Graduate Thesis Or Dissertation
 

Diversity and Evolution of Sexual Traits in Ground Beetles (Family Carabidae)

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  • Biologists have long been fascinated with reproductive traits, in part because they are frequently exaggerated or showy, and they commonly vary among closely related species. Exaggerated sexual traits, in particular, have been the focus of intense empirical and theoretical research, but most of this work has focused on traits involved in precopulatory interactions such as male display traits (e.g., feathers), male weapons (e.g, horns), female resistance traits, and female preference traits. Notably less is known regarding the diversification of traits involved in postcopulatory interactions. There is, however, increasing recognition that postcopulatory interactions can have major evolutionary consequences, and postcopulatory sexual selection is thought to be an engine of species diversification. Sperm-female interactions are the primary focus in the study of postcopulatory sexual selection. Sperm are launched away from the male’s soma to live out their days in the foreign environment of the female reproductive tract (FRT), and it is here where sperm compete and where females choose sperm via a variety of mechanisms. The design of sperm is thought to be driven by the female tract, and some sperm traits are now recognized as ornaments (e.g., the long flagella of some Drosophila flies) or male persistence traits (e.g., bristles in the sperm of Macrostomum flatworms). However, the adaptive significance behind most of the morphological diversity of sperm is unknown. For my dissertation I chose to examine patterns in the morphological evolution of sperm by taking a comparative approach in a large clade of terrestrial insects and by incorporating female tract morphological data. I studied sperm-female co-diversification in ground beetles (family Carabidae, 40,000 species) as they are a very promising system for studying the evolution of traits involved in postcopulatory interactions. First, I conducted a survey across ground beetles in order to better understand the morphological diversity of sperm in this group. I used light microscopy and DNA-staining to visualize sperm and sperm heads from 177 species of ground beetles from 61% of the tribal diversity of the group. I found that ground beetle sperm vary across several axes and that many ground beetles join their sperm together in groups termed conjugates. I scored the variation I observed and recorded data on 1 qualitative trait and 7 quantitative traits. I paired these data with a low-resolution phylogenetic hypothesis for relationships across the family in order to conduct analyses. Because a high-resolution phylogeny of Carabidae is lacking, I used a supertree approach to account for the evolutionary relationships among my sampling, and I used several approaches to account for the lack of branch length information. Results from comparative analyses suggest that sperm have experienced much divergence and convergence across this diverse assemblage of terrestrial insects. Ancestral state reconstruction of sperm conjugation suggests that ground beetles made sperm conjugates early in their evolutionary history and that conjugation has been lost independently at least three times. Sperm differ from sperm conjugates in some aspect of their underlying process, and sperm form has shaped conjugate form and vice versa. I also tested for stationary selection in these data by comparing the fit of macroevolutionary models for sperm head length and number of sperm in a conjugate. Although I found some support for more complex models, simulations suggested that my sampling does not have the power to distinguish between Brownian motion or more complex Ornstein-Uhlenbeck (OU) models. Sperm do not evolve in isolation; they compete within the environment of the female reproductive tract. The expectation is that sperm and FRT are morphologically integrated, and comparative studies have identified a correlation between sperm phenotype and FRT design. I incorporated female tract morphological data into my studies at a shallower scale within ground beetles. I focused on sperm-female co-diversification within the ground beetle tribes Clivinini and Dyschiriini. Clivinini and Dyschiriini together contain about 1200 species worldwide, and many species possess morphologically distinctive sperm and genitalia. There is almost no modern molecular phylogenetic data on relationships among these beetles beyond the inclusion of one or few species in broader phylogenetic studies. In order to test for evolutionary correlations between sperm traits and female tract traits, I estimated the phylogeny of Clivinini and Dyschiriini by sampling 70 species for two nuclear loci: the nuclear ribosomal gene 28S and the protein-coding gene CAD. I estimated divergence times by analyzing these molecular data with node-based dating methods in order to have a tree suitable for phylogenetic comparative analyses. I then gathered sperm-female tract morphometric data from 15 species across my sampling. I explored these data by assessing the phylogenetic signal per trait, fitting different models of evolution, and reconstructing ancestral states. In general these traits show low to moderate levels of phylogenetic signal, but sperm head width, the minimum width of the female tract, and the maximum width of the female tract exhibit high levels of phylogenetic signal. Model testing revealed that Brownian motion can describe the variation among species well except for spermatostyle length, the number of sperm in a conjugate, the length of the spermatostyle that bears sperm, and female tract length, which are better explained by an OU process. I tested for evolutionary correlations between sets of male and female traits using phylogenetic regressions, model selection, and partial least squares. I evaluated the statistical power of my dataset using simulations to assess its ability to distinguish among macroevolutionary models and recover correlations. I did not find a significant association between the first partial least squares axes for sperm and FRT datasets, but there were several significant associations among the pairwise comparisons between individual sperm and FRT traits. Unlike many prior studies of sperm-FRT co-diversification, sperm length did not correlate significantly with FRT length, but several conjugate traits are correlated with different dimensions of the FRT. Simulations showed that my dataset lacks sufficient statistical power to distinguish among models of evolution, but it does have enough power to identify correlations when the strength of correlation is at least moderately strong. The results are consistent with a scenario where FRTs are driving sperm phenotype and suggest that the sperm conjugate is a more frequent target of selection by the FRT than are individual sperm.
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  • Lab website of the Maddison lab research group at Oregon State University.
Funding Statement (additional comments about funding)
  • This work was partially funded by an National Geographic Society early career grant.
  • This work was partially funded by a NSF GRF grant.
  • This work was partially funded by an American Museum of Natural History Theodore Roosevelt Memorial grant..
  • This work was partially funded by an Entomological Society of America travel grant.
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  • Pending Publication
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  • 2021-06-19 to 2023-07-19

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