000			03206ntm a22002897a 4500
003			AT-ISTA
005			20250915114917.0
008			250915s2024 au ||||| m||| 00| 0 eng d
040			_cISTA
100			_aMrnjavac, Andrea _91084224
245			_aEarly stages of sex chromosome evolution
260			_bInstitute of Science and Technology Austria _c2024
500			_aThesis
505			_aAbstract
505			_aAcknowledgements
505			_aAbout the Author
505			_aList of Collaborators and Publications
505			_aTable of Contents
505			_a1 Introduction
505			_a2 Slower-X: Reduced efficiency of selection in the early stages of X chromosome evolution
505			_a3 Effects of sheltering on sex chromosome degeneration and sex-biased gene content evolution
505			_a4 Evidence of a slower-Z effect in Schistosoma japonicum
505			_a5 Discussion
520			_aSex 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.
856			_uhttps://doi.org/10.15479/at:ista:18531
942			_2ddc
999			_c768060 _d768060