynonymous to synonymous substitution ratios (dN/dS) for X-linked and autosomal genes.
Analysis of gene expression data and polymorphism (Tajima's D) to infer selection pressures.

Main Results:

Pea aphid X-linked genes exhibit faster evolution compared to autosomal genes.
Relaxed selection, indicated by low expression of many X-linked genes, is the main contributor to faster-X evolution.
Evidence for a minor role of positive selection on male-biased X-linked genes was observed.

Conclusions:

Relaxed selection is the predominant force driving faster X chromosome evolution in aphids.
The unique biology of aphids provides insights into the general mechanisms of sex chromosome evolution.
Understanding genome evolution requires studying diverse organisms with alternative chromosomal systems.