Print PageThe Red Queen Hypothesis is one of the most important contributions to ecological theory explaining the evolution and maintenance of sex. The hypothesis predicts that recombination of genetic material following sexual reproduction allows organisms to evolve resistance to parasites, disease, predators and adverse environmental conditions. Especially the role of sex in the evolutionary arms race between parasites and their hosts has received attention. However, data supporting the generality of this hypothesis are not yet available. In particular, data on the role of parasites on the evolution of sex in vertebrates are missing. To address this knowledge gap, we compared the parasite fauna of two closely related mollies (Poecilidae) co-existing in South Texas. Poecilia formosa exclusively reproduces parthenogenetically while P. latipinna only reproduces sexually. According to the Red Queen Hypothesis we predicted that 1) P. formosa harbored more parasites compared to P. latipinna due to a lower genetic variability in the former, and 2) Parasite infections incurred a greater fitness cost in the parthenogenetic P. formosa because they are less likely to evade parasite exploitation with subsequent impact on host fitness. Both species of fish were collected from irrigation canals in the Rio Grande Valley by seining in fall 2009 and in winter 2010 to compare the number of trematodes in the eyes, gills, heart, and brood sac. In addition, at least 30 females of the two species were sampled in spring 2011 from two irrigation canals to compare the number of developing embryos in relation to their parasite load.
Results/Conclusions
We found significant effects of both location and season. Thus, winter samples of both species contained significantly fewer parasites compared to the summer samples indicating a die-off of heavily infected individuals during winter. Contrary to the Red Queen Hypothesis, sexually reproducing mollies generally harbored more parasites compared to their parthenogenetic relatives. In both species, brood size was positively related to body mass and negatively impacted by the number of trematode metacercariae lodged in gill tissue. Despite the presumably strong selection pressure by gill parasites on host fitness, sexually reproducing mollies harbored a significantly larger number of gill parasites and were equally negatively affected by this group of parasites when compared to the parthenogenetic species. The Red Queen Hypothesis is therefore not supported by our data on the molly-trematode model system.
