Hybridization of checkerspot butterflies in the Great Basin

17 Pages·2003·10.5 MB·English

by  George T Austin

JournaloftheLcpidopterists'Society
57(3),2003,176-192
HYBRIDIZATION OF CHECKERSPOT BUTTERFLIES IN THE GREAT BASIN
George T. Austin
NevadaState MuseumandHistoricalSociety,700TwinLakesDrive, LasVegas,Nevada89107,USA
Dennis D. Murphy
DepartmentofBiology/314,UniversityofNevada,Reno, Nevada89557,USA
John F. Baughman, Alan E. Launerand EricaFleishman
CenterforConservationBiology, StanfordUniversity, Stanford,California94305,USA
ABSTRACT. TwoputativespeciesofEuphydryasbutterflies,E. aniciaandE. colon,maybehybridizinginthenorth-centralGreatBasin
afteranextendedperiodofgeographicseparation. Surveyswereconductedthroughoutnorthern Nevadatoestimatethedistributionofeach
speciesandofapparenthybrids. Moredetailedmark-recapturestudiesweremadeatonesiteinthePequopMountainsinordertoexamineeco-
logicalinteractionsbetweenthespecies.Thetwoarelargelyallopatricandreadilyseparatedbywingcolorandgenitalmorphology.Althoughin-
terbreedingwasapparentfromtheoccurrenceofintermediatephenotypesandknownmatingattempts,thetaxaarelargelytemporallysegre-
gatedandpreferdifferentlarvalhostplants. Thereisalsoasuggestionofanunbalancedsexratioofphenotypicallyintermediateindividuals.
TheseEuphydryas,althoughcloselyrelatedandnotstrictbiologicalspecies,areundoubtedhistoricalentitiesandseemtobebesttreatedas
phylogeneticspecies.
Additionalkeywords: distribution,Euphydryas,genitalia,hostplants,hybridization, Nevada,Nymphalidae,phenology,sexratio.
Manytaxaexistas intergradingpopulationsinwhich to the extent of interbreeding in these zones (e.g.,
individuals from neighboringpopulations are morpho- Blair 1950, Ficken &Ficken 1968, Collins 1984, John-
logically and ecologically similar, whereas individuals son&Johnson 1985, Nichols &Hewitt 1988, Malletet
from distantpopulations are quite distinct. Insomein- al. 1998, Benedict 1999). Ecological anddemographic
stances, however, phylogenetically distinct portions of data from hybrid zones may, however, provide infor-
a series ofintergrading populations, or oftwo closely mation crucial to reconstruction of paleoecological
related species, maybe sympatric. Such cases of"ring events, identification of biogeographic patterns, or
species" have been considered manifestations of al- predictionoffuturechanges incloselyrelatedlineages
lopatric speciation providingevidence ofthe differen- (e.g., Hafner 1992, Scriber & Gage 1995, Benedict
tiation ofpopulations along environmental gradients 1999). These data are particularly important since
(Irwin et al. 2001, Irwin & Irwin 2002). Interactions manyinteractingtaxa do not readilyfit into traditional
between contiguous orsympatricpopulations not only taxonomic schemes (e.g., Cracraft 1989, Templeton
augmentthe understandingofspeciation phenomena, 1989, Sperling 1990).
