Mitochondrial DNA sequence divergence among greenbug (Homoptera : Aphididae) biotypes: evidence for host-adapted races

The full complement of known greenbug, Schizaphis graminum (Rondani), bioty pes found in the USA were subjected to a molecular phylogenetic analysis ba sed on a 1.2-kb portion of the cytochrome oxidase I mitochondrial gene. In 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 Ely mus canadensis (L.) (CWR), and an isolate from Germany (EUR) were included. Schizaphis rotundiventris (Signoret) was included as an outgroup. Genetic distances among S. graminum biotypes ranged from 0.08% to 6.17% difference in nucleotide substitutions. Neighbour-joining, maximum parsimony and maxim um likelihood analyses all produced dendrograms revealing three clades with in S. graminum. Clade 1 contained the 'agricultural' biotypes commonly foun d on sorghum and wheat (C, E, K, I, plus J) and there were few substitution s among these biotypes. Clade 2 contained F, G and NY, and Clade 3 containe d B, CWR and EUR, all of which are rarely found on crops. The rarest biotyp e, H, fell outside the above clades and may represent another Schizaphis sp ecies. S. graminum biotypes are a mixture of genotypes belonging to three c lades and may have diverged as host-adapted races on wild grasses.