Ecological speciation without host plant specialization; possible origins of a recently described cryptic Papilio species

North American Papilio canadensis and P. glaucus (Lepidoptera: Papilionidae, these Papilio = Pterourus) have previously been described as having allopatric distributions separated by a narrow hybrid zone running from Minnesota to southern New England, and southward in the Appalachian Mountains (possibly to northern Georgia). Recent patterns of hybridization and introgression suggest a more complex interaction between the two, possibly even resulting in the formation of a new species (Pterourus appalachiensis Pavulaan & Wright, 2002). Recently, extensive northward interspecific introgression of P. glaucus-diagnostic traits has been observed in the hybrid zone. These include wing bands and other color patterns, the ability to feed on tulip tree leaves, and Hk-100 allozymes; all are autosomally encoded. However, there has been little northward introgression of certain other P. glaucus traits (such as facultative diapause and bivoltinism, and Ldh-100 allozymes, both X-linked; and the Y-linked melanic mimicry gene in females). Interspecific recombination of the X-chromosome has evidently occurred, as shown by discordant patterns of X-linked markers. The P. glaucus X-linked Pgd-100 and Pgd-50 alleles have introgressed 200–400 km north of the historical hybrid zone, yet the P. glaucus X-linked Ldh-100 allele has not. The allele frequency shift for both genes is more closely related to the ‘thermal landscape’ (i.e., accumulated degree-days above a developmental base threshold of 50 °F (=10 °C)) than to latitude. Delayed post-diapause eclosion of cohorts within the hybrid zone, e.g., the New York/Vermont border area, has produced a natural ‘false-second generation’ flight (a hybrid swarm of synchronous males and females, where 2300–2700 °F degree-days have accumulated each year since 1998) that is reproductively isolated from flights of both parental species. Moreover, the newly described P. appalachiensis exhibits a unique combination of traits. These include obligate diapause, a univoltine habit, and the Ldh-80 or Ldh-40 alleles (as for P. canadensis), the Pgd-100 or Pgd-50 alleles (as for P. glaucus), and a delayed ‘false-second generation’ reproductive flight period (as observed in the hybrid zone). Since 2001, a rare allele or ‘hybrizyme’ (Ldh-20) has appeared in this false second generation at high frequencies (40–50%). We hypothesize that strong selection against the facultative diapause (od-)trait (and the linked Ldh-100 allele) in regions with 2800 °F degree-days or less, and divergent selection in favor of Pgd-100 (or a closely linked trait) combined with allochronic reproductive isolation, has resulted in recombinational, parapatric, hybrid speciation. There is no evidence at present that host-plant shifts or changes in sex pheromones have driven this process, in contrast to many other speciation events in the Lepidoptera.