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Sex Determination

Nonmammalian Mechanisms



Sex determination and differentiation occur in virtually all complex organisms, but the mechanisms used by various animal classes, and even by various vertebrates, differ significantly. Birds, for instance, lack a clear homolog of SRY. In birds it is the female, rather than the male, that has two different sex chromosomes, with males being ZZ, and females ZW. In many reptiles, environmental conditions, rather than genetic factors, are the primary determinant of sex. The temperature at which eggs are incubated determines sex in some lizard, turtle, and alligator species.



In both the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans, the primary sex determination mechanisms and the molecular cascades controlling sexual differentiation have been studied extensively. Primary sex determination in these animals does not involve the Y chromosome but instead is determined by the ratio of the number of X chromosomes to autosomal (nonsex) chromosomes. By examining individuals with unusual numbers of various chromosomes it has been determined that in Drosophila those with one or fewer X chromosomes per diploid autosome set develop as males while those with two or more X chromosomes develop as females. Individuals with intermediate ratios such as those with two X chromosomes and a triploid set of autosomes develop as intersexes with both male and female characteristics. Although this ratio serves as the primary determinant of sex in both of these organisms, the specific gene products that influence this ratio assessment are different, demonstrating that different molecular mechanisms can be used for a similar purpose.

There is also significant variability in the strategies by which the outcome of sex determination is communicated to the various tissues that undergo sexual differentiation. In humans and most other mammals, the presence or absence of SRY protein in cells of the gonad specifies their sexual differentiation and which hormones are secreted by the gonad to direct the sexual differentiation of most other cells in the individual.

In Drosophila hormones have little effect on sexual differentiation. Instead, with only a few exceptions, each cell decides its sex independently of other cells and tissues. This "cell autonomous" mechanism is demonstrated in experimentally produced mosaic organisms called gynandromorphs ("male-female forms") in which some cells are XX (female) and others XO (male). Such individuals develop into adults with a mix of male and female cell types that match each cell's genotype. The lack of evolutionary conservation of sex-determining mechanisms among animals is particularly interesting because of the similarities that exist in other major switch genes for basic developmental processes.

Jeffrey T. Villinski

and William Mattox

Bibliography

Berta, Philippe, et al. "Genetic Evidence Equating SRY and the Testis-Determining Factor." Nature 348 (1990): 448-450.

Cline, Thomas W., and Barbara J. Meyer. "Vive la Difference: Males vs. Females inFlies vs. Worms." Annual Reviews of Genetics 30 (1996): 637-702.

Gilbert, Scott F. Developmental Biology, 6th ed. Sunderland, MA: Sinauer Associates,2000.

Hodgkin, Jonathan. "Genetic Sex Determination Mechanisms and Evolution." BioEssays 4 (1992): 253-261.

Sinclair, Andrew H., et al. "A Gene from the Human Sex-Determining Region Encodes a Protein with Homology to a Conserved DNA-Binding Motif." Nature 346 (1990): 240-224.

Zarkower, David. "Establishing Sexual Dimorphism: Conservation amidst Diversity?"Nature Reviews Genetics 3 (2001): 175-185.

Additional topics

Medicine EncyclopediaGenetics in Medicine - Part 4Sex Determination - Mammalian Mechanisms, Nonmammalian Mechanisms