Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)




Scott Pitnick


cryptic female choice, post-copulatory sexual selection, sperm competition, sperm conjugation, sperm heteromorphism, sperm morphological evolution

Subject Categories



Sperm form evolves rapidly and dramatically, particularly in taxa with internal fertilization. Post-copulatory sexual selection at the level of individual sperm has been suggested to explain the evolution of two enigmatic sperm phenotypes: sperm heteromorphism, where more than one type of sperm is produced by a male, and sperm conjugation, where multiple sperm join together for motility and transport through the female reproductive tract before dissociation prior to fertilization. I explore the taxonomic distribution of sperm heteromorphism and conjugation, speculate on the potential developmental origins and discuss functional hypotheses for evolutionary maintenance of these remarkable traits. I subsequently focus on the patterns of sperm morphological evolution in diving beetles (Dytiscidae), an excellent model to examine the evolution of sperm heteromorphism and conjugation. I use phylogenetically controlled regression and Bayesian estimation to infer ancestral sperm traits, identify both the rate and directionality of probable evolutionary transitions and test if the evolution of female reproductive tract design might have driven the evolution of complex, multivariate sperm form. I found sperm conjugation to be the ancestral condition in diving beetles, with subsequent diversification into three qualitatively unique forms (i.e., aggregates, pairs and rouleaux), each exhibiting varying degrees of evolutionary loss and convergence. Evolution of sperm head shape was correlated with conjugation, consistent with statistical support for non-random patterns of evolutionary transitions betIen the different forms of conjugation. The results suggest that both sperm length and sperm heteromorphism have independent evolutionary trajectories from conjugation. Results of Bayesian analyses suggest that the evolution of sperm morphology tracks changes in female reproductive structures. Data from both behavioral studies and transmission electron microscopic images of sperm conjugates stored in the female tract support the interpretation that sperm conjugation is an adaptation for maintaining favored positions for fertilization in the female reproductive tract. The results show that although sperm often have complex and varied sperm morphology, diversification of sperm form might also be constrained along particular evolutionary pathways. Moreover, the results highlight the importance of sperm-female interactions as an agent of diversification.


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