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Abstract Host–parasite interactions may often be subject to opposing evolutionary forces, which likely influence the evolutionary trajectories of both partners. Natural selection and genetic drift are two major evolutionary forces that act in host and parasite populations. Further, population size is a significant determinant of the relative strengths of these forces. In small populations, drift may undermine the persistence of beneficial alleles, potentially impeding host adaptation to parasites. Here, we investigate two questions: (a) can selection pressure for increased resistance in small, susceptible host populations overcome the effects of drift and (b) can resistance be maintained in small host populations? To answer these questions, we experimentally evolved the hostCaenorhabditis elegansagainst its bacterial parasite,Serratia marcescens, for 13 host generations. We found that strong selection favouring increased host resistance was insufficient to counteract drift in small populations, resulting in persistently high host mortality. Additionally, in small populations of resistant hosts, we found that selection for the maintenance of resistance is not always sufficient to curb the loss of resistance. We compared these results with selection in large host populations. We found that initially resistant, large host populations were able to maintain high levels of resistance. Likewise, initially susceptible, large host populations were able to gain resistance to the parasite. These results show that strong selection pressure for survival is not always sufficient to counteract drift. In consideration ofC.elegans natural population dynamics, we suggest that drift may often impede selection in nature.
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Gene flow accelerates adaptation to a parasite
Abstract Gene flow into populations can increase additive genetic variation and introduce novel beneficial alleles, thus facilitating adaptation. However, gene flow may also impede adaptation by disrupting beneficial genotypes, introducing deleterious alleles, or creating novel dominant negative interactions. While theory and fieldwork have provided insight into the effects of gene flow, direct experimental tests are rare. Here, we evaluated the effects of gene flow on adaptation in the nematode Caenorhabditis elegans during exposure to the bacterial parasite, Serratia marcescens. We evolved hosts against nonevolving parasites for 10 passages while controlling host gene flow and source population. We used source nematode populations with three different genetic backgrounds (one similar to the sink population and two different) and two evolutionary histories (previously adapted to S. marcescens or naive). We found that populations with gene flow exhibited greater increases in parasite resistance than those without gene flow. Additionally, gene flow from adapted populations resulted in greater increases in resistance than gene flow from naive populations, particularly with gene flow from novel genetic backgrounds. Overall, this work demonstrates that gene flow can facilitate adaptation and suggests that the genetic architecture and evolutionary history of source populations can alter the sink population’s response to selection.
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- PAR ID:
- 10418078
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Evolution
- Volume:
- 77
- Issue:
- 6
- ISSN:
- 0014-3820
- Format(s):
- Medium: X Size: p. 1468-1478
- Size(s):
- p. 1468-1478
- Sponsoring Org:
- National Science Foundation
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