A gradual change in the reproductive mode has been observed in two lake populations of the freshwater New Zealand snail, Potamopyrgus antipodarum. Clonal females dominate deep habitats, while cross-fertilizing males and females dominate the shallow regions. Snails in both lakes are infected by a trematode parasite, Microphallus sp., which sterilizes all individuals, and is more prevalent in shallow regions. A cline in the snail's life history (LH) traits has also been found where shallow-water snails mature at a smaller size than deep-water snails. This project will determine whether the difference in parasite frequency is positively correlated with the frequency of cross-fertilization and with differences in LH traits for snail populations in 20 lakes on the South Island of New Zealand. The PIs will determine reproductive mode, parasite prevalence, and whether resistance to infection can evolve in cross-fertilizing individuals using laboratory experiments.
The P. antipodarum study system is well suited to test important evolutionary theories using field data, providing information on the extent to which parasitism shapes a host population. Moreover, trematode worms are important disease agents in both humans and domesticated animals. According to the World Health Organization, 200 million people are infected by the trematodes that cause Schistosomiasis, which is second to malaria in its public health impact. Nonetheless, little is known about the effects of the snail populations in spreading these worms, which have similar life cycles to the worms studied in this research. Thus, this international, multi-lab investigation should interest organizations that aim to control these diseases.
With the funding obtained from NSF’s Doctoral Dissertation Improvement grant, we examined several aspects of the coevolutionary dynamics between hosts and parasites. One aim in the study of coevolution is to understand how natural selection mediates interactions through space and time. According to the Geographic Mosaic Theory of Coevolution (GMTC), there exist both hotspots, where interacting species are under reciprocal selection, and coldspots, where there is no reciprocal selection and no coevolution. This change in selection imposed by interacting species creates a geographic mosaic where the coevolutionary interactions vary across space and time. If we consider the close dynamics between parasites and their hosts, the geographic mosaics generated by this interaction could change due to variations in densities of the interacting species. The combination of GMTC and parasite-based theories like the Red Queen Hypothesis (RQH) may provide a possible explanation for the distribution of cross-fertilization in individuals within and across populations. The existence of cross-fertilization is an enigma in evolution, since clonal population that produces only females can produce twice as many daughters and four times as many granddaughters than a population that cross-fertilizes. The RQH predicts that parasites will impose a strong selective pressure against clonal genotypes that become common within the host population. Therefore genetic variation in the cross-fertilizing population is important to impede the parasite’s ability to adapt to host defenses, giving an advantage to offspring produced via cross-fertilization. Along with the widespread presence of a sterilizing trematode parasite Microphallus sp., the mixture of clonal and crossed-fertilized individuals within populations of the New Zealand freshwater snail Potamopyrgus antipodarum makes this host-parasite system ideal for the empirical evaluation of both the RQH and GMTC. Microphallus infection risk varies within and among lake populations due to ecology of the definitive hosts, ducks. Data consistent with a role for parasites in the maintenance of cross-fertilization has been reported for the P. antipodarum-Microphallus sp. host-parasite system. The relative frequency of crossed-fertilized P. antipodarum correlates positively with the incidence of infection by Microphallus and the shallow and deep regions of three lakes constitute coevolutionary hotspots and coldspots respectively. Using funding from NSF’s DDIG, we tested the predictions of the RQH and GMTC. We investigated the distribution of crossed-fertilized and clonal forms and parasites through space and time–two crucial aspects in evolutionary biology–combining field observations, lab experiments and molecular analysis. We described how parasites are necessary for the maintenance of cross-fertilization, independent of geographic location. We examined snail populations collected in fifteen lakes from the South Island of New Zealand from both the shallow and deep habitats and from a mid-depth habitat for ten of these lakes. We analyzed the data taking into account three distinct spatial scales: across the fifteen lakes, within one lake where we had numerous samples and within the shallow, mid-depth and deep habitats of that one lake. The results of this project show that independent of the spatial scale, Microphallus generates a hotspot that allows for the coexistence of crossed-fertilized and clonal snails. This project, which provides empirical examples of both the GMTC and the RQH, was published in the journal The American Naturalist. Another project that we accomplished thanks to NSF’s funding provides an empirical example that corroborates the theoretical expectation that clonal individuals are will be periodically more infected by parasites than crossed-fertilized individuals. We used snails collected during five consecutive years, and determined the temporal infection rate of clonal and cross-fertilized snails. The clonal individuals, consistent with theory, have periodic events where they are over-infected by Microphallus. The mean fitnesses of both crossed-fertilized and clonal snails are not significantly different, consistent with coexistence. This project, which is now in pres in the journal The American Naturalist, shows that parasites counter the advantage that clonal individuals have in reproduction, thus allowing for coexistence between clonal and crossed-fertilized snails. A third project conducted with NSF’s funding was an in-lab experiment to test whether snails that were exposed to Microphallus increased their mating activity compared to snails that were not exposed to the parasite. According to theory, individuals that are at risk of being infected by a detrimental parasite should increase their mating frequency to assure reproductive success. Our finding, which has been accepted for publication in the journal Biology Letters, corroborates the theory and shows that snails under presence of Microphallus increase their mating activity. Finally, we also quantified the morphologic differences from snails to test whether these changes correlate with the presence of cross-fertilization and of parasites. We found that neither the presence of parasites or cross-fertilization have an effect on the shape of the snails. A remarkable finding of this study, which is in preparation for publication, is that even though the snail populations differ in size and the presence of spines, their shape is very similar.
