Stronger positive and purifying selection on X-linked genes in a species with paternal genome elimination

A recent study has revealed that X-linked genes may evolve more rapidly than autosomes due to the effect of hemizygosity. This phenomenon, known as the faster-X effect, suggests that selection on expressed recessive alleles on the X chromosomes plays a significant role in sex chromosome evolution. To investigate this, researchers generated whole-genome resequencing and RNA-Seq data for the globular springtail Allacma fusca, which possesses unique reproductive characteristics that allow for the direct observation of the faster-X effect.

Despite the presence of surprising depleted neutral X-linked diversity and the need to account for the deleterious distribution of fitness effects, the study found evidence of both stronger purifying and positive selection on the sex chromosomes. This supports the idea that hemizygosity, the condition of having only one copy of a gene, can drive sex chromosome evolution.

The findings also provide the first direct evidence of the loss of DNA methylation in Collembola, a group of small hexapod insects that includes springtails.

The results of this study are significant because they provide valuable insights into the relative contribution of dominance effects to sex chromosome evolution. By eliminating the confounding sex-autosome differences that typically obscure the faster-X patterns, the researchers have shown that hemizygosity is a genuine driver of sex chromosome evolution.

Moreover, the findings suggest that sex-linked genes could contribute disproportionately to adaptive potential, which has important implications for the understanding of evolutionary processes and the genetic basis of sexual reproduction.