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Evolution of Hermaphroditism

Christopher C. Baldi
Bachelor of Science, 2001
The Richard Stockton College of New Jersey

Thesis Advisor: Ronald Ellis, Ph.D.

Cell and Molecular Biology Program

Science Center, Room 290

Wednesday, May 26, 2010
12 PM


Although the nematode C. elegans produces self-fertile hermaphrodites, it descended from a male/female species, so hermaphroditism provides a model for the origin of novel traits. In the related species C. remanei, which has only male and female sexes, lowering the activity of tra-2 by RNA interference created XX animals that made spermatids as well as oocytes, but their spermatids could not activate without the addition of male seminal fluid. However, by lowering the expression of both tra-2 and swm-1, a gene that regulates sperm activation in C. elegans, we produced XX animals with active sperm that were self-fertile. Thus, the evolution of hermaphroditism in Caenorhabditis probably required two steps a mutation in the sex-determination pathway that caused XX spermatogenesis, and a mutation that allowed these spermatids to self-activate.

In recent years, the discovery of conserved developmental regulatory pathways has raised interest in how forces other than selection might affect evolution. When we created artificial hermaphrodites in the species C. remanei, we discovered a developmental bias leading to the production of sexually dimorphic sperm in these animals. By using several sex-determination mutants in the male/hermaphrodite species C. elegans, we determined that this bias is caused by competition between spermatocytes and oocytes, and by the need for several male somatic tissues to support the production of larger sperm. Finally, analysis of all species in the elegans group showed that the male/female ancestor probably had large sperm, and that selection has decreased the average size of both male and hermaphrodite sperm in each of the hermaphrodite lineages.

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