Submitted to: Insect Molecular Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/22/1999
Publication Date: 2/1/2000
Citation: Shufran, K.A., Burd, J.D., Anstead, J.A., Lushai, G. 2000. Mitochondrial DNA sequence divergence among greenbug (Homoptera: Aphididae) biotypes: evidence for host-adapted races. Insect Molecular Biology. 9(2):179-184.
Interpretive Summary: The greenbug is a primary pest aphid of wheat and sorghum in the US. One management strategy used against greenbug is breeding crops naturally resistant to infestation and/or damage. Unfortunately, this strategy has only been effective for short periods of time, as unknown changes in the greenbug population occurred, rendering resistant crops vulnerable again. These new forms of greenbug able to damage resistant crops are termed biotypes. To date, 11 greenbug biotypes are known to exist in the US, and are identified by ability to damage an array of resistant wheat, barley, and sorghum lines. For the last 40 years, the origin, evolution, and taxonomic status of greenbug biotypes have been debated and remain controversial. Most importantly, factors that promote biotype occurrence were not understood, which hindered the development of durable plant resistance. We addressed this problem by using the modern genetic technique eof DNA sequencing. Looking at differences in a specific gene in the DNA of all known biotypes, we were able to infer the evolutionary relationship between the various biotypes. Within the greenbug species, we found only 3 evolutionary distinct subgroups. All biotypes commonly found on wheat and sorghum belong to one subgroup. The other two subgroups contain the rare biotypes, found on non-crop species of grasses. We concluded that greenbug biotypes actually represent members of specific, host-plant-adapted races that evolved independently from cultivated crops. Planting modern resistant crops has not caused biotypes and has had little impact on greenbug genetics. The greenbug evolved naturally over time into a genetically diverse species. This variation is maintained on wild grasses, and only a very small subset of greenbugs is able to utilize and damage crops.
Technical Abstract: The full complement of known greenbug, Schizaphis graminum (Rondani), biotypes found in the United States were subjected to a molecular phylogenetic analysis based on a 1.2 kb portion of the cytochrome oxidase I gene located in the mitochondrial DNA. An addition to these nine biotypes (B, C, E, F, G, H, I, J, and K), a probable isolate of the enigmatic biotype A (NY), a "new biotype" collected from Elymus canadensis (L.) (CWR), and an isolate from Germany (EUR) were included. Schizaphis rotundiventris (Signoret) was included as an outgroup. Substantial divergence among S. graminum biotypes was found, with genetic distances ranging from 0.08 to 6.17% difference in nucelotide sequences. Neighbor- joining, maximum parsimony, and maximum likelihood analyses all produced dendograms revealing three sub-specific clades within S. graminum. Clade 1 contained the "agricultural" biotypes commonly found on sorghum and wheat (C, E, K, I, plus J). This was the most homogeneous group. Clade 2 contained F, G, and NY; and Clade 3 contained B, CWR, and EUR, all of which are rarely found on crops. The rarest biotype, H, was the most divergent, resided outside the above clades, and thus may represent another Schizaphis species. S. graminum biotypes appear to be in reality a mixture of genotypes that belong to three sub-specific categories, which probably evolved as host-adapted races on wild grasses.