A role for male genitalia in mate recognition: Aedeagus shape evolution results in pseudocopulation in the Drosophila mojavensis species cluster. Maxi Polihronakis Richmond, Therese Markow. Cell and Developmental Biology, University of California, San Diego, La Jolla, CA.

   The primary role of the aedeagus during copulation is to transmit sperm to the female. However, due to the vast morphological diversity of these structures, especially among arthropods, the aedeagus also has been hypothesized to play a role in mate recognition through mechanical and/or sensory mechanisms. In a previous analysis quantifying patterns of aedeagus variation in the Drosophila mojavensis species cluster, we found evidence that this structure is involved in mate recognition due to a pattern consistent with oscillating bouts of stabilizing selection between divergence events in combination with directional selection occurring at the time of divergence. In order to test this prediction from a mechanistic perspective, we conducted reciprocal mating experiments between all taxa in the D. mojavensis species cluster and measured the degree of pseudocopulation, or the ability of males to achieve the appropriate copulatory position after females agreed to mate. We also recorded time to each copulatory attempt, copulation duration, and resulting progeny. The results of the pseudocopulation experiment revealed varying degrees of post-copulatory isolation among D. mojavensis cluster taxa. Copulatory attempts between the sister species D. arizonae and D. mojavensis resulted in the highest frequency of pseudocopulation and often ended with the male getting stuck such that neither party could terminate copulation. The degree of pseudocopulation between the D. mojavensis subspecies was variable and dependent on the population of origin of both the male and the female. These results support a mate recognition role for the aedeagus. Whether this role is sensory and/or mechanical is discussed in light of the known shape differences among the taxa studied, and whether certain shape combinations are more likely to result in failed copulation attempts.