Hot summer temperatures may stop movement of Papilio canadensis butterflies and genetic introgression south of the hybrid zone in the North American Great Lakes Region

The reasons that the northern tiger swallowtail butterfly Papilio canadensis does not move south of the Great Lakes hybrid zone, where it meets P. glaucus, may be largely due to natural temperature-induced stress on diapausing pupae, Temperatures of 36degreesC for only four days killed all P. canadensis (from northern Michigan) and most of the Papilio troilus, a species that lives south of the hybrid zone (sympatrically with P. glaucus). In contrast, interspecific hybrids (P. glaucus mother x P. canadensis father) had significant adult eclosion or pupal survival at both 30degrees and 36degreesC. All surviving hybrid pupae were only females (the heterogametic sex, which are known to express the prolonged diapause due to Haldane effects). The southern species, P. troilus, had almost all (81%) normal (non-deformed) adults eclose at 30degreesC, whereas only 20% of the A canadensis females and ca 30% or the A canadensis males eclosed to produce non-deformed adults. Unlike the case with P. troilus and P. canadensis, no hybrid females eclosed at 30degreesC (only males did). Unlike P. troilus and P. canadensis pupae, hybrid (female) pupae remain viable, some of which have already successfully emerged after the chamber experiments. A follow-up study using P. glaucus, P. canadensis (from Vermont), and their hybrids with more normal lower thermal regimes included (27degrees, 30degrees, 33degrees, and 36degreesC) again showed higher hybrid survival as uneclosed (living) pupae at 36degreesC. In addition, A glaucus and P. canadensis showed high mortality and wing deformity of eclosing adults at 36degreesC, suggesting that geographic source of the P. canadensis may reflect differential tolerances of the extreme 36degreesC temperature.