Activity of the Sex-Determining Gene tra-2 Is Modulated to Allow Spermatogenesis in the C. ELEGANS Hermaphrodite

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In the nematode C. elegans, there are two sexes, the self-fertilizing hermaphrodite (XX) and the male (XO). The hermaphrodite is essentially a female that makes sperm for a brief period before oogenesis. Sex determination in C. elegans is controlled by a pathway of autosomal regulatory genes, the state of which is determined by the X:A ratio. One of these genes, tra-2, is required for hermaphrodite development, but not for male development, because null mutations in tra-2 masculinize XX animals but have no effect on XO males. Dominant, gain-of-function tra-2 mutations have now been isolated that completely feminize the germline of XX animals so that they make only oocytes and no sperm and, thus, are female. Most of the tra-2(dom) mutations do not correspondingly feminize XO animals, so they do not appear to interfere with control by her-1 , a gene thought to negatively regulate tra-2 in XO animals. Thus, these mutations appear to cause gain of tra-2 function in the XX animal only. Dosage studies indicate that 5 of 7 tra-2( dom) alleles are hypomorphic, so they do not simply elevate XX tra-2 activity overall. These properties suggest that in the wild type, tra-2 activity is under two types of control: (1) in males, it is inactivated by her-1 to allow male development to occur, and (2) in hermaphrodites, tra-2 is active but transiently inactivated by another, unknown, regulator to allow hermaphrodite spermatogenesis; this mode of regulation is hindered by the tra-2(dom) mutations, thereby resulting in XX females.

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