Behavioral, molecular, and morphological evidence for a hybrid zone between Chrysochus auratus and C-cobaltinus (Coleoptera : Chrysomelidae)

In this article, we describe a hybrid zone between the chrysomelid beetles, Chrysochus auratus (F.),and C. cobaltinus LeConte, which have historically been considered as having allopatric distributions. By combining field stu dies with surveys of museum specimens, we documented that in western North America there are two legions in which these beetles are sympatric, and fou r additional legions in which populations of the two species are <100 km ap art. In south-central Washington, we found an <approximate to>25 km wide ar ea of sympatry in which the two species freely intel breed. Morphological a nd allozyme differences between the species allowed us to demonstrate that individuals with intermediate coloration in this area are indeed hybrids; a ll 22 putative hybrids we assayed for allozyme variation were heterozygous at each of three species-specific loci. Museum specimens revealed that the two species have been hybridizing in this region at least since 1952. Withi n the hybrid zone, approximate to 10-15% of the beetles is apparently F-1 h ybrids. At one focal site, 22.9% of all matings involved heterospecific pai rs and 20.8% of all matings involved at least one hybrid individual. Althou gh we found no molecular evidence of introgression between the two species. morphometric results and preliminary ecological data suggest possible past introgression or weak ongoing introgression. We discuss the implications o f our findings for the specific status of these two species. This system ap pears well suited to provide answers to long-standing questions concerning the evolution of premating barriers between hybridizing species. In additio n, hybridization between these two beetle species with differing host range s will allow us to test the hypothesis that ecologically significant traits such as diet breadth can be gained via introgression.