Population Structure and Host Use in Hybridizing Subspecies Ofpapilio Glaucus(lepidoptera: Papilionidae)

Two parapatric subspecies of the eastern tiger swallowtail butterfly, Papilio glaucus glaucus and P. g. canadensis, differ greatly in larval survival and growth on host plants in the Magnoliaceae, Salicaceae, and Betulaceae. In the first part of this study, butterflies were collected from 17 sites along a transect from Georgia to Quebec and used for allozyme electrophoresis. Two X- (or Z-)linked enzyme loci (Ldh and Pgd) showed allele frequency differences that delineated a hybrid zone between the subspecies in northern Pennsylvania and south-central New York. No significant linkage disequilibrium could be detected among allozyme loci within the hybrid zone samples. Genetic differentiation at Ldh and Pgd coincides with subspecies differences in diapause control and female mimicry, which are also sex-linked. Larval offspring of butterflies from 13 of the sites were then tested in the laboratory for survival and growth on Liriodendron tulipifera (Magnoliaceae), Populus deltoïdes (Salicaceae) and Betula lutea (Betulaceae). Steep clines in survival rates matched the position of the hybrid zone. Hybrid zone larvae showed intermediate survival rates and significant heterogeneity among families on all three plants, indicating presence of substantial genetic variation. The results suggest that differential host use by P. g. glaucus and P. g. canadensis is maintained primarily by independent clines in selection intensity for ability to use allopatric sets of host plants, coupled with restricted gene flow across the hybrid zone.