Submitted to: Evolution
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/8/2002
Publication Date: 1/8/2002
Citation: Sword, G.A. 2002. The importance of the ontogenetic niche in resource-associated genetic divergence: evidence from a generalist grasshopper. Evolution. 56(4):731-740. Interpretive Summary: Interpretive Summary: Grasshoppers as a group considered to be generalist feeds and they can eat plants from a number of different plant families. This perspective is based primarily on studies of adult grasshopper host plant use. The possibility that patterns of juvenile host plant use may be different from those of adults has been largely unexplored. At least one grasshopper species, Schistocerca emarginata (=lineata) exhibits changes in its diet breadth during development. Juvenile S. emarginata are dietary specialists, but the adults are generalist feeders. Juveniles in different populations may also specialize on different plants. This study uses mitochondrial DNA sequence variation to demonstrate that populations of juvenile S. emarginata specializing on different plants are genetically-distinct host plant-associated lineages. This pattern of host plant-associated genetic structuring of populations is common among specialist herbivorous insects such as Lepitopterans and Coleopterans, but had not been previously demonstrated among grasshoppers. Accounting for this genetic pattern required knowledge of juvenile patterns of host plant use. This study suggests that potentially overlooked developmental changes in host plant use can have substantial impacts on the ecology and evolution of herbivorous insects.
Technical Abstract: In the bird-winged grasshopper (Schistocerca emarginata = (lineata)), we investigate whether genetic data exist in accordance with geographic variation in host plant use and coloration.In TX,USA, juvenile grasshoppers feed almost exclusively on one of two host plants, Rubus trivialis (Rosaceae) or Ptelea trifoliata (Rutaceae). Adults of both forms are dietary generalists and consume many plants from unrelated families (Sword and Dopman 1999). Along with differences in juvenile feeding, differences in a density- dependent color polyphenism are concordant with genetic (mtDNA) variation among 8 different populations. Forms feeding on R. triv- ialis and those feeding on P.trifoliata represent monophyletic mtDNA lineages. Character state optimization of host plant acceptability on a phylogeny containing S.emarginata and outgroup taxa indicates that populations consuming R. trivialis gave rise to popu-lations consuming P. trifoliata.Juvenile grasshoppers that consume P. trifoliata acquire deterrence against predation(Sword 1999), suggesting that "enemy-free space" facilitated this host shift.In extant populations, adaptations stemming from alternative resource use during ontogeny present possible barriers to gene exchange.This study represents the first demonstration of resource-associated divergence in an otherwise generalist insect that exhibits temporal variation in resource use, characterized as developmental changes in host specialization.Our findings suggest that exploitation of diff- erent resources may have unexplored significance for generalist species that compartmentalize specialization to particular life stages.