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Journal of The Lepidopterists' Society

Volume 45                                     1991                                     Number 4

Journal of the Lepidopterists' Society 45(4), 1991, 245-258

PAPILIO CANADENSIS AND P. GLAUCUS (PAPILIONIDAE) ARE DISTINCT SPECIES

Robert H. Hagen,1 Robert C. Lederhouse, J. L. Bossart and

J. Mark Scriber

Department of Entomology, Michigan State University, East Lansing, Michigan 48824

ABSTRACT. Papilio canadensis Rothschild & Jordan is recognized as a distinct species, not a subspecies of P. glaucus L., on the basis of physiological and genetic differences despite great similarity in adult appearance of these two taxa. Interspecific hybrids are found in a well-marked zone where the ranges of the species come into contact. The ecological or genetic factors that maintain species integrity despite this natural hybridization are as yet uncertain.

Additional key words: diapause, electrophoresis, hybrid zone, Michigan, Great Lakes region.

In their revision of American Papilionidae, Rothschild and Jordan (1906) described a northern subspecies of Papilio glaucus L., P. g. canadensis, distinguishable from P. g. glaucus by differences in the details of wing color pattern and by its smaller size. Although evidence was scanty, Rothschild and Jordan suggested that glaucus and canadensis "completely intergrade" in the Great Lakes region. Presumably, the parapatric distribution of these two taxa was a major factor in their decision to treat canadensis as a subspecies of P. glaucus.

Despite the morphological similarity between P. glaucus and canadensis adults and the occurrence of natural hybridization, our recent studies lead us to conclude that canadensis does warrant recognition as a distinct species. Three lines of evidence support our interpretation: first, there is significant differentiation between glaucus and canadensis in characters besides adult morphology; second, there appears to be a closer phylogenetic relationship between glaucus and P. alexiares Hoppfer than between glaucus and canadensis-, and third, intergra-

1 Current address: Department of Entomology, Haworth Hall, University of Kansas, Lawrence, Kansas 66045.

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dation between glaucus and canadensis is restricted to a narrow hybrid zone. We discuss this evidence and its implications below.

In contrast to canadensis, another currently recognized subspecies of P. glaucus, P. g. australis Maynard, appears to fully intergrade morphologically and genetically with P. g. glaucus in the southeastern United States (Hagen & Scriber 1991, R. C. Lederhouse, J. L. Bossart & J. M. Scriber unpubl.). We suggest that australis should be treated as only a form of P. glaucus. A more complete discussion of relationships between australis and typical glaucus populations is in preparation (R. C. Lederhouse, J. L. Bossart & J. M. Scriber unpubl.). In addition, our studies provide no evidence whatsoever to justify resurrection of an Alaskan subspecies, arcticus Skinner; all specimens from Fairbanks, Alaska, that we have examined are clearly P. canadensis.

Papilio canadensis Rothschild & Jordan, new status

Papilio glaucus canadensis Rothschild & Jordan (1906).

Diagnostic characters for the species are discussed below, and include those which have been used previously for the subspecies.

Diagnostic Characters Color Pattern and Morphology

Overall similarity of adults characterizes taxa within species groups of Papilio and contributes to frequent uncertainties in species-level taxonomy of Papilionidae (Rothschild & Jordan 1906, Munroe 1961, Sperling 1987, 1990). Two features of the wing pattern originally noted by Rothschild and Jordan appear to be fairly reliable characters for distinguishing adult glaucus and canadensis, along with overall size.

The first feature is the form of the forewing underside submarginal spots (Luebke et al. 1988). In P. glaucus the yellow spots centered in each cell are distinct; in P. canadensis, the spots form a nearly continuous yellow band with black pigment confined to a narrow line of scales at each vein. The yellow band in canadensis is similar to that on the forewing underside of P. rutulus Lucas. However, both canadensis and glaucus can be distinguished from rutulus by the presence of orange in the first submarginal spot of the hindwing (Rothschild & Jordan 1906).

The second diagnostic feature for canadensis is the width of the black band along the anal margin of the hindwing (Scriber 1982). For P. glaucus males, widths are in the range 10 to 50 percent of the width from wing margin to the CuA2 vein; whereas for P. canadensis males, the range is 50 to 90 percent (Scriber 1982, R. Hagen, unpubl. data). The width of the black band is greater in females than males, though the relative difference between species persists.

