Sex determination in the animal kingdom is as complex and variable as its many species. For invertebrate organisms, however, three basic mechanisms draw most of our current scientific interest, and are thus considered representative. For the insect order of Hymenoptera, the choice lies between uniparental haploid males and biparental diploid females, originating from unfertilized and fertilized eggs accordingly, a mechanism also known as single-locus Complementary Sex Determination (sl-CSD). Relevant information has recently been obtained by the extensive study of genetic sex determination in the honeybee. 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. Thus, the nematodes are also an important tool for the study of sex-specific genes, some of which are, at least partially, homologous to genes also observed in vertebrates. Some of the latter have been associated with sex determination and differentiation in other species as well, which supports the theory of a common evolution for the modern animal kingdom and reservation of the qualities of some basic genetic domains. This is a three-fold comparative review concerning the basic molecular patterns of sex determination in invertebrate species, according to the most recent research findings in this field.

Sex determination has been known to exhibit a remarkable variability of mechanisms across the animal kingdom (Beye et al. 2003). Invertebrate organisms perhaps follow that rule more than any others. However, it is beyond our ability or even intention to study each and every one of these mechanisms in detail. Indeed, there are numerous species of invertebrates, and though sex differentiation has been studied in a minority of them, it appears that even morphologically similar and genetically closely related species may still present entirely different sex determination and differentiation patterns. Thus, the ability to examine the whole group of invertebrates in terms of sex determination is indeed a tedious task, having to surpass extreme difficulties in terms of financial burden and technical/methodological complexity.

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