Relationship between Intrapopulation Processes and Reproductive Isolation

For sexually reproducing organisms, an important part of their fitness is a product of how well they compete for mating opportunities. This “sexual selection” was suggested to be a strong driver of evolution by Darwin, and has since been invoked to describe broad scale patterns of biodiversity. Only by studying the mechanistic and genetic links between sexual selection and reproductive isolation, will biologists be able to determine the influence of sexual selection on evolutionary processes.

My research focuses on the genetics and evolution of sexual interactions, mating behavior and speciation, especially prezygotic reproductive isolation, leveraging the power of multiple empirical systems in conjunction with new statistical approaches and theory. A challenge for understanding the evolution of prezygotic reproductive isolation is that many different forces shape the complex behaviors involved in sexual interactions. Thus, it is paramount to understand how traits, and their underlying genes, respond to these various selective pressures.  My goal is to integrate understanding of behavioral phenotypes with the genes and molecular mechanisms underlying these traits.

speciation diagram
Sexual selection, adaptation, and drift operate throughout lineage diversification and speciation. A major challenge is to determine which forces contribute to the evolution of reproductive isolation

Postmating Prezyogitc Isolation

One common supposition in evolutionary biology is that sexual selection unilaterally drives the evolution of reproductive isolation. Given a shared genetic basis, however, selection for increased reproductive isolation could constrain or facilitate sexual selection, when closely related species co-occur.

I demonstrated that genes important for intraspecific sperm competition also contribute to conspecific sperm precedence in Drosophila melanogaster (Castillo & Moyle 2014; Proc. Roy. Soc.). In this experiment, I used sets of null and knock-out strains of D. melanogaster that each lacked function of an important component of sperm competition; in competitive matings mutant males sired fewer progeny compared to controls. I then demonstrated that two of the three genes also impact the level of reproductive isolation, via conspecific sperm precedence, between D. melanogaster and D. simulans.

I subsequently tested the hypothesis that selection for increased reproductive isolation in sympatry can affect intrapopulation sexual selection and sexual interactions in natural populations. Using the sister species D. pseudoobscura and D. persimilis, I found a pattern consistent with reinforcement where conspecific sperm precedence was stronger in sympatric populations compared to allopatric populations and that, unexpectedly, reinforcement had constrained sexual selection in sympatric populations (Castillo and Moyle 2016; bioRxiv). I have sequenced both male and female transcriptomes from all genotypes used in my previous work and I am currently evaluating population genetic patterns of a set of a priori candidate loci (associated with intraspecific sperm competition) and identifying new candidate genes for sexual selection and reinforcement.

CSP plot2 copy 2
Reinforcing selection has increased mean CSP in sympatric populations of Drosophila pseudoobscura


Experimental Evolution of Premating Isolation

The use of experimental evolution in model systems is a powerful method to uncover the underlying mechanisms controlling evolution, and to reveal historical processes that contribute to adaptation and speciation. When populations adapt to new environments, ecological divergence and/or behavioral divergence are often correlated with reproductive isolation. Experimental evolution is one of the few ways that trait divergence can actually be connected with the selective forces shaping these populations.

Using experimental evolution, I determined the relative contribution of ecological divergence and sexual selection to the evolution of premating isolation in Caenorhabditis remanei populations (Castillo et al. 2015; Evolution). I found that sexual selection led to the rapid evolution of assortative mating and copulation latency. One prevalent expectation is that segregating variation facilitates rapid evolution. Using the power of a model system in which an ancestral population can be retained and retested, I was able to provide evidence in support of this expectation by demonstrating significant genotypic variation between male x female genotype combinations for mating traits (Castillo & Delph 2016; Evolution).


Transposable Elements and Speciation

Using D. virilis and relatives I am trying to determine whether TEs can have any effect on reproductive isolation between species via dysgenesis (a phenomenon seen in intraspecies crosses) by completing controlled crosses where I know the identity of a specific TE that causes dysgenesis within species

TE expectation
One model describing the expectation that increased reproductive isolation will only be seen in interspecific crosses where one species has a unique TE (blue circles)

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