Zusammenfassung
Natural populations of Heliothis virescens (F.) and Helicoverpa zea (Boddie) were studied using starch gel electrophoresis as part of a program to understand gene flow, insecticide resistance, and evolutionary relationships in the group. This paper treats the electrophoretic techniques developed and used in our laboratories, compares our results with those of other workers on Heliothis and Helicoverpa, suggests enzyme loci that might be used in chemical tests for species identification, and proposes a new model for evolution in Helicoverpa. Several new or little-known enzyme staining recipes are included. Of the 23 interpretable loci revealed, 14 (61%) were polymorphic in H. virescens and 12 (52%) were polymorphic in H. zea. We find considerably less enzyme polymorphism in both species that did T. P. Sluss and co-workers in either species, but we suggest that some of the polymorphisms found by the latter were artifactual. Based on the 23 loci run here, average expected heterozygosity in H. virescens (17.2%) was significantly greater than in H. zea (5.5%). Eleven loci were diagnostic between the two species. However, our results on diagnostic loci also differ from those of T. P. Sluss et al., again suggesting scoring problems in their laboratory. Our results with H. zea were compared with those for the Old World species Helicoverpa armigera (Hübner) and Helicoverpa punctigera (Wallengren) from Australia. At polymorphic loci, H. zea has a 61% reduction in heterozygosity compared with its probable closest Old World relative, H. armigera and a similar reduction compared with H. punctigera. If caused by genetic drift, this reduction in heterozygosity would have required a severe bottleneck, equivalent to ≍10 generations with an effective population size of only six individuals, in the population ancestral to the entire species of H. zea. Our results are consistent with a founder effect leading to the evolution of the species H. zea during colonization of the Americas by stray individuals of H. armigera or a close relative.
