Submitted to: Entomological Research Bulletin of
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/23/2001
Publication Date: 4/1/2002
Citation: ANSTEAD, J.A., BURD, J.D., SHUFRAN, K.A. MITOCHONDRIAL DNA SEQUENCE DIVERGENCE AMONG SCHIZAPHIS GRAMINUM (HEMIPTERA: APHIDIDAE) CLONES FROM CULTIVATED AND NON-CULTIVATED HOSTS: HAPLOTYPE AND HOST ASSOCIATIONS. 2002. BULLETIN OF ENTOMOLOGICAL RESEARCH. V. 92. P. 17-24.
Interpretive Summary: The greenbug (Schizaphis graminum) is an aphid pest of wheat and sorghum in the Great Plains. Currently, control depends on the use of insecticides and the planting of resistant crop varieties. The development of "biotypes" or populations of greenbugs that are capable of injuring the previously resistant crops has hampered management by resistant crops. Scientists have been attempting to understand the genetic and evolutionary processes by which new biotypes arise and render resistant crops vulnerable to damage. One theory is that biotypes are pre-existing in nature, and live on non-crop, and wild grasses, of which about 70 species have been identified. We found that the genetic diversity of greenbug populations on wild grasses was greater than on wheat and sorghum. Populations on crops were the most genetically uniform. We also found a greater number of different and "new biotypes" on wild grasses, which supports the theory that biotypes are already present in nature, and simply move onto crops which they then easily exploit. Therefore, the planting of resistant varieties has little effect on the genetic diversity of greenbug populations and does not drive the formation of new biotypes. This information will be of particular use to plant breeders and IPM specialists who are developing and implementing resistant crop varieties in greenbug management systems.
Technical Abstract: The greenbug (Schizaphis graminum (Rondani)) has been recorded from over 70 species of grasses (including crops such as wheat (Triticum aestivum L.) and sorghum (Sorghum bicolor (L.) Moench.), but its population genetics on non-cultivated hosts has rarely been considered. A 1.0-kb region of the mitochondrial cytochrome oxidase subunit I gene was sequenced for 24 field-collected isolates from non-cultivated and cultivated hosts. Maximum likelihood, maximum parsimony, and neighbor- joining phylogenies were estimated for these isolates, plus 12 isolates previously sequenced by Shufran et al. (2000), and all produced trees with identical topologies. The presence of 3 clades within S. graminum was further confirmed. Isolates showed no relationship between biotype and clade. At least one biotype was found in all three clades, suggesting exchange or sharing of genetic material between clades conditioning for crop virulence. On the other hand, there was a relationship between host and clade. Clade 1 was the most homogenous and contained 12 of 16 isolates collected from cultivated hosts and 5 of the 6 collected from johnsongrass (Sorghum halepense (L.) Pers.), a close relative of sorghum. Four of the 6 isolates collected from Agropyron spp. were found in Clade 2. Clade 3 contained two isolates from wheat and four from other hosts other than Agropyron spp. These clades probably diverged on grasses prior to the advent of human agriculture. Schizaphis graminum biotypes have no meaning in a taxonomic or evolutionary context.