Molecular phylogeny and genetic diversity of Lygus
1Insect Genetics & Biochemistry, Red River Valley Agricultural Research Center, ARS, USDA, Fargo, ND, USA
2Canadian National Collection of Insects, AAFC, Ottawa, ON, Canada and
3Department of Entomology, North Dakota State University, Fargo, ND, USA
Correspondence: richard.roehrdanz@ars.usda.gov
The genus Lygus (Heteroptera: Miridae) in North America includes several species of polyphagous insect pests. The most common of these in the eastern half of the USA and Canada is the tarnished plant bug, Lygus lineolaris, which has been observed feeding on over 300 different plants. In western North America it is replaced primarily by the western tarnished plant bug, L. hesperus, and to a lesser extent L. elisus, L. keltoni, and L. borealis. There are additional Lygus species that are not agricultural pests. Inter- and intraspecific genetic diversity in North American Lygus was examined using nuclear and mitochondrial DNA. DNA sequences have been obtained from the mitochondrial cox1 and cox2 genes, the nuclear ITS1 spacer, and some microsatellite flanking regions (MSFR). The Fargo lab sequenced a region overlapping the cox1-cox2 boundary, whereas the Ottawa lab sequenced a region near the 5-prime end of cox1. These data sets are being merged by reciprocal swapping and sequencing of key DNA samples. L. lineolaris has been widely sampled in the eastern US and southern Canada. The cox1-cox2 boundary sequence identified two major clades and a small third group of very divergent individuals. One of the MSFRs and cox1 together also form two clades. The three marker sets appear to be defining the same populations. Both clades are widely distributed and sympatric. Clade 1 is most common in southern Ontario and eastern North Dakota. Clade 2 predominates in the rest of Canada from Quebec to Alberta and also represents the majority of insects from New England and the Gulf Coast in the USA. The data raises the possibility of cryptic species within L. lineolaris. Except for the paucity of Clade 1 at most Canadian sites, well-defined geographic based population structure has not been uncovered in L. lineolaris. The molecular and morphological data both support the monophyly of the genus Lygus with HenryLygus as the sister taxon. At the interspecific level within the genus both ITS1 and MSFRs roughly divide the species into eastern and western groups. L. borealis is eastern with ITS1 and western with MSFRs. Strict monophyly of the species has been difficult to demonstrate. With cox1-cox2 the majority of sequences from individuals identified as L. lineolaris, L. elisus, L. borealis, L. hesperus, and L. keltoni cluster as species. However a few individuals from all of these species are scattered among other branches of the trees. Morphological species assignment is difficult because of overlapping characters and the misplaced individuals are most likely the result of these difficulties. The L. hesperus and L. keltoni clusters are intermixed in both cox1-cox2 and some MSFR. Further evaluation of their species status is in progress. The complete mt genome (~14,150 bp), except for the control region and some sequences adjacent to the control region has been sequenced from L. lineolaris. Stephen Cameron (Canberra, Australia) has sequenced the same region from L. hesperus. The nucleotide sequence divergence between them is about 3.4% and the gene order of both corresponds to that for Drosophila yakuba.