I received my PhD in Ecology and Evolutionary Biology at the University of Oklahoma, in the lab of J.P. Masly. My dissertation research addressed how rapidly evolving genital structures contribute to reproductive isolation which can eventually give rise to new species.
I am currently a postdoctoral research associate in Michael Patten’s lab at the Oklahoma Biological Survey. I am applying my love of data visualization and analysis in a new direction, analyzing long-term tracking data in Oklahoma and New Mexico to assess how the Lesser Prairie-Chicken responds to human modification of its habitat.
A widespread pattern in nature is that among internally-fertilizing animals, male genitalia tend to evolve rapidly. This pattern is observed in intromittent organs, and also secondary reproductive structures that contact the female during mating but do not transfer sperm.
Enallagma damselflies experienced several radiations within the past ~250,000 years, which are characterized by striking differentiation in male grasping appendages (cerci) and female thoracic plates, which males grasp to initiate mating. In contrast, many of these species are ecologically indistinct and have diverged little in their overall morphologies. These features make Enallagma an ideal system to study the role of rapid divergence in mating structures in causing reproductive isolation and the genetic architecture of sexually coevolved morphologies.
How do females evaluate potential mates?
Female Enallagma have presumed mechanoreceptors on their thoracic plates, and it is suspected that the way in which male cerci stimulate these receptors aids female species recognition and affects female mating decisions. However, this idea has not been studied quantitatively.
Based on the assumptions that females with more or denser mechanoreceptors have greater tactile acuity and that females in sympatry are under selection to correctly identify appropriate mates and avoid hybridizing, I am testing the hypothesis that quantitative variation in female mechanoreceptor traits affects female species recognition. This hypothesis predicts that sympatric females will have more and/or denser mechanoreceptors than allopatric females, or have different spatial distributions of receptors than allopatric females.
To test this, I am using scanning electron microscopy to compare the number and location of mechanoreceptors among multiple populations of female E. anna and E. carunculatum. In the future, I would like to use electrophysiology to characterize the firing patterns of sensilla in different locations within the thoracic plate.
Has genetic coupling promoted species-specific male and female mating structure evolution?
Genetic coupling (pleiotropy or physical linkage) is implicated in the coevolution of male traits and female preferences in a number of taxa, including crickets (song), fruit flies (sex pheromone), and butterflies (wing color). Enallagma male cerci and female thoracic plates display coevolutionary divergence, and reproductive isolation may evolve especially rapidly if these coevolving male and female genital morphologies share a common genetic basis.
I am using RADseq to genotype a large collection of E. anna, E. carunculatum, and hybrids at many loci throughout the genome. I will use these genotypes to characterize the genetic architecture of male-female coevolved morphologies and to identify whether the same genetic loci are associated with both male and female morphology.
Is the rapid divergence of genital morphologies an important cause of reproductive isolation?
Because the genitalia often evolve rapidly between populations, this raises the question of whether genital divergence can cause RI that may eventually give rise to new species. To investigate whether rapid evolution of male and female reproductive structures been a major cause of reproductive isolation in Enallagma, we quantified 19 potential pre- and postzygotic RI barriers between Enallagma anna and E. carunculatum, two species with many traits in common but conspicuously different reproductive structure morphology.
Despite these differences, these species hybridize in a sympatric region. Because these sister species are relatively young and have incomplete RI, we can infer which reproductive isolating barriers are among first to evolve in early stages of speciation, and we can also examine the role of reproductive structures in causing RI compared to other isolating barriers.
Overall, we found that species-specific differences in the reproductive structures account for nearly all reproductive isolation between E. anna and E. carunculatum. The secondary genitalia reduce interbreeding via both mechanical incompatibilities, which impair heterospecific coupling, and tactile incompatibilities, which reduce female willingness to mate with heterospecific males. These same mechanisms also impair hybrid males’ ability to backcross with females of either parental species. In comparison, other postzygotic isolating barriers appear to have little effect on hybrid viability or fertility. A preprint version of this article is available on BioRxiv, here.
How do male and female reproductive structures vary among parental species and interspecific hybrids?
In collaboration with Mark McPeek at Dartmouth College, I obtained high-resolution digital representations of male cerci and female thoracic plates from ~300 E. anna, E. carunculatum, and their hybrids. I used spherical harmonics to quantify and compare the three-dimensional shapes of male cerci, and 3-D geometric morphometrics to do the same for female plates.
For each sex, principal component analysis shows that morphologies of the two parental species are distinct and non-overlapping. Hybrid morphologies, on the other hand, display broad variation ranging from similarity to either parental species to intermediate. The next step is to identify genetic loci responsible for variation in these male and female morphologies.