EVOLUTIONARY CONSEQUENCES OF SEXUAL VERSUS ASEXUAL REPRODUCTION IN LATE CRETACEOUS OSTRACODE SPECIES

Asexual reproduction is more efficient than sexual reproduction because only the female portion of sexual offspring are themselves capable of bearing offspring. Despite this “two-fold cost of sex,” the overwhelming majority of eukaryote species reproduce sexually. There are many hypotheses that address the evolution and maintenance of sex, but most focus on genetic recombination and its importance in producing genetic variation. Bearing more varied offspring can be beneficial for individual organisms in variable environments, and, at the species level, the greater genetic variation that results from sex may lead to more rapid adaptive responses and to lower risk of extinction. It has been difficult to document the long-term evolutionary consequences of sex, however, because it is rare to be able to infer the mode of reproduction in fossil lineages. The ostracode genus Phacorhabdotus from the Late Cretaceous of the U.S. Coastal Plain presents an exceptional opportunity for exploring this issue because it includes species exhibiting various reproductive modes, including sexual, asexual, and a mixture of the two. We documented reproductive mode across populations in this genus; using distinct male and female clusters in carapace shape as evidence of sexual reproduction, in accordance with previous work. We found that, of the five species of Phacorhabdotus common enough to consider, three (P. pokornyi, P. simplex, P. texanus) reproduce sexually in all samples thus far encountered, one (P. bicostimilus) seems to be completely asexual, and the final species, P. venodus, has both sexual and asexual populations. A large number of samples from the Annona Chalk were analyzed in greater detail as three species occur throughout this interval. We found that the sexual species, P. texanus, was the most abundant, followed by P. venodus (mixed reproduction), with the asexual species P. bicostimulus rarest. Preliminary morphometric analyses indicate that the asexual P. bicostimulis accumlated significantly less shape variation than the two species that sexually reproduce (Levene’s test, F = 3.073, df =2 , P = 0.048). Continued investigation of evolutionary variation and extinction risk will help address if asexual reproduction limits evolutionary potential, acting as a macroevolutionary dead end.