EFFECTIVE POPULATION SIZE AND THE FASTER-X EFFECT: EMPIRICAL RESULTS AND THEIR INTERPRETATION

TLDR

The X chromosome differs from autosomes by hemizygosity and potentially smaller effective population size, which may accelerate adaptive evolution—an effect known as Faster‑X—whose strength varies among species and is influenced by demography and mating system. This study integrates genomic data on Faster‑X evolution with demographic, mating system, and sex‑chromosome regulatory factors to investigate their influence on the effect. The authors compiled genomic datasets across diverse animals and examined them alongside demographic parameters, mating systems, and dosage‑compensation mechanisms. They found that effective population size and dosage‑compensation mechanisms modulate the extent of Faster‑X evolution, accounting for clade‑specific patterns observed.

Abstract

The X or Z chromosome has several characteristics that distinguish it from the autosomes, namely hemizygosity in the heterogametic sex, and a potentially different effective population size, both of which may influence the rate and nature of evolution. In particular, there may be an accelerated rate of adaptive change for X-linked compared to autosomal coding sequences, often referred to as the Faster-X effect. Empirical studies have indicated that the strength of Faster-X evolution varies among different species, and theoretical treatments have shown that demography and mating system can substantially affect the degree of Faster-X evolution. Here we integrate genomic data on Faster-X evolution from a variety of animals with the demographic factors, mating system, and sex chromosome regulatory characteristics that may influence it. Our results suggest that differences in effective population size and mechanisms of dosage compensation may influence the perceived extent of Faster-X evolution, and help to explain several clade-specific patterns that we observe.