Sex determination is an integral part of reproduction and an essential process for the evolvement and enrichment of the genome. It has thus been the subject of many studies in reference to species across the entire animal kingdom. From insects to mammals, there is much to learn from the many mechanisms employed to determine sexual fate. This is no lost cause, since the study of sex determination and differentiation is only the natural expansion of comparative biology and reproductive physiology in the modern, molecular Era. Interestingly, data so far accumulated by a variety of model organisms has shown a relative economy in the molecular regulation of sex determination. More specifically, sex determination has so far proven to be a result of one of the following three mechanisms:
a) Environmental action on the embryo at a crucial stage of development. To the extent that this interaction is associated with temperature alterations, the process is also described as temperature-dependent sex determination and the developmental stage of sex determination is referred to as the thermosensitive period (TSP). This mechanism is mainly observed in reptiles and fish.
b) Genetic action, when at least one specific gene is considered to be the central regulator in a cascade of events leading to the determination of sexual phenotype. This mechanism is already known to apply in the case of several animals, including invertebrates (insects, worms) and amphibians. Moreover, it is a proposed regulatory mechanism for several species, whose study has so far been limited or led to inconclusive data as to the attempt to detect a single, specific, sex-determining gene.
c) The presence of distinct sex chromosomes or gonosomes. The identical pair may be present in both males (birds) and females (mammals) and their major sex-determining gene may be either known (e.g. mammalian SRY) or still suspected [1-3].
Although sex determination has been suggested to promote specific functions at a universal level, such as selective cell proliferation (Mittwoch) or steroid hormone accumulation (Howard), this issue remains debatable [1,4]. What is even more intriguing is the fact that the conservation of relatively limited regulatory patterns in sex determination may suggest the presence of a single general regulatory scheme, at least in vertebrates, potentially involving or incorporating both hormonal elements and dosage compensation epigenetic regulatory phenomena, whenever necessary . Such a discovery would bear great implications for comparative biology studies and might also allow important applications in the field of reproductive endocrinology. The study of more model organisms is a necessity to investigate this hypothesis and the consolidation of both recent and classic data from the relevant research work may significantly facilitate this discussion. This essay is dedicated to the brief and yet compact presentation of some of the better studied animal models of sex determination, in an attempt to approach that knowledge.