Bovine X and Y Chromosomes

Abstract

Sex chromosomes evolved from a pair of autosomes (Muller 1914) and are believed to be the result of genetic sex determination that originated when a sex-determining gene was acquired by one member of the pair to become the sex specific determining chromosome. This gave origin to the male heterogamety, XX female:XY male, and female heterogamety, ZW female:ZZ male, systems. The former is observed in mammals, some species of turtles, insects, lizards, and even some plants and the latter in birds, amphibians, snakes, and some species of fish, turtles, insects, and lizards (Modi and Crews 2005). Sex chromosomes show a relative gradient of morphological and size differentiation moving from undifferentiated sex chromosomes in fishes and amphibians (Ohno 1967) to those of mammals and birds. The X and Z chromosomes are large and gene rich, while, in comparison, the Y and W chromosomes are significantly smaller, gene poor, and contain large heterochromatic blocks. However, the gene content of the XY and ZW systems is different (Nanda et al. 1999).In the broad sense, the X and Y chromosomes have two regions: (1) the pseudoautosomal region (PAR), which is the recombining region, and (2) the X-specific and Y-specific regions that do not pair and therefore do not recombine during meiosis.Our current understanding of X chromosome evolution in mammals is that it is formed by four evolutionary strata and the PAR (Graves 2006). The first evolutionary layer known as the X conserved region (XCR) was identified by the comparison of human X orthologous genes across mammals. This XCR, a conserved block of euchromatin, represents the original autosome pair from which sex chromosomes evolved (Glas et al. 1999) about 166 million years ago (MYA) (Veyrunes et al. 2008). The second X chromosome evolutionary strata is defined by genes that are orthologous to autosomal genes in marsupials and monotremes; therefore, this region was only added about 90-50 MYA and is known as the X added region (XAR). However, comparisons of chicken homologs to the human X chromosome subdivided XCR into two strata and the XAR (Nanda et al. 1999;Kohn et al. 2004). A further refinement of our understanding of these evolutionary strata was achieved by comparing gene sequences between the human Y and X chromosomes. The oldest group of genes (more divergent) corresponds to the XCR stratum I, and the second oldest to XCR stratum II. Similarly, the XAR contains two clusters of genes (evolutionary strata III and IV) differentiated on the basis of their homology/divergence with copies found on the Y chromosome and the PAR (Lahn and Page 1999b).