Why do some host-parasite interactions become less antagonistic over evolutionary time? Vertical transmission can select for reduced antagonism. Vertical transmission also promotes coevolution between hosts and parasites. Therefore, we hypothesized that coevolution itself may underlie transitions to reduced antagonism. To test the coevolution hypothesis, we selected for reduced antagonism between the host Caenorhabditis elegans and its parasite Serratia marcescens. This parasite is horizontally transmitted, which allowed us to study coevolution independently of vertical transmission. After 20 generations, we observed a response to selection when coevolution was possible: reduced antagonism evolved in the co-passaged treatment. Reduced antagonism, however, did not evolve when hosts or parasites were independently selected, without coevolution. In addition, we found strong local adaptation for reduced antagonism between replicate host/parasite lines in the co-passaged treatment. Taken together, these results strongly suggest that coevolution was critical to the rapid evolution of reduced antagonism.
Frequency of the ruby-throated phenotype
Frequency of the ruby-throated phenotype assayed in different combinations of host and parasite lines. "Pairing" indicates the specific pairing of host and parasite lines being made. "Host" and "Parasite" columns provide additional details. In these columns, Ancestor and Ancestor-sm2170 refer to the ancestral populations of host and parasite, respectively, used to initiate experimental evolution. Similarly, control, singly, and co-passaged refer to the products of 20 generations of experimental selection under the Control, Singly-passaged, and Co-passaged treatments, respectively. "Line" indicates the replicate lineage being tested, and "Replicate" indicates the replicate assay plate within each replicate lineage. "Frequency" indicates the proportion of adult hermaphrodites displaying the ruby-throated phenotype in the assay population.
1-Frequency.csv
Fecundity of ruby-throated hermaphrodites
Fecundity of ruby-throated hermaphrodites, resulting from different combinations of host and parasite lines. "Pairing" indicates the specific pairing of host and parasite lines being made. "Host" and "Parasite" columns provide additional details. "Line" indicates the replicate lineage being tested, and "Replicate" indicates the replicate assay plate within each replicate lineage. For each replicate assay plate, 10 hermaphrodites were observed - column "Hermaphrodite." The number of offspring produced by each hermaphrodite is reported in the column "Offspring."
2-Fecundity.csv
Fecundity of healthy hermaphrodites
Fecundity of healthy hermaphrodites, assayed in otherwise the same way as fecundity of ruby-throated hermaphrodites. "Host" indicates the host experimental evolution treatment being assayed, and "Line" the replicate lineage. Fifteen hermaphrodites were sampled per replicate lineage, identified under "Hermaphrodite." The number of offspring produced by each hermaphrodite is reported in column "Offspring."
3-Fecundity_healthy.csv
Mortality rate
Mortality rate induced by parasite populations following experimental evolution. All parasite populations were evaluated against the ancestral host lineage of matching number. “Parasite” indicates the parasite experimental evolution treatment being assaying, and “Line” the replicate lineage. “Replicate” indicates the replicate assay plate. “Mortality_Rate” gives the proportion of hosts found to be dead or morbid following 24 hours of exposure to the parasite population.
4-Mortality.csv
Local Adaptation
Local adaptation data. Fecundity of hosts from the Co-passaged treatment paired with sympatric and allopatric parasites from the Co-passaged treatment. A “1” in the “Sympatric” column indicates a sympatric pairing; “0” indicates allopatric. “Line_host” and “Line_parasite” indicate the replicate lineages of host and parasite, respectively, combined in the assay. “Replicate” identifies the 15 hermaphrodites assayed for fecundity per pairing. The number of offspring produced by each hermaphrodite is reported in the column “Offspring.”
5-LocalAdaptation.csv
