Early stages of sex chromosome evolution
Mrnjavac A. 2024. Early stages of sex chromosome evolution. Institute of Science and Technology Austria.
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Thesis
| PhD
| Published
| English
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Corresponding author has ISTA affiliation
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ISTA Thesis
Abstract
Sex chromosomes and autosomes exhibit very different evolutionary dynamics.
The Y chromosome usually degenerates, leaving many X-linked loci hemizygous in
males. Since recessive X-linked mutations are always exposed to selection in males,
selection is more efficient on the X chromosome than on autosomes on recessive
mutations, leading to faster adaptation on the X chromosome than other genomic
regions, if beneficial mutations are on average recessive (known as the Faster-X
effect). In the presence of the functional, but non-recombining gametolog on the Y (as
is often the case in young non-recombining regions), recessive mutations are
sheltered from selection on the X chromosome. We model this scenario and show that
the efficiency of selection is reduced on diploid X loci due to sheltering by the Y
chromosome. Reduced efficiency of selection leads to slower adaptation and
increased accumulation of deleterious mutations (Slower-X effect). We extended this
model to explore the effect of sex-specific selection on degeneration of sex
chromosomes, showing theoretically that male-limited genes degenerate on the X
chromosome and female-biased genes degenerate on the Y chromosome. This
prediction depends on the effective population size and the mutation rate, explaining
the variety of sex chromosome degeneration patterns observed in nature.
To test for direct evidence of a Slower-X (or Slower-Z) effect, we analyzed the
ZW sex chromosomes of the flatworm Schistosoma japonicum, which have a very
young non-recombining region with non-degenerated W. Diploid Z-linked genes have
higher ratios of non-synonymous to synonymous polymorphisms than autosomal
genes, supporting reduced efficiency of selection on the diploid Z region. These results
provide evidence of sheltering by the W chromosome, a mechanism that could
contribute to Z (X) chromosome degeneration, and illustrate contrasting evolutionary
patterns in old and young sex chromosome regions. In addition, genes with sexspecific patterns of expression show opposite patterns of selection in the young
(diploid) and old (hemizygous) Z, showing the complex manner in which sex-specific selection shapes the evolutionary patterns of sex chromosomes.
Keywords
Publishing Year
Date Published
2024-11-11
Publisher
Institute of Science and Technology Austria
Acknowledged SSUs
Page
181
ISSN
IST-REx-ID
Cite this
Mrnjavac A. Early stages of sex chromosome evolution. 2024. doi:10.15479/at:ista:18531
Mrnjavac, A. (2024). Early stages of sex chromosome evolution. Institute of Science and Technology Austria. https://doi.org/10.15479/at:ista:18531
Mrnjavac, Andrea. “Early Stages of Sex Chromosome Evolution.” Institute of Science and Technology Austria, 2024. https://doi.org/10.15479/at:ista:18531.
A. Mrnjavac, “Early stages of sex chromosome evolution,” Institute of Science and Technology Austria, 2024.
Mrnjavac A. 2024. Early stages of sex chromosome evolution. Institute of Science and Technology Austria.
Mrnjavac, Andrea. Early Stages of Sex Chromosome Evolution. Institute of Science and Technology Austria, 2024, doi:10.15479/at:ista:18531.
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