but also potentiallyprovide important information on This work focuses on butterflies ofthe Euphydryas
several aspects of population biology (e.g., Endler chalcedona (Doubleday) complex (Lepidoptera:
1977, Harrison 1993, Bull 1991, Futuyma & Shapiro Nymphalidae) in the north-central Great Basin, an
1995, Jiggins et al. 1996). areawhere two morphologically distinct forms ofthe
Hybrid zones have long intrigued biologists and an group appearto hybridize. The objectiveswere to de-
abundantliteraturespeculatingonthegenetic, ecolog- terminetheirgeographicaloverlapinnorthern Nevada
ical, and evolutionary significance ofinteractions be- and the present extent and nature ofinteraction. To
tween closelyrelated taxain such areas has developed addressthese, populations ofEuphydryaswereplaced
(e.g., Sibley 1961, Mayr 1963, Moore 1977, Grant & into a broad biogeographic context byconducting re-
Grant 1992, Harrison 1993) including one for Lepi- gional surveys ofwing colorpatterns and male genital
doptera (e.g., Remington 1968, Oliver 1979, Porter morphology. To examine ecological interactions be-
1997, Sperling 1990, Scriber et al. 1995, Porter et al. tween forms in greater detail, concentrated investiga-
1995, 1997,Jiggins etal. 1996). Morefieldresearchon tions were conducted on a site in the Pequop Moun-
the interactions between taxa in zones of overlap is tains (ElkoCounty, Nevada) atwhichboth formswere
needed; most investigations infer ecological interac- present. At this site, datawere collected on wingphe-
tionsbasedonmorphologicalandgeneticdata(butsee notypes, genital morphology, andspatial andtemporal
Otte & Endler 1989, Lindroth et al. 1988a, b, Porter distributionsoftheforms. Naturallyoccurringmatings
1997, Malletetal. 1998). Littleattentionhasbeenpaid were quantified and oviposition hostplantpreferences
Volume 57, Number 3
were tested as an indicatorofrecent evolutionaryhis-
tory and potential overlap in hostplant utilization be- 1514tr25"
tween the forms. In addition, the apparent sex ratioof
the Pequop population was compared with sex ratios *»A6 .3 Vis
of allopatric populations to determine ifthis may be .2
unbalancedinthe Pequop Mountains (Haldane 1922). 3^-~v -•Ooo1•o• °
These dataweresupplementedwithinformation from
othersiteswhere more than one form occurs.
°o
Study System
<Po
ThetaxaoftheEuphydryaschalcedona complexare
distributed across much of western North America, 016 ,7Au£stin%
from Alaska to Mexico and east to the Great Plains °°8 8 ° o 8
(Scott 1986). The group consists of three nominally „ IS 6b o
no^Hawthorne o
distinct species, E. chalcedona, Euphydryas colon (W.
H. Edwards), and Euphydryas anicia (Doubleday &
Hewitson) (Miller& Brown 1981). Thesespecieswere
defined primarily by the shape of the male genitalia Fig. 1. Map ofnorthern Nevada showing distributions ofEu-
and by wing shape and coloration (Gunder 1929, phydryas anicia (open circles), E. colon (closed triangles), and E.
Bauer in Ehrlich & Ehrlich 1961). Within each ofthe chalcedona (opentriangles); closedcircles indicate siteswith more
three putative species, there is considerable between- tishtahneoHnuemfboorml.dtIrRrievgeurl.arNulimnbeeprasssirnegfetrhtrooulogchatEiloknosacnoddeWdiinnnTeambulecsea1
population phenotypic variation in both wing color and3.
(Austin & Murphy 1998b) and male genital morphol-
'
ogy(Scott 1978). ondary from primary contact and/or intergradation
Largely in response to this phenotypic variation, (Mayr 1942, Endler 1977, but see Hammond 1990).
nearly 80 names have been proposed for the various Based, however, ontheknownpaleoelimateandpaleo-
forms within the E. chalcedona group, manyofwhich vegetation of western North America (Mifflin &
represent aberrations. At present, 11 subspecies are Wheat 1979, Wells 1983, van Devender et al. 1987,
recognized forE. chalcedona, five forE. colon, and22 Benson & Thompson 1987, Grayson 1993), allozyme
forE. anicia (Miller & Brown 1981). Although Ferris data (Brussard et al. 1989), andpresent distributions,
(1989) synonymizedE. colon withE. chalcedona, there itappears thatE. chalcedona andE. colonwere south-
is no consensus on their taxonomic status. Allozyme ernandnorthern Pacificisolates, respectively, andthat
studies,however, suggestedthatvariationinwingcolor E. anicia was isolated somewhere between the Bock)7
andpatternwas not accompaniedbygenetic differen- MountainsandSierra Nevada. Itisprobablethatpres-
tiationtojustifyspecies-levelcharacterization, andthe entdistributions reflectpost-Pleistocene dispersaland
three groups were tentatively lumped into one mor- rejunction andnotthe resultofasingle rampantlvdif-
phologically diverse species, E. chalcedona (Brussard ferentiatinglineage.
etal. 1985, 1989, Scott 1986).
