In most discussions of the evolution of sex chromosomes, it is presumed that the morphological differences between the X and Y were initiated by genetic changes. An alternative possibility is that, in the early stages, a key role was played by epigenetic modifications of chromatin structure that did not depend directly on genetic changes. Such modifications could have resulted from spontaneous epimutations at a sex‐determining locus or, in mammals, from selection in females for the epigenetic silencing of imprinted regions of (...) the paternally derived sex chromosome. Other features of mammalian sex chromosomes that are easier to explain if the epigenetic dimension of chromosome evolution is considered include the relatively large number of X‐linked genes associated with human brain development, and the overrepresentation of spermatogenesis genes on the X. Both may be evolutionary consequences of dosage compensation through X‐inactivation. BioEssays 26:1327–1332, 2004. © 2004 Wiley Periodicals, Inc. (shrink)

Mammals have an XX:XY system of chromosomal sex determination in which a small heterochromatic Y controls male development. The Y contains the testis determining factor SRY, as well as several genes important in spermatogenesis. Comparative studies show that the Y was once homologous with the X, but has been progressively degraded, and now consists largely of repeated sequences as well as degraded copies of X linked genes. The small original X and Y have been enlarged by cycles of autosomal addition (...) to one partner, recombination onto the other and continuing attrition of the compound Y. This addition–attrition hypothesis predicts that the pseudoautosomal region of the human X is merely the last relic of the latest addition. Genes (including SRY) on the conserved or added region of the Y evolved functions in male sex determination and differentiation distinct from the general functions of their X‐linked partners. Although the gonadogenesis pathway is highly conserved in vertebrates, its control has probably changed radically and rapidly in vertebrate – even mammalian – evolution. (shrink)

The genomes of virtually all sexually reproducing species contain transposable elements. Although active elements generally transpose more rapidly than they are inactivated by mutation or excision, their number can be kept in check by purifying selection if its effectiveness becomes disproportionately greater as their copy number increases. In sexually reproducing species, such synergistic selection can result from ectopic crossing-over or from homologous recombination under negative epistasis. In addition, there may be controls on transposon activity that are associated with meiosis. Because (...) a sexual lineage that abandons sex must lack such mechanisms, it may be driven to extinction by the unchecked proliferation of deleterious transposons inherited from its sexual progenitor. An important component of the evolutionary advantage of sex over asex may therefore lie in the ability of sex, despite facilitating the spread of deleterious elements within interbreeding populations, also to restrain their intragenomic proliferation. BioEssays 27:76–85, 2005. © 2004 Wiley Periodicals, Inc. (shrink)