DOI: 10.5061/dryad.rf47cp2
title: Data from: Turnover in local parasite populations temporarily favors host outcrossing over self-fertilization during experimental evolution

description: The ubiquity of outcrossing in plants and animals is difficult to explain
 given its costs relative to self-fertilization. Despite these costs,
 exposure to changing environmental conditions can temporarily favor
 outcrossing over selfing. Therefore, recurring episodes of environmental
 change are predicted to favor the maintenance of outcrossing. Studies of
 host–parasite coevolution have provided strong support for this
 hypothesis. However, it is unclear whether multiple exposures to novel
 parasite genotypes in the absence of coevolution are sufficient to favor
 outcrossing. Using the nematode Caenorhabditis elegans and the bacterial
 parasite Serratia marcescens, we studied host responses to parasite
 turnover. We passaged several replicates of a host population that was
 well-adapted to the S. marcescens strain Sm2170 with either Sm2170 or one
 of three novel S. marcescens strains, each derived from Sm2170, for 18
 generations. We found that hosts exposed to novel parasites maintained
 higher outcrossing rates than hosts exposed to Sm2170. Nonetheless, host
 outcrossing rates declined over time against all but the most virulent
 novel parasite strain. Hosts exposed to the most virulent novel strain
 exhibited increased outcrossing rates for approximately 12 generations,
 but did not maintain elevated levels of outcrossing throughout the
 experiment. Thus, parasite turnover can transiently increase host
 outcrossing. These results suggest that recurring episodes of parasite
 turnover have the potential to favor the maintenance of host outcrossing.
 However, such maintenance may require frequent exposure to novel virulent
 parasites, rapid rates of parasite turnover, and substantial host gene
 flow.
description: Figure-1-dataMortality rates suffered by the ancestral host population (EW2-30) after 24 hours of exposure to the four parasite strains we used for experimental evolution. Each of the five replicate host populations was tested against each parasite strain with three technical replicates and the average number of dead nematodes was calculated for each combination. All plates started with 200 nematodes and dead nematodes were counted 24 hours later.Figure-2-dataCounts of male and hermaphrodite hosts in each replicate population in the ancestral generation and generations 6, 12, and 18 of experimental evolution. Treatment = parasite strain each population was passaged with during experimental evolution. Male Frequency = # males / (# males + # hermaphrodites). Outcrossing Rate = 2*(Male Frequency – 0.002).Figure-3-dataPercent changes in mean fitness for generation 18 hosts relative to their ancestors during exposure to the parasite strain each generation 18 host population was passaged with, as determined by competitive fitness assays against a GFP-marked tester strain. See README for further details.Figure-4-dataMortality rates suffered by evolved generation 18 host populations after 24 hours of exposure to Sm2170. Each replicate host population was tested in two technical replicates and the average number of dead nematodes was calculated. All plates started with 200 nematodes and dead nematodes were counted 24 hours later. Host = parasite strain the host population was passaged with during experimental evolution. Parasite = parasite strain used for the mortality assays.Table-3-dataSurvival rates of hermaphrodites and males from the ancestral host population (EW2-30) within the selection regime. Both host sexes were tested against each of the four parasite strains used for experimental evolution in five replicates. Groups of nematodes (222 hermaphrodites or 50 males) were placed on the parasite side of Serratia selection plates (Worms Plated). Nematodes that migrated out of the parasite lawn and were alive 48 hours after exposure to the parasite were counted as survivors (Worms Alive).