1. Coevolutionary interactions with parasites constrain the spread of self-fertilization into outcrossing host populations
Given the cost of sex, outcrossing populations should be susceptible to invasion and replacement by self-fertilization or parthenogenesis. However, biparental sex is common in nature, suggesting that cross-fertilization has substantial short-term benefits. The Red Queen hypothesis (RQH) suggests that coevolution with parasites can generate persistent selection favoring both recombination and outcrossing in host populations. We tested the prediction that coevolving parasites can constrain the spread of self-fertilization relative to outcrossing. We introduced wild-type Caenorhabditis elegans hermaphrodites, capable of both self-fertilization and outcrossing, into C. elegans populations that were fixed for a mutant allele conferring obligate outcrossing. Replicate C. elegans populations were exposed to the parasite Serratia marcescens for 33 generations under three treatments: a control (avirulent) parasite treatment, a fixed (non-evolving) parasite treatment, and a copassaged (potentially coevolving) parasite treatment. Self-fertilization rapidly invaded C. elegans host populations in the control and the fixed-parasite treatments, but remained rare throughout the entire experiment in the copassaged treatment. Further, the frequency of the wild-type allele (which permits selfing) was strongly positively correlated with the frequency of self-fertilization across host populations at the end of the experiment. Hence, consistent with the RQH, coevolving parasites can limit the spread of self-fertilization in outcrossing populations.
Mean selfing rates (± one standard error) over the course of the experiment. Host populations were exposed to three different treatments: control (heat-killed S. marcescens; dotted line and triangular markers), fixed-parasite treatment (fixed strain of S. marcescens; dashed line and circle markers), copassaged (copassaged S. marcescens; solid line and square markers) for 33 generations.
2. Sedentary dark eyed-juncos (Junco hyemalis) maintain a higher prevalence of Plasmodium infections throughout the winter and early spring than migratory juncos in a seasonally sympatric population
In temperate climates, some avian Plasmodium (malarial) parasites alternate between an active stage in the host blood stream in the spring and summer, and a dormant stage in the host organs in the winter. We predicted that Plasmodium parasites should re-emerge into the host blood stream earlier in the spring in host populations that breed at lower (warmer) latitudes where vectors likely emerge earlier. We compared Plasmodium infection densities in the bloodstream in 2 closely related populations of dark-eyed juncos (Junco hyemalis): one population (sedentary) winters and breeds in Virginia while the other population (migrant) winters in Virginia and migrates North to Canada to breed. We captured male sedentary and migrant juncos in December in Virginia, and brought them into a common garden indoor aviary (vector-free) environment where they were maintained under a natural (Virginia) photoperiod. We extracted DNA from blood samples collected in December, early March, and late March, and measured the relative densities of Plasmodium parasites using a quantitative Polymerase Chain Reaction. We found no evidence for spring emergence of Plasmodium parasites into the host blood stream; parasite prevalence did not increase over the course of the study. However, we found that our sedentary population of juncos sustained a higher prevalence of Plasmodium parasites throughout the study. Thus, our results are consistent with the migratory culling hypothesis, which posits that long distance migration weeds out infected animals, and which predicts that parasite infection prevalence should be higher in sedentary host populations than in closely related migratory host populations.
Prevalence of Plasmodium infections (as determined by qPCR) by population and time point. Red bars represent infection prevalence in the sedentary population, blue bars represent infection prevalence in the migrant population. Error bars represent 95% binomial confidence intervals. Asterisks above the bars indicate significant differences in prevalence between populations at each time point. Horizontal bars indicate comparisons in prevalence across time points.