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Smaller adult size is a third criterion useful for distinguishing specimens of P. canadensis from P. glaucus. Luebke et al. (1988) estimated size from the forewing chord length (FCL), the distance from wing apex to base: FCL for male P. canadensis ranged from 41 to 50 mm (median = 45 mm; n = 91); FCL for male P. glaucus ranged from 46 to 59 mm (median = 54 mm; n = 72). (Females were not measured in this study.) In both species the upper limits to size appear to be determined by intrinsic developmental constraints, but variation in larval food plant quality can produce wide variation among individuals below this limit; thus, small P. glaucus may be similar in size to P. canadensis.

Use of wing pattern characters for species diagnosis also must be tempered with caution. Adult P. glaucus that diapaused as pupae have greater anal band widths than individuals that did not undergo diapause, and more closely resemble P. canadensis adults in other wing pattern features (Scriber 1990). This effect undoubtedly contributes to similarity between P. canadensis and "spring brood" (i.e., overwintered) P. glaucus, noted by Rothschild and Jordan (1906) and discussed at length by Clark and Clark (1951). It is even possible that some early season individuals collected within the range of P. glaucus may be P. canadensis, or hybrids (Scriber 1990).

At least one larval character appears useful for distinguishing glaucus and canadensis. Very young larvae (1st or 2nd instar) of canadensis possess three transverse white bands on the dorsal side, whereas glaucus larvae have only a single, central, band (Fig. 1). A possible difference in the mature larvae (4th and 5th instar) is the presence of a distinct pale yellow or white pigment around the dorsal anal tubercles of canadensis that is fainter or absent in glaucus larvae. However, we are uncertain of the reliability of this character difference. The form and color of the metathoracic "eyespots" of mature larvae do not differ between glaucus and canadensis, though they differ markedly from those of P. eurymedon Lucas, P. rutulus, and P. multicaudatus Kirby larvae (Brower 1959).

Male genitalia of glaucus and canadensis cannot be distinguished by shape, independent of overall size (Brower 1959). Female genitalia and internal morphology have yet to receive careful study.

Diapause

The most striking differences between glaucus and canadensis involve physiological and developmental characters (Table 1).

Populations of canadensis and glaucus differ in the control of pupal diapause (Scriber 1982, Hagen & Lederhouse 1985, Rockey et al. 1987a, Hagen & Scriber 1989). In P. glaucus, diapause appears to be determined environmentally: larvae that experience long daylength, good

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Fig. 1. First instar larvae of (A) P. glaucus, from Lawrence County, Ohio and (B) P. canadensis, from Cheboygan County, Michigan, showing diagnostic banding patterns (reared June-July 1991).

quality food, and perhaps warm temperatures are much more likely to develop directly into adults, without diapause, than are larvae that experience short days, poor food, or low temperatures. Under standard rearing conditions of 16 h light: 8 h dark, the proportion of larvae developing directly decreases among populations of P. glaucus with increasing latitude (Hagen & Lederhouse 1985, Rockey et al. 1987a).

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Table 1. Summary of physiological and biochemical differences between P. glaucus and P. canadensis, and their modes of inheritance, if known. See text for fuller explanation.

1. Hagen and Scriber (1989); 2. Rockey et al. (1987a); 3. Kukal et al. in press; 4. Scriber (1986); 5. Scriber (1988); 6. Scriber et al. 1989; 7. Hagen 1990; 8. ClarkeandSheppard(1962);9. J. M. Scriber, R. H. Hagen and R. C. Lederhouse, unpublished; 10. Scriber et al. (1987); 11. Hagen and Scriber 1991.

This pattern is presumably an adaptive response to latitudinal variation in day length, similar to that shown by many other insects (Tauber et al. 1986).

In P. canadensis, however, pupal diapause appears to be obligate: regardless of rearing conditions, all individuals enter pupal diapause (Rockey et al. 1987a). The genetic basis for obligate diapause is an X-linked, recessive allele present in P. canadensis (Rockey et al. 1987b, Hagen & Scriber 1989). (The sex chromosomes of males are here denoted as XX; those of females as XY.)

The obligate diapause allele of P. canadensis has an obvious benefit for individuals of this species, since the growing season throughout its range is too short to permit more than one generation to develop successfully each year (Hagen & Lederhouse 1985, Scriber 1988). The obligate diapause allele would be highly disadvantageous for P. glaucus, because glaucus occurs in areas where the growing season should permit two or more generations per year.

