Thursday, March 13, 2025
3/13/2025
One of the most important questions in evolutionary biology is how new traits originate. To find answers, this proposal focuses on the origin of self-‐‑fertility in some species of roundworms. These species produce hermaphrodites that look like females, but make their own sperm and fertilize their own eggs. Surprisingly, these types of hermaphrodites evolved independently on many separate occasions. Thus, genetic comparisons between different roundworm species provide a natural system for learning about evolution in the laboratory. Two of these species are particularly useful. C. nigoni is male/female , but C. briggsae makes hermaphrodites. Despite this dramatic difference, the two species are so closely related that they can mate and produce fertile offspring. Hence, genetic comparisons between them should reveal which genes control self-‐‑fertility. The central hypothesis in this proposal is that new traits like self-‐‑fertility originate in three steps. The first produces preconditions that are required for the new trait. The unequal distribution of these preconditions means that some species are more likely to evolve a particular trait than others. The second step involves a precipitating change that produces the new trait in an unrefined form, by co-‐‑opting older genetic processes. The third stage consists of reinforcing changes that optimize the trait. The mutations controlling each step can be identified by their behavior in the tests described below. Aim #1: Determine how sexual development is controlled in male/female species of roundworms. These results will define the ancestral sex-‐‑determination pathway and identify preconditions for self-‐‑fertility. Aim #2: Identify C. briggsae genes that are necessary and sufficient for XX spermatogenesis. Swapping genes between female and hermaphroditic species will reveal which genes are sufficient to make herma-‐‑ phrodites. These genetic changes might have precipitated the transition to self-‐‑fertility. Other genes that only affect the number or quality of sperm in hermaphrodites probably helped optimize this trait after it first arose. Aim #3: Use forward and reverse genetics to learn how self-‐‑fertility is regulated in C. tropicalis. Dissecting the genetic regulation of self-‐‑fertility in a third species will test these predictions about how it is controlled. In roundworms, the Gli protein TRA-‐‑1 plays a key role in the control of sexual development, and interacts with other medically relevant genes to control which germ cells form eggs and which make sperm. Thus, these studies will provide new information about the regulation of genes that play important roles in human health.
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