Research
From behavioral ecology to bioinformatics and genomics to conservation in the field, I have studied a wide variety of taxa. All of my work is at its core focused on two things - evolution and speciation!
PhD Dissertation
Reinforcement is the process by which natural selection directly promotes the evolution of prezygotic isolation where two species co-occur in sympatry, preventing the formation of unfit hybrids. This often occurs when two species diverged in allopatry and later come back together in sympatry, termed secondary contact. Theory predicts that regions of reduced recombination - such as sex chromosomes - may facilitate reinforcement by preventing the break-up of alleles associated with prezygotic and postzygotic isolation. However, empirical tests of this prediction are lacking. Darters (Percidae: Etheostominae) are a highly diverse group of North American stream fishes and provide a model system for studying the genetic basis of speciation via reinforcement.
Differences in sex chromosome identity act as a postzygotic isolation barrier between two species of hybridizing darters
Orangethroat and rainbow darters are two wide-ranging species that commonly hybridize in sympatry. Previous work has shown that behavioral isolation is enhanced in sympatry compared to allopatry, consistent with predictions for reinforcement, but the genetic basis of sex determination is unknown. Here, we leveraged whole genome re-sequencing and population genomic approaches to identify sex chromosomes in darters for the first time. We found strong evidence indicating divergent sex chromosomes are present among sympatric and allopatric populations of orangethroat and rainbow darters. This suggests that sex chromosome turnover prior to secondary contact may have played an important role in finalizing speciation via reinforcement in darters and provides new insight into how aspects of genomic architecture predispose some lineages to rapid diversification and speciation.
Genome-wide association study and FST analyses reveal orangethroat darters (left) have a sex chromosome on chromosome 9 while rainbow darters (right) have a sex chromosome on chromosome 23.
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Next, I want to further investigate the mechanisms of sex determination in darter species.
First, I want to know when and how these sex chromosome turnover events happened. We currently are in the process of sequencing new darter species for which we have not identified a sex chromosome, and the end goal is to identify as many darter sex chromosomes as possible, then using phylogenetic methods to elucidate the timeline of turnover.
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I am also interested in the specific genes that may be involved in sex determination. We have found certain genes of interest that align with sex-associated regions in these two species, but we do not know the functional implication of these genes. I want to utilize a novel method of CRISPR/Cas9 which has been done successfully in adult guppy brains to test the role of certain genes in sex determination and thus species or male-female recognition.
Texas Parks & Wildlife Grant
Non-native suckermouth armored catfish (referred to as SAC, Family: Loricariidae) have invaded freshwater spring systems, powerplant cooling reservoirs, and southeastern bayous in Texas through illegal introductions. For sensitive ecosystems inhabited by threatened and endangered species, the best hope of reducing the negative ecological and economic effects of introduced armored catfish is through population control efforts aimed at reducing abundance, biomass, or distribution. While spearfishing tournaments in the San Marcos River have reduced short-term population size, a method of complete eradication has not yet been found. One aspect of conservation that has not been explored for the SAC is management informed by genetic data.
Our work had three main objectives. First, we used genetics as a basis of species identification. Knowing which species occupy which river will inform management decisions as conservation guidelines tend to require separate management plans for each individual species. Second, we described the genetic population structure of SAC in three Texas waterways in order to get a better understanding of whether there is genetic diversity and hybridization among populations. Finally, we determined whether the SAC in Texas have genetic sex determination with male heterogamety (XX/XY) in order to utilize a novel method of population control involving release of YY “supermales”, which will only be able to produce genotypically male offspring, into natural populations, thus driving effective population size down. New advances in genomic technologies presented here, including the development of reference genomes and high-throughput whole genome resequencing, could help develop the resources needed to advance alternative management strategies.
University of Kentucky - Honors Thesis Project
Host-identity influences on speciation and related morphological characteristics in pine sawflies.
Among insects, there are many reproductive strategies for how eggs are laid. In the Neodiprion sawfly genus, the females hatch from their cocoons with their entire cadre of eggs. The environment that these females are in when they lay their eggs has been shown to impact the reproductive behaviors and strategies that are utilized. Individuals need to allocate energy in such a way that the trait that will provide them with the highest fitness is the “strongest”, placing them in an evolutionary trade-off.
​Fitness trade-offs are ubiquitous in nature. A well-studied trade-off in evolutionary ecology is investment in dispersal versus reproduction. In insect systems, relative investment in flight capability and reproductive potential can be influenced by conditions during development, such as food availability and temperature. However, plasticity of dispersal-reproduction trade-offs has rarely been studied in host-specialized insects, which include the most diverse insect taxa. To fill this gap, we manipulated the diet of red-headed pine sawfly (Neodiprion lecontei) larvae and reared them to adulthood. We measured several life history and morphological traits, including: survival, development time, cocoon weight, and adult female body size, wing size, and egg size and number. N. lecontei larvae and adults attack many different pines in eastern North America but tend to avoid white pine. We found that relative to larvae fed foliage from a preferred pine (Virginia pine), larvae fed foliage from a non-preferred pine (white pine) had lower survival, longer development times, and produced smaller cocoons. Morphological data from adult females also revealed differential investment in body size, wing size, and reproductive potential (egg size and number). Together, these data expand our understanding of fitness trade-offs in ecological specialists.
Neodiprion lecontei (red-headed pine sawfly)
Neodiprion pinetum (white pine sawfly)
We then applied the same measurements to a species of specialist, the white pine sawfly (Neodiprion pinetum), to see if the trade-off is consistent across species!
In a similar pattern to the N. lecontei crosses, N. pinetum females displayed a differential investment into development of wing and egg size when raised on their preferred pine (white pine) versus their non-preferred pine (Virginia pine).
Individuals across each colony had larger egg size relative to wing area when raised on their non-preferred pine. Similarly, more energy was invested into developing fewer larger eggs rather than more smaller eggs when raised on Virginia (non-preferred) pine.