Sex Chromosome Turnovers in Evolution


Animals and plants display a remarkable diversity in the way they determine sex. This implies that changes in the mode of sex determination occur over evolutionary time. Changes that involve a modification of the identity of sex chromosomes are called sex chromosome turnovers, and are caused by the spread of a mutant gene with sex‐determining properties or the translocation of the ancestral sex determiner to a new genomic location. Through both empirical and theoretical studies, the mysteries surrounding the proximate and ultimate mechanisms responsible for turnovers, and their consequences on sex chromosome evolution, are gradually being unravelled. Nevertheless, many questions surrounding these evolutionary transitions remain unanswered: Why do turnover rates vary across taxa? What are the driving evolutionary forces? Why do certain types of transitions seem more common than others?

Key Concepts

  • In many species with separate sexes, sex is determined genetically by a single gene, located on the sex chromosomes.
  • A sex chromosome turnover is a change in the identity of the sex chromosomes, resulting from a change in identity and/or location of the sex‐determining gene.
  • The study of sex chromosome turnovers in evolution has been driven mainly by theoretical work, but empirical studies are gradually catching up.
  • Different types of turnovers have been described, which have contrasting consequences for sex determination.
  • Rates of transitions vary dramatically across taxa: they are rare in some taxa and very frequent in others.
  • Many evolutionary mechanisms have been proposed as potential drivers of turnovers, but their relative implication and importance is still poorly understood.
  • Certain types of transitions seem more frequent than others, and some lineages experiencing frequent turnovers show nonrandom transitions.

Keywords: sex determination; chromosomes; transition; heterogamety; drift; sexually antagonistic selection; sex ratio; genomic conflicts

Figure 1. Sex chromosome turnovers driven by sex‐determining mutations linked to the ancestral sex determiner (homologous transitions). Coloured blocks show the ancestral and new recurrent pairs of genotypes forming simple heterogametic systems (blue: male heterogamety; red: female heterogamety). Genotypes in white are only present during the polygenic phase. Arrows indicate the chromosome lost once the transition is over.
Figure 2. Sex chromosome turnovers driven by autosomal sex‐determining mutations (nonhomologous transitions). Coloured blocks show the ancestral and new recurrent pairs of genotypes forming simple heterogametic systems (blue: male heterogamety; red: female heterogamety). Genotypes in white are only present during the polygenic phase. Arrows indicate the chromosome lost once the transition is over.
Figure 3. De Finetti diagram of frequencies of sex‐determination alleles throughout a trans‐heterogamety homologous transition driven by a dominant feminising mutation (case b1 in Figure ). The dashed line shows the allelic frequencies expected along the path of neutral equilibria that connects the ancestral sex‐determination system (XX/XY: male heterogamety) to the new one (FY/YY: female heterogamety). Along this curve, population sex ratio is balanced, female biased above the curve and male biased below. Dotted lines indicate isofrequencies of 0.25, 0.5 and 0.75.


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Further Reading

Abbott JK, Norden AK and Hansson B (2017) Sex chromosome evolution : historical insights and future perspectives. Proceedings of the Royal Society B 284 (1854): 20162806.

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How to Cite close
Saunders, Paul A(Oct 2019) Sex Chromosome Turnovers in Evolution. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0028747]