Methods
The "messy" systematic situationis compoundedby
the predominantlyallopatric geographic distributions Dataonthe distributionofbutterflies ofdieE. chal-
of the forms. Although distribution maps suggest cedona group in northern Nevada were collected as
broad sympatry in some areas ofthe western United part of ongoing field studies throughout the region.
States (Stanford & Opler 1993), the seemingly geo- These surveys, initiated in the mid-1970s, were ex-
graphically sympatric taxa are usually ecologically al- panded specifically to investigate the E. chalcedona
lopatric, mayhave somewhat different flight seasons, complex duringthe late 1980s and early 1990s.
andoftenhavedifferentlarvalhostplants. At mostspe- In order to further clarify the distribution of the
cific locations, therefore, only a single member ofthe three E. chalcedona group species, die genitalia of
complex is present. There are a few locations, how- more than 800 male butterflies from 32 sites within
ever, at which two of the three named entities co- Nevada (including one reported by Scott 1978) were
occur (Ehrlich & Murphy 1982, Austin & Murphy scored according to Scott (1978). At seven of these
1987, 1998b, Ferris 1988, Brussard et al. 1989). The sites, two forms or intermediates ofthe Euplii/dryas
phylogenetichistoryofeach ofthe forms has notbeen chalcedona complexhave been recorded (Fig. 1). The
fullyclarifiedanditmaybe difficultto distinguishsec- remaining25 sites supportedonlvasingle form.
178 Journalofthe Lepidopterists' Society
Table 1. Number of individuals with each wing phenotype sumedthatan apparentsexratiowithin ataxon should
amongmuseumspecimensoftheEuphydryaschalcedona complex be correlative across populations using comparable
from locations with sympatry or intermediates in Elko County,
Nevada (location numbers refertothose in Fig. 1). R = red form collection techniques (Boyd et al. 1999). Accordingly,
("pure"E. anicia),RI =intermediatephenotype(moresimilartoE. the sex ratios of museum specimens and individuals
anicia),I="true"intermediate,BI =intermediatephenotype(more captured as part ofthe MRR studiesweretabulated.
similartoE.colon),B =blackform("pure"E. colon). The MRRstudyin 1989 and 1990 alsoproducedin-
Wingphenotype formationon relative spatialandtemporaldistribution
Location Sex B BI I RI R ofred and black forms. In 1989, the site was divided
into 36 areas ofsimilar size. Thirty sites straddled the
1.PequopMountains male 35 3 3 5 17 road and the remaining six were located adjacent to
female 8 1 1 3
2.WindemereHills male 42 1 1 the road in the broadest area ofthe canyon. In 1990,
female 11 1 onlythe 10subsites thathadthehighestbutterflyden-
3. SnakeMountains male 11 1 1 1 sities in 1989were again sampled. Forthepurposes of
4. IndependenceMts.
(MaggieSummit) male 16 1 2 this spatial and temporal delineation, R and RI indi-
female 1 1 viduals were considered "red", B and BI were consid-
5. IndependenceMts. ered "black", and the few true intermediates were
(JackCreek) male 14 2 2
6. OwyheeRiverValley male 32 2 omitted.
female 2 Probable differences in emergence timingbetween
thetwoforms areindicatedbydifferences in meanage
At a site in the Pequop Mountains, south of 1-80 ofindividuals. At each handling, all individuals were
abouttwo miles eastofPequop Summitatamean ele- scoredforwingwear, acommon estimatorofbutterfly
vation ofapproximately 2200 m (41'05"N 114'33"W; age (Orive & Baughman 1989). Butterflies were
seeTable 1, Fig. 1), comparativelydetailed determina- scoredin0.5intervals from 0.5 (newlyemerged) to3.5
tions of wing color were made during mark-release- (worn, indicatingextendedflight).
