187-3 Is Asexual Reproduction an Evolutionary Dead-end? Using Fossil Ostracodes to Infer Evolutionary Outcomes of Sexual versus Asexual Species

Session: Phylogenetic and Computational Approaches in Paleobiology and Paleoecology (Posters)

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Abstract:

The ubiquity of sex in the natural world is a puzzle. Not only is it both risky and time-consuming to find a mate, but sexual reproduction is also much less efficient than asexual reproduction. Despite its advantages, asexual reproduction has long been thought to be an evolutionary dead-end because of its limited ability to generate genetic variation. This hypothesis has remained largely untested, however, as indicators of reproductive mode are rarely preserved in the fossil record. Because it is known to contain species with sexual, asexual, and mixed reproductive modes, the ostracode genus Phacorhabdotus from the Late Cretaceous and Paleocene of the U.S. Coastal Plain presents an opportunity to test this hypothesis. Here, we inferred reproductive mode, compared trait variation within species, and traced the evolutionary history of reproduction in this clade using a phylogenetic analysis. We can infer reproductive mode because cytheroid ostracodes are sexually dimorphic, with male carapaces that are more elongate than those of females. To evaluate dimorphism at the population-level and determine reproductive mode, we fit size and shape data obtained from digitized specimen outlines to mixture models. We interpreted populations best-fit by two clusters as sexual and one-cluster populations as female-only and therefore asexual. We inferred phylogenetic relationships in these species via a parsimony analysis of 15 discrete morphological characters observed from scanning electron and optical microscopy. The resulting phylogeny was time-scaled using stratigraphic occurrences compiled from the published literature and museum samples. We found that sexual reproduction was lost once, in the branch leading to the only strictly asexual species, P. bicostilimus, and that there was at least one gain and one loss of asexual reproduction. Notably, the asexual P. bicostilimus showed the least evolutionary variation in shape (L/H ratio), while also having the shortest stratigraphic range. In the two species with mixed sexual and asexual reproduction, the most morphologically divergent populations were sexual. These results suggest a higher capacity for adaptation in sexual populations and reduced evolutionary potential in asexual populations, but given limited sample sizes, a broader analysis is required to confirm whether asexual reproduction is generally a macroevolutionary dead-end. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7787

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