%0 Journal Article
%T Repetitive Sequence and Sex Chromosome Evolution in Vertebrates
%A Tariq Ezaz
%A Janine E. Deakin
%J Advances in Evolutionary Biology
%D 2014
%R 10.1155/2014/104683
%X 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. 1. Introduction 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). How these functionally important chromosomes evolve has been a topic for debate for more than a century, since the discovery of the first sex chromosomes in the late 1800s [1¨C3]. 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¨C7] or the evolutionarily young sex chromosomes of fishes such as the half-smooth tongue sole [8], three-spine stickleback [9], and medaka [10]. The comparison between species of evolutionarily advanced Y chromosomes is not ideal for gaining insight into the mechanisms driving sex chromosome evolution as the chromosomes have undergone extensive changes and degeneration, perhaps even losing key clues required to unravel their evolution. In contrast, the sequencing of young sex chromosomes should help in elucidating these driving mechanisms [11], particularly if comparisons can be made to more highly diverged sex chromosomes that share a common ancestry. Although we have some understanding of the molecular organization of sex chromosomes in model vertebrate species, it is largely unknown for the
%U http://www.hindawi.com/journals/aeb/2014/104683/