The C. elegans hermaphrodite is essentially identical to females of related gonochoristic species, except for spermatogenesis in its otherwise female body. This androdioecious mode of sexual reproduction is rare in both plants and animals, but has evolved independently from gonochoristic ancestors multiple times in different Rhabditid nematode species. We would like to discover the genetic controls that distinguish sex determination in hermaphrodites and females. To this end we are characterizing
tra-2 in the closest gonochoristic relative of C. elegans, C. remanei. As in C. briggsae (Kuwabara and Shah, 1994), a divergent C. remanei
tra-2 homologue exists as part of a conserved operon. RNAi demonstrates that the function of
Cr-tra-2 in repressing male fates in both germ line and soma is conserved. Given that downregulation of
Cr-tra-2 by RNAi results in sperm, we assessed the conservation of two known negative-regulatory sites required for hermaphrodite spermatogenesis in C. elegans. One, the mx region of TRA-2A and TRA-2B (Kuwabara et al., 1998), is as well conserved in C. remanei as in C. briggsae, suggesting that this control is not unique to hermaphroditic species. Another, the TGE element in the 3' UTR of the
tra-2 mRNA (Goodwin et al., 1993; Jan et al., 1997), appeared, by inspection, to be missing in C. remanei. However, RNA gel-shift assays demonstrate that there is a factor in C. remanei that specifically binds both the C. elegans and C. remanei 3' UTRs. This suggests that TGE-mediated downregulation of
tra-2 is also conserved in C. remanei, a hypothesis we are now testing. Several researchers have proposed that the mx and/or TGE controls may be novel features of hermaphroditic species. However, our data suggest that while they are necessary for hermaphrodite spermatogenesis, they are general and do not constitute the actual "switch mechanism" per se. androdioecy: Mating system with hermaphrodite and male sexes. Used in nematodes, shrimp, and a few odd flowers. gonochorism: (a.k.a. dioecy) Mating system with male and female sexes; very popular. Goodwin, E.B., et al. (1993) Cell 75: 329-339. Jan, E., et al. (1997) EMBO J. 20: 6301-13. Kuwabara, P.E. and Shah, S. (1994) Nucl. Acids Res. 22: 4414-4418. Kuwabara, P.E., et al. (1998). Dev. Biol. 204: 251-262.