title: Host race evolution in Schizaphis graminum (Hemiptera: Aphididae): nuclear DNA sequences.
abstract: The greenbug aphid, Schizaphis graminum (Rondani) was introduced into the United States in the late 1880s, and quickly was established as a pest of wheat, oat, and barley. Sorghum was also a host, but it was not until 1968 that greenbug became a serious pest of it as well. The most  effective control method is the planting of resistant varieties; however, the occurrence of greenbug biotypes has hampered the development and use of plant resistance as a management technique. Until the 1990s, the evolutionary status of greenbug biotypes was obscure. Four mtDNA cytochrome  oxidase subunit I (COI) haplotypes were previously identified, suggesting that S. graminum sensu lato was comprised of host-adapted races. To elucidate the current evolutionary and taxonomic status of the greenbug and its biotypes, two nuclear genes and introns were sequenced; cytochrome  c (CytC) and elongation factor 1-α (EF1-α). Phylogenetic analysis of CytC sequences were in complete agreement with COI sequences and demonstrated three distinct evolutionary lineages in S. graminum. EF1-α DNA sequences were in partial agreement with COI and CytC  sequences, and demonstrated two distinct evolutionary lineages. Host-adapted races in greenbug are sympatric and appear reproductively isolated. Agricultural biotypes in S. graminum likely arose by genetic recombination via meiosis during sexual reproduction within host-races. The 1968 greenbug  outbreak on sorghum was the result of the introduction of a host race adapted to sorghum, and not selection by host resistance genes in crops.
authors.affiliation: USDA-ARS, Wheat, Peanut, and Other Field Crops Research Unit, Stillwater, OK 74075, USA. kashufran@aol.com
authors.forename: Kevin A
authors.initials: KA
authors.lastname: Shufran
mesh_terms: D000818:Animals; D001042:Aphids / Q000235:genetics*; D018556:Crops, Agricultural / Q000469:parasitology; D045304:Cytochromes c / Q000235:genetics*; D004272:DNA, Mitochondrial / Q000235:genetics; D019143:Evolution, Molecular; D058507:Host Specificity / Q000235:genetics*; D020648:Peptide Elongation Factor 1 / Q000235:genetics*; D010802:Phylogeny; D017422:Sequence Analysis, DNA
chemical_list: D004272:DNA, Mitochondrial; D020648:Peptide Elongation Factor 1; D045304:Cytochromes c