---
OA_embargo: '6'
OA_place: publisher
_id: '18531'
abstract:
- lang: eng
  text: "Sex chromosomes and autosomes exhibit very different evolutionary dynamics.\r\nThe
    Y chromosome usually degenerates, leaving many X-linked loci hemizygous in\r\nmales.
    Since recessive X-linked mutations are always exposed to selection in males,\r\nselection
    is more efficient on the X chromosome than on autosomes on recessive\r\nmutations,
    leading to faster adaptation on the X chromosome than other genomic\r\nregions,
    if beneficial mutations are on average recessive (known as the Faster-X\r\neffect).
    In the presence of the functional, but non-recombining gametolog on the Y (as\r\nis
    often the case in young non-recombining regions), recessive mutations are\r\nsheltered
    from selection on the X chromosome. We model this scenario and show that\r\nthe
    efficiency of selection is reduced on diploid X loci due to sheltering by the
    Y\r\nchromosome. Reduced efficiency of selection leads to slower adaptation and\r\nincreased
    accumulation of deleterious mutations (Slower-X effect). We extended this\r\nmodel
    to explore the effect of sex-specific selection on degeneration of sex\r\nchromosomes,
    showing theoretically that male-limited genes degenerate on the X\r\nchromosome
    and female-biased genes degenerate on the Y chromosome. This\r\nprediction depends
    on the effective population size and the mutation rate, explaining\r\nthe variety
    of sex chromosome degeneration patterns observed in nature.\r\nTo test for direct
    evidence of a Slower-X (or Slower-Z) effect, we analyzed the\r\nZW sex chromosomes
    of the flatworm Schistosoma japonicum, which have a very\r\nyoung non-recombining
    region with non-degenerated W. Diploid Z-linked genes have\r\nhigher ratios of
    non-synonymous to synonymous polymorphisms than autosomal\r\ngenes, supporting
    reduced efficiency of selection on the diploid Z region. These results\r\nprovide
    evidence of sheltering by the W chromosome, a mechanism that could\r\ncontribute
    to Z (X) chromosome degeneration, and illustrate contrasting evolutionary\r\npatterns
    in old and young sex chromosome regions. In addition, genes with sexspecific patterns
    of expression show opposite patterns of selection in the young\r\n(diploid) and
    old (hemizygous) Z, showing the complex manner in which sex-specific selection
    shapes the evolutionary patterns of sex chromosomes. "
acknowledged_ssus:
- _id: ScienComp
- _id: CampIT
alternative_title:
- ISTA Thesis
article_processing_charge: No
author:
- first_name: Andrea
  full_name: Mrnjavac, Andrea
  id: 353FAC84-AE61-11E9-8BFC-00D3E5697425
  last_name: Mrnjavac
citation:
  ama: Mrnjavac A. Early stages of sex chromosome evolution. 2024. doi:<a href="https://doi.org/10.15479/at:ista:18531">10.15479/at:ista:18531</a>
  apa: Mrnjavac, A. (2024). <i>Early stages of sex chromosome evolution</i>. Institute
    of Science and Technology Austria. <a href="https://doi.org/10.15479/at:ista:18531">https://doi.org/10.15479/at:ista:18531</a>
  chicago: Mrnjavac, Andrea. “Early Stages of Sex Chromosome Evolution.” Institute
    of Science and Technology Austria, 2024. <a href="https://doi.org/10.15479/at:ista:18531">https://doi.org/10.15479/at:ista:18531</a>.
  ieee: A. Mrnjavac, “Early stages of sex chromosome evolution,” Institute of Science
    and Technology Austria, 2024.
  ista: Mrnjavac A. 2024. Early stages of sex chromosome evolution. Institute of Science
    and Technology Austria.
  mla: Mrnjavac, Andrea. <i>Early Stages of Sex Chromosome Evolution</i>. Institute
    of Science and Technology Austria, 2024, doi:<a href="https://doi.org/10.15479/at:ista:18531">10.15479/at:ista:18531</a>.
  short: A. Mrnjavac, Early Stages of Sex Chromosome Evolution, Institute of Science
    and Technology Austria, 2024.
corr_author: '1'
date_created: 2024-11-11T08:40:45Z
date_published: 2024-11-11T00:00:00Z
date_updated: 2026-04-07T13:22:45Z
day: '11'
ddc:
- '576'
degree_awarded: PhD
department:
- _id: GradSch
- _id: BeVi
doi: 10.15479/at:ista:18531
file:
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  checksum: 3e48b163c22114ef5d5371f758668289
  content_type: application/vnd.openxmlformats-officedocument.wordprocessingml.document
  creator: amrnjava
  date_created: 2024-11-13T12:15:28Z
  date_updated: 2025-05-11T22:30:04Z
  embargo_to: open_access
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  file_name: AMrnjavac_thesis_library.docx
  file_size: 26870629
  relation: source_file
  title: Early stages of sex chromosome evolution
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  checksum: 3ead60c1b678e7dcf018043aef3b5db2
  content_type: application/pdf
  creator: amrnjava
  date_created: 2024-11-13T12:15:54Z
  date_updated: 2025-05-11T22:30:04Z
  embargo: 2025-05-11
  file_id: '18552'
  file_name: AMrnjavac_thesis_library.pdf
  file_size: 4228766
  relation: main_file
  title: Early stages of sex chromosome evolution
file_date_updated: 2025-05-11T22:30:04Z
has_accepted_license: '1'
keyword:
- Sex chromosomes
- evolution
- selection
- sheltering
language:
- iso: eng
license: https://creativecommons.org/licenses/by-nc-nd/4.0/
month: '11'
oa: 1
oa_version: Published Version
page: '181'
publication_identifier:
  issn:
  - 2663-337X
publication_status: published
publisher: Institute of Science and Technology Austria
related_material:
  record:
  - id: '12521'
    relation: part_of_dissertation
    status: public
  - id: '18549'
    relation: part_of_dissertation
    status: public
status: public
supervisor:
- first_name: Beatriz
  full_name: Vicoso, Beatriz
  id: 49E1C5C6-F248-11E8-B48F-1D18A9856A87
  last_name: Vicoso
  orcid: 0000-0002-4579-8306
title: Early stages of sex chromosome evolution
tmp:
  image: /images/cc_by_nc_nd.png
  legal_code_url: https://creativecommons.org/licenses/by-nc-nd/4.0/legalcode
  name: Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International
    (CC BY-NC-ND 4.0)
  short: CC BY-NC-ND (4.0)
type: dissertation
user_id: ba8df636-2132-11f1-aed0-ed93e2281fdd
year: '2024'
...