Evolution of Sex Chromosomes in Plants

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Abstract Sex chromosomes are known from a minority of flowering plants (angiosperms), and from some haploid plants, but the sex‐determining chromosomes of many dioecious plants (i.e. species with the sexual stage individuals being either purely male or female) are either unstudied, or are not morphologically different between the two sexes. Both these observations and the taxonomically scattered distribution of species with sex chromosomes suggest that many plant sex chromosome systems evolved recently and independently in different taxa. A fundamental characteristic of sex chromosome pairs is possession of a nonrecombining region. Some plants may be in an early stage of evolving separate sexes, so that this has not yet evolved. Other plants appear to have small nonrecombining regions, or ‘proto‐sex chromosomes’, whereas a few species have cytologically detectable sex chromosome heteromorphism, and probably have large nonrecombining regions. With modern molecular approaches, these different states can now be identified and studied. Key Concepts: The evolution of separate sexes (males and females) from an initial state in which individuals have both sex functions requires mutations of at least two genes. Sex chromosome pairs are defined as homologous chromosomes (that pair in meiosis, and part of the pair recombines) but which also include a nonrecombining region involved in sex determination. During the evolution of separate sexes, after male‐ and female‐sterility mutations arise, selection will promote reduced recombination between the different genes, in order to avoid producing sterile individuals (with both male‐ and female‐sterility mutations) that will arise through recombination between the genes. The existence of evolutionary strata (see Glossary) shows that recombination suppression between sex chromosomes has occurred in several distinct events, at different times, and the main hypothesis to explain this is that sexually antagonistic polymorphisms may arise in the recombining regions of sex chromosome pairs, and that this situation again generates selection for reduced recombination.