Unisexual species consist of only females (there are no males), and their persistence is an evolutionary paradox because they do not benefit from the increase in genetic variation that results from sexual reproduction. Gynogenesis is a type of unisexual reproduction that occurs when an all-female species must mate with males of closely related species (hosts) to stimulate the development of their eggs, but a male's sperm does not fertilize the eggs. Gynogenetic species face two major challenges. Because they produce only female offspring, they reproduce at a higher rate than sexual species, which can cause extinction of the sexual host via competitive exclusion (followed afterward by extinction of the gynogens). Furthermore, males of the host species should evolve to avoid mating with unisexuals because they do not pass on their genes when mating with gynogens. Solving this paradox can shed light on costs and benefits of sexual and unisexual reproduction. This research addresses hypotheses about maintenance of unisexuals in a mating system consisting of the gynogenetic fish, the Amazon molly, and its hosts, the sailfin molly and the Atlantic molly. This project investigates how hormone levels of the male hosts and the relative abundance of Amazon mollies affect the propensity of males to mis-mate with Amazon mollies, and will determine whether extinction and colonization of inter-connected populations can explain the maintenance of Amazons mollies, even though they should outcompete their host in a local population. The project will provide information about how complex interactions between species influence the evolution of reproductive strategies. The interesting nature of this system makes it appealing for K-12 and college students. This work will include participation by students and the investigators will give talks and conduct outreach activities related to these studies. Finally, this project will involve underrepresented groups in biology.
Understanding the evolutionary maintenance of alternative modes of reproduction in closely related species is a fundamental area of research in evolutionary biology. This area of inquiry is important, as it can shed light on the balance of costs and benefits of both sexual and unisexual reproduction. One model system for evaluating the coexistence of unisexuals with their sexual hosts is composed of a gynogenetic fish, the Amazon molly, which is a sexual parasite of its parental species: the sailfin molly and the Atlantic molly. This species relies on matings with the parental species to start the development of their eggs but the male’s sperm does not fertilize the female’s eggs. Because Amazon mollies due not use the sperm to fertilize their eggs, their offspring are thought to be clonal. The final intellectual merit outcomes of this project involved testing hypotheses concerning the origin, evolutionary history and persistence of Amazon mollies, as well as examining male mate choice in the sexually parasitized species, and the hormonal effects on mating behavior. First, we found that male sailfin mollies prefer to mate with their own species and that a primary sex steroid hormone in fishes, 11-ketotestosterone (KT), plays a role in species recognition for males, but other hormones such as estradiol and testosterone do not (Gabor & Grober 2010). We also found that male Atlantic mollies did not prefer to mate with conspecifics and males did not show a KT response to mating (Gabor et al. 2012). These results indicate that hormones can play an important role in mate recognition, and that persistence of Amazon mollies is attributable, at least in part, to the failure of Atlantic molly males to distinguish Amazon mollies from their conspecific females. Next, we (Alberici da Barbiano et al. 2013) assessed genotypic variation within Amazon mollies and found that the genomes of individuals Amazon mollies are intermediate between the parental species, which is consistent with their hybrid origin. But unlike the results of all prior studies, we found that Amazon mollies are not first generation hybrids. We also found unexpectedly high levels of genetic variation among Amazons suggesting that there was a period of backcrossing with both parent species before the onset of gynogenesis or that some form of recombination might occur in this asexual species. These results indicate that Amazons have more genetic variation than previously suspected, that they have the genetic variation to respond to natural selection, and that ecological and behavioral studies might need to account for this genetic variation. Finally, we (Alberici da Barbiano et al. 2011) found that male sailfin mollies inseminated a significantly higher proportion of females of sailfin mollies and fertilized a significantly higher number of eggs of their own species across treatments that varied in the frequency of Amazons and female sailfin mollies. Nonetheless, preference for conspecific females was not exclusive, as a range of 20 to 50% of Amazon mollies were fertilized. Social interactions among males may best explain these results and may therefore play an important role in the maintenance of unisexual – bisexual mating complexes. The broader impacts of our work were substantial. First, along with our REU supplements, we were able to train eight minority students in the techniques we used for all of the experiments and to mentor them in STEM. Most of these students have applied for and received external awards for their research and have presented their results at local or national conferences. Two of these students are now working on Doctorates in STEM and another plans on doing the same when he graduates. Two doctoral students and three masters students have also been trained in all of the laboratory methods and have had the opportunity to work closely with and train undergraduate students. One post-doc also learned next generation DNA sequencing protocols, developing the bioinformatics skills to manage these data, and learning the statistical framework for analyses. We have also been involved in outreach to other community groups, including Master Naturalists, and Master Gardeners, local school teachers, American Women in Science and Engineering and the Animal Behavior Society Broadening Perspectives group.
