Abstract
Sex chromosomes are the most dynamic entity in any genome having unique morphology, gene content, and evolution. They have evolved multiple times and independently throughout vertebrate evolution. One of the major genomic changes that pertain to sex chromosomes involves the amplification of common repeats. It is hypothesized that such amplification of repeats facilitates the suppression of recombination, leading to the evolution of heteromorphic sex chromosomes through genetic degradation of Y or W chromosomes. Although contrasting evidence is available, it is clear that amplification of simple repetitive sequences played a major role in the evolution of Y and W chromosomes in vertebrates. In this review, we present a brief overview of the repetitive DNA classes that accumulated during sex chromosome evolution, mainly focusing on vertebrates, and discuss their possible role and potential function in this process.
Highlights
IntroductionTwo major types of sex chromosome systems exist in vertebrates, XX female/XY male (e.g., human and salmon) and ZZ male/ZW female (birds and snakes)
Two major types of sex chromosome systems exist in vertebrates, XX female/XY male and ZZ male/ZW female
This has resulted in the most detailed research on vertebrate sex chromosomes being carried out on species which have had at least part of the euchromatic region of their Y or W chromosome sequenced, for example, the evolutionary old sex chromosomes of eutherian mammals [4,5,6,7] or the evolutionarily young sex chromosomes of fishes such as the halfsmooth tongue sole [8], three-spine stickleback [9], and medaka [10]
Summary
Two major types of sex chromosome systems exist in vertebrates, XX female/XY male (e.g., human and salmon) and ZZ male/ZW female (birds and snakes). Despite the interest in this area, difficulties in sequencing highly repetitive Y and W chromosomes have hampered progress towards gaining a fuller understanding of the mechanisms involved in their evolution. This has resulted in the most detailed research on vertebrate sex chromosomes being carried out on species which have had at least part of the euchromatic region of their Y or W chromosome sequenced, for example, the evolutionary old sex chromosomes of eutherian mammals (three primates and two carnivores) [4,5,6,7] or the evolutionarily young sex chromosomes of fishes such as the halfsmooth tongue sole [8], three-spine stickleback [9], and medaka [10]. As the technical advances in this field are relatively recent, we call upon data from organisms outside the vertebrate phylogeny to identify important areas for future research in vertebrates
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