Differences in the regulation of diapause induction are accompanied by differences in the cold tolerance of diapausing pupae (Table 1: Kukal et al. in press). Diapausing P. canadensis pupae are capable of surviving

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lower temperatures than are P. glaucus pupae. The species also differ in primary metabolism: labelled glucose injected into diapausing pupae is converted to different compounds in each species (Kukal et al. in press). The genetics of cold tolerance and pupal physiology have not been studied in these species.

Larval Food Plant Use

Larval food plant use by P. glaucus and P. canadensis also differs (Table 1: Scriber 1988). Laboratory studies have revealed that P. glaucus larvae develop poorly on foliage from aspen (Populus spp.: Salicaceae) or birch (Betula spp.: Betulaceae), good hosts for P. canadensis. Conversely, P. canadensis larvae are unable to grow on foliage from tulip-tree (Liriodendron tulipifera L.: Magnoliaceae), an important host for P. glaucus.

These reciprocal inabilities are due to differences in the larval de-toxication systems of each species. Lindroth et al. (1988) showed that P. canadensis larvae possess high levels of a specific esterase enabling detoxication of a glycoside present in extracts of trembling aspen (Populus tremuloides Michx.) foliage. Larvae of P. glaucus lack this esterase activity and are poisoned by the glycoside; esterase activity is heritable (Scriber et al. 1989). Extracts of tuliptree foliage are similarly toxic to P. canadensis larvae, but are tolerated by P. glaucus (Lindroth et al. 1986). The toxic components in tuliptree have not yet been characterized, but clearly differ from those present in aspen.

Mimicry

As noted by Rothschild and Jordan (1906), mimetic (black or dark brown) females occur as a polymorphism only in glaucus; canadensis females are always yellow. The expression of this polymorphism in P. glaucus is determined by genes on both the X and Y chromosome (Hagen & Scriber 1989). The phenotypic polymorphism in P. glaucus results from a genetic polymorphism at the Y-linked locus (Clarke & Sheppard 1962). Thus, in P. glaucus, yellow females produce yellow daughters and black females produce black daughters. (Both produce yellow sons, since males do not carry the Y chromosome.)

However, an X-linked gene is also required for expression of the mimetic form (Hagen & Scriber 1989). A female who inherits the Y-linked allele for black color from her mother but also inherits the X-linked "suppressor" allele from her father will be yellow. The X-linked suppressor allele appears to have no effect on the yellow phenotype of males or of females that inherit the Y-linked yellow allele.

Genetic analysis of P. glaucus and P. canadensis hybrids suggests that P. canadensis populations lack the Y-linked allele for black color

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and may be fixed for the X-linked suppressor allele (J. M. Scriber, R. H. Hagen & R. C. Lederhouse unpubl.). Conversely, P. glaucus populations appear to lack the X-linked suppressor allele.

Biochemical and Molecular Characters

Papilio glaucus and P. canadensis show fixed genetic differences at three enzyme loci detected by allozyme electrophoresis: Hexokinase (Hk), Lactate dehydrogenase (Ldh), and 6-Phosphogluconate dehydrogenase (Pgd) (Table 1: Hagen & Scriber 1989, Hagen 1990, Hagen & Scriber 1991). The distribution of allele frequencies at these loci in samples of butterflies collected from Michigan are given in Table 2 and shown in Fig. 2 (discussed below). Differences between the species may represent either chance fixation of neutral variants or the results of natural selection favoring different enzymes in each species.

Analysis of 26 allozyme loci, including the three noted above, yielded an estimate of genetic identity (Nei 1972) also consistent with species-level differentiation between glaucus and canadensis (Hagen & Scriber 1991). The estimate, 0.86, was lower than that between P. eurymedon and P. rutulus (0.91), and the same as that between P. glaucus and P. alexiares. A similar range of genetic identities was reported by Sperling (1987) for North American species of the Papilio machaon species group. The allozyme data gave no support for the proposal (Scott 1986) that P. rutulus is conspecific with P. canadensis or P. glaucus.

The mitochondrial DNA's of P. glaucus and P. canadensis also differ (F. Sperling & R. Hagen unpubl.).

Discussion Phylogenetic Evidence

A phylogenetic analysis of tiger swallowtails using allozyme loci gave strongest support for a monophyletic lineage consisting of P. glaucus and P. alexiares (Hagen & Scriber 1991). The characters most clearly supporting this lineage were Hk and Ldh. In both cases, P. glaucus and P. alexiares share a form of the enzyme that differs from that present in all other P. glaucus group taxa.

