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Geographic variation in the evolution and coevolution of a tritrophic interaction
Timothy P Craig1, Joanne K Itami, John D Horner
1Department of Biology, University of Minnesota-Duluth, Duluth, Minnesota 55803, USA. tcraig@d.umn.edu
Summary
Geographic variation in natural enemies drives differences in Eurosta solidaginis gall size and shape between prairie and forest biomes. Coevolution is evident, with the parasitic wasp Eurytoma gigantea showing longer ovipositors in the prairie.
Area of Science:
- Ecology
- Evolutionary Biology
- Zoology
Background:
- The geographic mosaic theory of coevolution posits that local variations in species interactions create unique selective pressures.
- Eurosta solidaginis (a fly) induces galls on Solidago altissima goldenrod, serving as a model system to study coevolutionary dynamics.
- Natural enemies of E. solidaginis, including birds and insects, exert differential selection on gall morphology.
Purpose of the Study:
- To test the geographic mosaic theory by examining geographic variation in selection on E. solidaginis gall morphology.
- To identify the specific natural enemies responsible for differential selection in prairie and forest biomes.
- To investigate evidence of coevolution between E. solidaginis and its natural enemies, particularly Eurytoma gigantea.
Main Methods:
- Field study conducted over 11 years in prairie and forest biomes (Minnesota and North Dakota).
- Measurement of E. solidaginis gall size and shape.
- Analysis of natural enemy attack frequencies and their impact on gall morphology and E. solidaginis survival.
- Comparison of Eurytoma gigantea ovipositor length between biomes.
Main Results:
- Galls were significantly larger and more spherical in the prairie compared to the forest.
- Differences in the frequency of attacks by natural enemies, not similar selection pressures, created distinct selection regimes between biomes.
- Bird predation selected for smaller galls in the forest, while Mordellistena convicta and Eurytoma gigantea selected for larger galls in both biomes.
- Eurytoma gigantea exhibited longer ovipositors in the prairie, suggesting adaptation to larger gall sizes.
Conclusions:
- Natural enemies of E. solidaginis are key drivers of geographic variation in gall morphology, supporting the geographic mosaic theory.
- Distinct selection regimes imposed by natural enemies in prairie and forest environments lead to divergent gall evolution.
- Evidence suggests coevolution between E. solidaginis and E. gigantea, with the parasitoid adapting its morphology to host gall size.
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