Male heterogamety: what we know and what we need to know

Abstract

Sex chromosomes have attracted the attention of geneticists and evolutionary biologists due to of their key role in sex determination and sexual development, and their special role in the speciation process. These chromosomes stand out in the chromosomal complement since they are structurally different, have a particular meiotic behavior, are observed in certain cases as heteropicnotic bodies in the interphase nuclei, and have an associated phenotype, female or male. The heterogametic sex has a chromosomal pair that it is morphologically different (XY, ZW), while the homogametic sex has a microscopically indistinguishable chromosomal pair (XX, ZZ). In male heterogametic systems, males are heterozygous for the sex determining region and carry an X and a Y chromosome (XY), whereas females are homozygous carrying 2 X?s (XX). The X and Y chromosomes are thought to have arisen from a pair of homologous autosomes that stopped recombining with each other after acquiring a sex-determining role. The accumulation of sexually antagonistic genes linked to the sex-determining genes, the suppression of recombination between the nascent sex chromosomes, and a series of progressive and evolutionary modifications, would be the mechanisms leading to the morphological and molecular differentiation between the X and Y sex chromosomes. A striking common feature of many animal taxa is the almost complete erosion of genes on the Y chromosome. The molecular and morphological differences between the X and Y chromosomes range from microscopically imperceptible (i.e. homomorphic sex chromosomes) to conspicuous (heteromorphic sex chromosomes). The diversity of sex chromosome systems both within and between groups of insects and mammals offers multiple potentialities to address the basic structural and molecular mechanisms of the evolution of these chromosomes and to analyze their degree of molecular differentiation. This chapter will address the common and distinctive characteristics of sex chromosomes in species of the orders Heteroptera (Insecta) and of primates of the suborders Catarrhini and Platyrrhini (Mammalia). The possible evolutionary processes involved in the differentiation of these chromosomes and the current theories that explain the mechanisms involved in their evolution will be discussed.