Use of Multivariate Discriminant Analysis of Male Wing Morphometrics to Delineate a Hybrid Zone for Papilio glaucus glaucus and P. g. canadensis in Wisconsin

Abstract

The eastern tiger swallowtail butterfly Papilio glaucus L. is distributed throughout Canada and the eastern half of the United States. Two of the three purported subspecies (Papilio glaucus glaucus and Papilio glaucus canadensis) occur in Wisconsin. In order to determine the extent of hybridization between the two subspecies, 18 wing characters were measured or scored for three reference groups: canadensis (from Canada, northern Minnesota and the Upper Peninsula of Michigan), glaucus (from southern Ohio and central Illinois) and known laboratory F, hybrids. These data were subjected to multivariate discriminant analysis, and the resulting rule was then used to statistically analyze field-captured adults from various counties of Wisconsin. Individuals collected from Wisconsin were entered as unclassified data and analyzed with regard to the reference groups in two ways. First, general body size (as indicated by forewing length) was included because of long recognized size differences between the subspecies. Second, the data were reanalyzed after body size was removed statistically. The results of the second analysis parallel those of the first, indicating that useful, nonsize related taxonomic characters are included in the measured traits. Of 15 wing traits (and three ratios), three wing characters alone gave an overall classification accuracy of 84%, which is close to the 88.5 7O correct classification with all traits used. The southern third of Wisconsin is very different from the northern two-thirds in tiger swallowtail composition. No field-captured individuals (unknowns) were classified as Papilio glaucus glaucus in the northern two thirds of the state, while most of the field captures from the southern third of the state were classified as such. Computerclassified reached their greatest frequency in the S-central part of Wisconsin and support the evidence of a hybrid between these Papilio subspecies. INTRODUCTION Hybrid zones have been defined as narrow regions in which genetically distinct populations meet, mate, and produce hybrids (Barton and Hewitt, 1985). Hundreds of suspected hybrid zones have been identified and a recent review concludes that it is difficult to distinguish whether the distinct forms in zones of parapatry are maintained by hybrid unfitness or differential adaptation to environmental factors (Woodruff, 1981; Barton and Hewitt, 1985). Central Wisconsin is particularly interesting in that it represents a tension zone for plant communities (Curtis, 1959) and a of overlap and suspected hybridization for many insects (Remington, 1968; Platt and Brower, 1968; Scriber, 1982, 1983). There is great interest in understanding the causal mechanisms and processes promoting or preventing gene exchange among populations and hybrid zones offer a valuable natural laboratory in this regard (e.g, Littlejohn and Watson, 1985; Slatkin, 1985). 'Department of Entomology, University of Wisconsin, Madison 53706. 2Current address: Department of Entomology, Michigan State University, East Lansing, 48824. 3Departments of Horticulture and Statistics, University of Wisconsin, Madison 53706. 4Current address: Department of Medical Genetics, University of Wisconsin, Madison 53706.