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 (nonevolving) 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.
Slowinski et al Evolution 2016 data Dryad
Key and description of the Slowinski et al. 2016 Evolution data submitted to Dryad
Key:
Treatments were abbreviated as follows:
C: Control (heat-killed) parasite treatment
F: Fixed-parasite (non-evolving parasite) treatment
R: Recycled parasite treatment (referred to as the “copassaged” treatment in the paper)
Genetic background data was abbreviated as follows:
C: Lines that were previously passaged for 30 generations in a control, or heat-killed, parasite treatment prior to their use in the Slowinski et al. 2016 Evolution study.
E: Lines that were previously passaged in an “Evolution” treatment for 30 generations during which they were passaged on plates with a live but non-evolving parasite prior to their use in the Slowinski et al. 2016 Evolution study.
F: Lines that were not previously subject to experimental evolution prior to the Slowinski et al 2016 Evolution study. The “F” genetic background lines are the ancestors of the “C” and “E” genetic background lines.
OR is an abbreviation for outcrossing rate.
SR is an abbreviation for selfing rate.
MF is an abbreviation for male frequency.
MMG is an abbreviation for mixed-mating genotype.
Male frequencies sheet:
The male frequencies data sheet contains the male frequency data from all the time points in the experiment in which male frequencies were measured. The generation 10, 15, 21, and 33 male frequencies were calculated by scoring the sexes of a random sample of 200 individuals along a transect in experimental populations. The generation zero male frequencies were not measured directly, but rather were estimated based on the starting frequency of males in the mixed-mating populations (which are listed in the sheet called “mixed mating pop gen zero MFs”), and on the male frequencies in the obligately outcrossing generation zero populations (which were assumed to be 50% male and 50% female). The male frequencies at generations 10, 15, and 21 were all measured during the experiment in populations that had not been frozen and rethawed prior to male frequency measures. The male frequencies in generation 33 populations were measured before and after the populations had been frozen and rethawed, as indicated in the data sheet.
Outcrossing rates data sheet:
This sheet indicates the outcrossing rates for each of the experimental lines. Outcrossing rates were calculated from the male frequencies using the equation:
outcrossing rate = (male frequency – 0.0015) X 2
The 0.0015 in the equation is to correct for males produced by non-disjunction of the X chromosome during meiosis, which we assumed occurs at a frequency of 0.0015.
Selfing rates data sheet:
The selfing rates data sheet indicates the selfing rates for each experimental line at each of the time points measured. Selfing rates were calculated using the following equation:
Selfing rate = 1 – Outcrossing rate
Mixed mating pop gen zero MFs sheet:
The Mixed mating pop gen zero MFs sheet indicates the male frequencies that we measured in the mixed mating populations shortly before the initiation of our experiment (i.e. these male frequencies were measured before the mixed-mating populations and the obligately outcrossing populations were combined). The male frequencies that we measured in the mixed-mating populations prior to the start of our experiment were used to estimate the male frequencies in the mixed (mixed-mating + obligately outcrossing) populations at the start (generation zero) of our experiment.
