Submitted to: Molecular Ecology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/17/2005
Publication Date: 4/22/2005
Citation: Sword, G.A., Joern, A., Senior, L. 2005. Host plant-associated genetic differentiation in the snakeweed grasshopper, hesperotettix viridis (orthoptera, acrididae). Molecular Ecology. 14(7):2197-2205. Interpretive Summary: Studies of herbivorous insects have played a major role in understanding how ecological divergence can facilitate genetic differentiation. In contrast to the majority of herbivorous insects, grasshoppers as a group are largely polyphagous. Due to this relative lack of intimate grasshopper-plant associations, grasshopper-plant systems have not played a large part in the study of host-associated genetic differentiation. The oligophagous grasshopper, Hesperotettix viridis (Thomas), is endemic to North America and feeds on composites (Asteraceae) within the tribe Astereae. Previous work has shown both preference and performance differences between H. viridis individuals feeding on either Solidago mollis or Gutierrezia sarothrae. We examined the genetic relationships among 38 H. viridis individuals feeding on these plants both in sympatry and allopatry using 222 AFLP markers. Neighbor-joining analysis resulted in two distinct host-associated clades with 71% bootstrap support for host'associated monophyly. Analyses of molecular variation (AMOVA) revealed significant genetic structuring with host plant accounting for 20% of the total genetic variance while locality accounted for 0%. Significant genetic differentiation was detected between S. mollis-feeders and G. sarothrae-feeders even when the two were present at the same locality. These results are consistent with observed differences in preference and performance between H. viridis grasshoppers feeding on either G. sarothrae and S. mollis and indicate that H. viridis is comprised of at least two genetically-distinct host plant-associated lineages.
Technical Abstract: The snakeweed grasshopper, Hesperotettix viridis (Orthoptera, Acrididae), feeds on at least 34 different host plant species in the family Astreaceae across its range in western North America. Detailed examination of local host plant preference and performance patterns on two of its hosts, Solidago mollis and Gutierrezia sarothrae, revealed substantial ecological divergence between insects utilizing the different plants as hosts. In this study, we utilized a multi-locus AFLP approach to examine the genetic relationships among H. viridis grasshoppers feeding on either S. mollis or G. sarothrae. We found that the insects fell out into two genetically-distinct groups associated with either S. mollis or G. sarothrae. Importantly, these genetic differences were maintained even when the insects were feeding on different plants at the same site. Insects feeding on S. mollis at one site in Nebraska were more closely related to other insects feeding on the same plant 30 km away than they were to conspecifics feeding on G. sarothrae at the same site. Similarly, grasshoppers feeding on G. sarothrae were more closely related to other grasshoppers feeding on the same plant over 900 km away in New Mexico than they were to conspecifics at the same site feeding on S. mollis. These results suggest that H. viridis grasshoppers in North America exist as at least two genetically-distinct host plant-associated lineages.