Papilio glaucus and P. alexiares also uniquely share the mimetic female form (Beutelspacher-Baigts & Howe 1984). Analysis of interspecific hybrids between P. glaucus and P. alexiares indicates that genetic control of female color is similar in the two species (Scriber et al. 1988[89]). It appears likely that presence of mimetic females is a derived character within the P. glaucus species group (Scriber et al. 1990).

The "Haldane Effect" is a commonly observed consequence of in-

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Table 2. Allele frequencies for Ldh, Pgd, and Hk allozyme loci from Michigan samples, separated by county. Counties are listed from north (P. canadensis range) to south (P. glaucus range), as shown in Fig. 2. N = number of males scored for each locus. Alleles at each locus are named according to enzyme mobility relative to the most common allele in P. glaucus (named the "100" allele: Hagen & Scriber 1989, Hagen & Scriber 1991). The canadensis alleles for Ldh are "80" and "40"; for Pgd, "125," "80," and "150"; and for Hk, "110." Hk was not scored from some samples ("—"). (Electrophoretic methods discussed in Hagen & Scriber 1989 and Hagen & Scriber 1991.)

LDH                                                                    PGD

terspecific hybridization (Haldane 1922, Coyne & Orr 1989). In the P. glaucus group, the Haldane Effect has been observed when P. glaucus females are hand-paired to males of P. eurymedon, P. rutulus, and P. multicaudatus, taking the form of reduced viability of female hybrid offspring (Scriber et al. 1990). The effect was not observed among hybrid offspring of P. glaucus females and P. alexiares males (Scriber et al. 1988[89]).

A significant Haldane Effect does occur among hybrid offspring of P. glaucus females and P. canadensis males. A total of 568 male and 370 female offspring eclosed successfully from 39 hybrid families reared in our laboratory from 1983-88 (39.4% female: Hagen & Scriber in press). There was a significant bias against female hybrids in these families: in 25 families more males than females eclosed, in three an equal number eclosed, and in only 11 did fewer males than females eclose (P = 0.026; Wilcoxon test). Backcrosses of hybrid males to P.

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Table 2. Extended.

PGD                                                                              HK

glaucus females also resulted in significantly lower eclosion success of female progeny (Hagen & Scriber 1989, in press).

Hybrid offspring from the reciprocal cross (P. canadensis female with P. glaucus male) did not show the Haldane Effect: from 35 families reared over the same interval, a total of 284 male and 300 female offspring eclosed (51.4% female: Hagen & Scriber in press). In 14 families more males eclosed, in seven families an equal number of males and females eclosed, and in 14 families fewer males eclosed (P = 0.829; Wilcoxon test). The same absence of Haldane Effect was observed in reciprocal crosses when P. glaucus males were paired with P. euryme-don or P. rutulus females (Scriber et al. 1990).

These observations raise a new question about the status of P. alexiares in relation to P. glaucus (Hagen & Scriber 1991). For the present, it seems preferable to retain species designation for alexiares, given the lack of information on interactions between glaucus and alexiares in

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16.®^. 20.QO

0OCV.

00Oi8.

©^21.

Ldh Pgd Hk

Fig. 2. Proportion of all canadensis alleles for Ldh, Pgd, and Hk loci in samples of butterflies from Michigan counties. The number next to each set of circles corresponds to a county listed in Table 2. Hk was not scored for samples from all counties; only 2 circles are shown in those cases. Filled circle = 100% canadensis; open circle = 0% canadensis (=100% glaucus); intermediate frequencies of canadensis alleles are indicated by the proportion of each circle filled. Sample sizes are given in Table 2, along with frequencies for the separate canadensis and glaucus alleles. The shaded region indicates the position of 1400-1500°C seasonal isotherm in Michigan, the predicted northern limit for a multivoltine life cycle.

northeastern Mexico, where they are reported to be sympatric (Beu-telspacher-Baigts & Howe 1984).

