POST-COPULATORY SEXUAL SELECTION AND GAMETIC ISOLATION IN STALK-EYED FLIES

Abstract

Understanding the forces that drive lineage splitting, i.e. speciation, has been a goal of evolutionary research since Darwin but remains poorly understood. Sexual selection is frequently invoked as a possible explanation, but focus is typically placed on precopulatory activities where males compete for access to females or females choose among males. The possibility that postcopulatory sexual selection, a powerful evolutionary force which involves interactions between sperm and the female reproductive tract, may contribute to reproductive isolation has only recently been considered. Using diopsid stalk-eyed flies as a model system, I examine divergence in fertilization systems among closely related populations of a single species (Teleopsis dalmanni), in order to assess whether gametic isolation has the potential to contribute to speciation.

In chapter 2, I measure a suite of reproductive and non-reproductive morphological traits in eight closely related populations to determine their relative rates of evolution. I find that reproductive traits have diverged more rapidly than non-reproductive traits and that male and female postcopulatory traits, i.e. sperm length and sperm storage organ dimensions, have coevolved.

Chapters 3 and 4 describe experiments aimed at elucidating the importance of gametic isolation among these populations. Chapter 3 is an examination of non-competitive gametic isolating barriers. I performed 275 crosses between four populations and measured mechanisms of non-competitive gametic isolation including sperm transfer, sperm survival, sperm motility and ability of sperm to reach the site of fertilization. I conclude that non-competitive gametic isolation exists among these population pairs and specifically identify the inability of sperm to reach the site of fertilization in between-population crosses as a mechanism of reproductive isolation.

Chapter 4 is an investigation of competitive gametic isolation which occurs when sperm of males from different populations compete for fertilization. Using two pairs of populations, I carry out every possible combination of crosses and genotype over 1200 offspring to determine paternity. The results demonstrate that sperm competition further inhibits successful hybridization among these closely related populations.

I conclude that postcopulatory sexual selection and gametic isolation have the potential to play an important role in the formation of new species in this system.