Is Faster-X Adaptation Due to Large-Effect Mutations? An Empirical Test of a New Theory.
- 1Institute of Science and Technology Austria, Klosterneuburg 3400, Austria.
Abstract
A widespread observation in molecular evolution is that X-linked genes evolve faster-and often adapt faster-than autosomal genes. Three main ideas have been put forward to explain "faster-X" adaptation. The first is that new beneficial mutations are typically partially recessive with respect to fitness, leading to more efficient selection in hemizygous males. The second is that sex differences in recombination, or sex differences in variance in reproductive success, enhance the effective population size on the X chromosome relative to autosomes. Both ideas have limitations: the first conflicts with theories of dominance, in which beneficial mutations are proposed to be partially dominant, while the second only applies to some taxa. Here we tested a third, newly proposed theory, in which mutations with large "scaled phenotypic effects" (ie large effects relative to the distance to a phenotypic optimum) experience more positive selection on the X. Specifically, we used three proxies for scaled phenotypic effects-amino acid dissimilarity, amino acid conservation, and gene age-and estimated X and autosomal rates of adaptation of nonsynonymous mutations across effect-size classes. We did this in three lineages with a well-documented faster-X: Drosophila melanogaster, Mus musculus, and Homo sapiens. As expected, we found that large-effect mutations were more likely to be under purifying selection and less likely to be under positive selection than small-effect mutations. However, contrary to the new theory, faster-X adaptation was not enriched among large-effect mutations. Overall, our results highlight an unresolved gap between patterns of sex chromosome evolution and theories of dominance for fitness.
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BibTeX
@article{ruzicka2026fasterx,
title = {{Is Faster-X Adaptation Due to Large-Effect Mutations? An Empirical Test of a New Theory.}},
author = {Filip Ruzicka and Beatriz Vicoso},
journal = {Genome biology and evolution},
year = {2026},
month = oct,
doi = {10.1093/gbe/evag259},
url = {https://europepmc.org/article/MED/42858367},
note = {CC BY-NC},
}
RIS
TY - JOUR
TI - Is Faster-X Adaptation Due to Large-Effect Mutations? An Empirical Test of a New Theory.
AU - Filip Ruzicka
AU - Beatriz Vicoso
PY - 2026
DA - 2026/10/01
JO - Genome biology and evolution
DO - 10.1093/gbe/evag259
UR - https://europepmc.org/article/MED/42858367
AB - A widespread observation in molecular evolution is that X-linked genes evolve faster-and often adapt faster-than autosomal genes. Three main ideas have been put forward to explain "faster-X" adaptation. The first is that new beneficial mutations are typically partially recessive with respect to fitness, leading to more efficient selection in hemizygous males. The second is that sex differences in recombination, or sex differences in variance in reproductive success, enhance the effective population size on the X chromosome relative to autosomes. Both ideas have limitations: the first conflicts with theories of dominance, in which beneficial mutations are proposed to be partially dominant, while the second only applies to some taxa. Here we tested a third, newly proposed theory, in which mutations with large "scaled phenotypic effects" (ie large effects relative to the distance to a phenotypic optimum) experience more positive selection on the X. Specifically, we used three proxies for scaled phenotypic effects-amino acid dissimilarity, amino acid conservation, and gene age-and estimated X and autosomal rates of adaptation of nonsynonymous mutations across effect-size classes. We did this in three lineages with a well-documented faster-X: Drosophila melanogaster, Mus musculus, and Homo sapiens. As expected, we found that large-effect mutations were more likely to be under purifying selection and less likely to be under positive selection than small-effect mutations. However, contrary to the new theory, faster-X adaptation was not enriched among large-effect mutations. Overall, our results highlight an unresolved gap between patterns of sex chromosome evolution and theories of dominance for fitness.
SN - 1759-6653
ER -