The Hybrid Zone Between glaucus and canadensis

Evidence of natural hybridization between glaucus and canadensis has come from studies of morphology (Rothschild & Jordan 1906, Luebke et al. 1988), host plant use (Scriber 1986,1988, Hagen 1990); inheritance of mimetic female color (Scriber et al. 1987, J. M. Scriber, R. H. Hagen & R. C. Lederhouse unpubl.) and allozyme electrophoresis (Hagen 1990). However, hybridization is confined to a relatively narrow zone between 41 and 44°N latitude, extending from New England through the Great Lakes region. The position of the hybrid zone appears to

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track closely the seasonal degree-day isotherm corresponding to the predicted northern limit for a multivoltine lifecycle in P. glaucus (Scrib-er 1982, 1983, 1988, Hagen 1990).

The narrowness of the hybrid zone is demonstrated best by allozyme characters, since their frequencies can be estimated more easily than frequencies of physiological or developmental characters. Allele frequencies for Hk, Ldh, and Pgd from samples of males collected in Michigan from 1986-89 are given in Table 2, grouped by county. The county locations are shown in Fig. 2.

The sharp transition in frequencies for all three loci in Michigan coincides with the predicted northern limits to a multivoltine life cycle. Laboratory experiments have shown that successful completion of two generations by P. glaucus or P. canadensis requires a minimum of 1400 to 1500°C "degree-days" above a threshold of 10°C, on good larval hosts (Hagen & Lederhouse 1985). The shaded band in Fig. 2 indicates the position of this 1400-1500°C band based on a 20 year (1950-70) average of climatic data (J. M. Scriber unpublished data).

Conclusions

Because there is no geographic barrier to prevent dispersal of P. glaucus and P. canadensis across the hybrid zone, maintenance of the sharp boundary between them must be attributed to more subtle factors that prevent extensive interbreeding or the establishment of sympatric populations. One possible factor may be inability of P. glaucus to adapt to the univoltine lifecycle that is essential for survival north of the hybrid zone (Hagen & Lederhouse 1985, Rockey et al. 1987a). This could prevent northward extension of P. glaucus' range, but would not prevent P. canadensis from extending its range further south.

Another possibility is disruption of development in the offspring of interspecies matings: for example, that which is responsible for the Haldane Effect observed in laboratory-generated hybrids. However, this too may be only a partial barrier, since we have evidence of developmental disruption affecting female offspring in only one direction of hybridization. Additional possibilities include differential mate preference by males or females of the two species, or differing tolerances for extremes of high—or low—temperature encountered on either side of the hybrid zone.

It is probable that a combination of factors will turn out to be responsible for maintaining the species7 identities. The range boundary between P. glaucus and P. canadensis coincides with a complex ecotone between boreal coniferous forest and temperate deciduous forest (Braun 1974, Curtis 1959). This ecotone coincides with a climatic transition between northern areas dominated by relatively cool, dry arctic air

JwUrvI\A.J_j KJr 1 JnLHi J—iJtLiJrlL/wx 1 Hirvlo 1 o

masses and southern areas dominated by warmer, wetter tropical air masses (Bryson & Hare 1974). It also coincides generally with the southern margins of Pleistocene continental glaciation and thus represents a transition in topography and history of occupancy by animals and plants (Braun 1974).

The range limits of a variety of animal taxa coincide with this ecotone (Remington 1968, Scriber & Hainze 1987). Among the best-known examples from Lepidoptera are subspecies of the admiral butterfly, Limenitis arthemis arthemis Drury and L. a. astyanax Fabricius (Nym-phalidae) (Piatt & Brower 1968, Waldbauer et al. 1988). We strongly suspect that similar patterns of geographic differentiation occur frequently among eastern North American Lepidoptera. Tiger swallowtails may be atypical only in that they have received the intensive study necessary to detect it.

Acknowledgments

We are grateful to Matt Ayres, Wade Hazel, James Nitao, Felix Sperling, and Fred Stehr for valuable discussion and criticism at various stages in preparation of this manuscript. For collecting assistance in Michigan, we also thank Jim Bess, Dave Biddinger, Charley Chilcote, Mark Evans, Bruce Giebink, Ted Herig, Mo Nielsen, Virginia Scott, and Mark Stehr. Doozie Snider cheerfully assisted with allozyme electrophoresis in the laboratory. The research contributing to this manuscript was funded by the Michigan State University Agricultural Experiment Station (Projects 8051, 8072, and 1644) and Graduate School (AURIG), NSF grants (BSR 8306060, BSR 87-18448, BSR 87-02332, and BSR 88-01184), and USDA-CRGO (87-CRCR-1-2851).

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Received for publication 26 May 1990; revised and accepted 24 July 1991.