Abstract
Determining sexual fate is an integral part of reproduction, used as a means to enrich the genome. A variety of such regulatory mechanisms have been described so far and some of the more extensively studied ones are being discussed.For the insect order of Hymenoptera, the choice lies between uniparental haploid males and biparental diploid females, originating from unfertilized and fertilized eggs accordingly. This mechanism is also known as single-locus complementary sex determination (slCSD). On the other hand, for Dipterans and Drosophila melanogaster, sex is determined by the ratio of X chromosomes to autosomes and the sex switching gene, sxl. Another model organism whose sex depends on the X:A ratio, Caenorhabditis elegans, has furthermore to provide for the brief period of spermatogenesis in hermaphrodites (XX) without the benefit of the "male" genes of the sex determination pathway.Many reptiles have no discernible sex determining genes. Their sexual fate is determined by the temperature of the environment during the thermosensitive period (TSP) of incubation, which regulates aromatase activity. Variable patterns of sex determination apply in fish and amphibians. In birds, while sex chromosomes do exist, females are the heterogametic (ZW) and males the homogametic sex (ZZ). However, we have yet to decipher which of the two (Z or W) is responsible for the choice between males and females.In mammals, sex determination is based on the presence of two identical (XX) or distinct (XY) gonosomes. This is believed to be the result of a lengthy evolutionary process, emerging from a common ancestral autosomal pair. Indeed, X and Y present different levels of homology in various mammals, supporting the argument of a gradual structural differentiation starting around the SRY region. The latter initiates a gene cascade that results in the formation of a male. Regulation of sex steroid production is also a major result of these genetic interactions. Similar observations have been described not only in mammals, but also in other vertebrates, emphasizing the need for further study of both normal hormonal regulators of sexual phenotype and patterns of epigenetic/environmental disruption.
Highlights
Sex determination is an integral part of reproduction and an essential process for the evolvement and enrichment of the genome
To the extent that this interaction is associated with temperature alterations, the process is described as temperature-dependent sex determination and the developmental stage of sex determination is referred to as the thermosensitive period (TSP)
Studies have shown that there seem to be no significant differences in the expression of sex-related genes
Summary
Sex determination is an integral part of reproduction and an essential process for the evolvement and enrichment of the genome. Its role mimicks that of a molecular switch, since its peak expression is limited in a specific time period that is still considered sufficient to induce male-type differentiation of the reproductive system, via downstream gene action The latter refers to several genes, including sox family members, SF1 (sex steroid regulation) and transcriptional factors, such as GATA4. The fact that members of the group have been detected in various species of vertebrates, such as fish (sox9) and all mammals (e.g. sox and sox 14 in monotremes) further emphasizes their importance for genetic sex determination [65] The observation of this gene family's evolutionary conservation adds further credit to the multistage model of sex chromosome evolution described above, since sox has been proposed as the autosomal ancestor of SRY, which places it among the chronologically first sex-related genes in the common evolutionary history of all vertebrates [66]. This mechanism may be evaluated by the analytical description of all epigenetic changes occuring at a chromatin level during the various stages of normal sex differentiation and their comparison with observations made in individuals with sex distortions [5]
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