recapture studies (MRR, techniques according to To testwhetherindividuals ofdifferent color forms
Ehrlich & Davidson 1961) overtwoseasons. The MRR at least attempt to interbreed, the genitalia of each
MRR
studieswereconductedalongadirtroadinthebottom male captured duringthe studies was dipped in
of a predominantly east-west oriented canyon. This a powdered fluorescent dye upon initial capture and
canyon is surrounded by mixed riparian and canyon oneach subsequentrecapture. Someofthis dyeis typ-
wash habitats with pifion {Finns monophylla) andju- icallytransferredtoafemale duringasubsequentmat-
niper(Junipernsosteospenna) onthesurroundinghill- ing, andmatedfemaleswereexaminedunderultravio-
sides. From 20 Mayto 12 June 1989 and from 1 to 12 let light for evidence ofdye (Wheye & Ehrlich 1985,
June 1990, 387 and 194 individual butterflies, respec- Fleishman etal. 1993). Use ofthis technique toinves-
tively were marked along this road. Upon initial cap- tigate matings between members ofdifferent experi-
ture, each butterfly was given a unique number and mentalclassesassumesthatdyedandundyedmales are
placed into one of five distinct categories based on equallylikelyto achieve copulations, dyes ofdifferent
wing coloration: red (R), red-intermediate (RI), inter- colorsareequallylikelytobetransferredduringmating
mediate (I), black-intermediate (BI), and black (B); and retained byfemales after mating, anddye transfer
voucherspecimensfromhere andothersites areatthe occurs atconsistent frequencies forsuccessful and un-
Nevada State Museum and Historical Society, Las Ve- successful matings. The protocol does not assume that
gas. Categories Rand B areheretermedas "pure", RI, all matings are equally viable or are equivalent in an
I, and BI as "intermediate", and I alone as "true inter- evolutionarysense. All Rand RI maleswere dippedin
mediate." All butterflieswere released in the centerof pinkdye, while B and BI maleswere dyedgreen.
the areainwhich theyhad been captured. Similar de- Some mated females were retained during the
terminations of phenotype were made on museum MRR studyto determine ovipositionpreference using
specimens collectedfrom several sites in northeastern techniques developed by Singer and co-workers (e.g.,
Nevada. Singer 1986). Each female was sequentially offered
Within field samples ofbutterflies, and accounting each of three locally available potential oviposition
fordevelopmental differences, males usuallyoutnum- hostplant species, Castilleja angustifolia (Nutt.) G.
ber females, often bya broad margin (e.g., Ehrlich et Don, Penstemon speciosus Dougl. ex Lindl. (both
al. 1984) duelargelytobehavioral differencesbetween Scrophulariaceae), and Symphoricarpos oreophihis
the sexes (e.g., Gall 1985). In general, with large sam- Gray (Caprifoliaceae) at five-minute intervals oratin-
ples taken throughout the flight season, it maybe as- tervals permitted by weather conditions. A plant was
Volume 57, Number 3 179
recordedas acceptedifthe females abdomenwas fully Table2. Wingphenotype ofEuphydryaschalcedona complex
curled and the ovipositor extruded for at least three individuals markedduringthe mark-reeapture-releasestudyinthe
seconds. Actual oviposition was not permitted. A fe- Pequop Mountains(wingphenotypeasinTable1).
malewas consideredtohavepreferredplant speciesA 1989 1990
overplantspecies B ifarejectionofBwas recordedaf-
Category Males Females Total Males Females Total
teran acceptance ofA (Singer 1982). Iffemales were
captured before 11:00, their preference-testing com- R 61 27 88 29 9 38
RI 48 3 51 23 4 27
menced on the day ofcapture. Iftheywere captured I 2 2 8 8
after 11:00, testing commenced on the following day. BI 64 5 69 35 3 38
In everycase, plant species thatwere acceptedon the B 165 12 177 75 8 83
Total 340 47 387 170 24 194
first day of testing were recorded. In addition, ex-
amples ofthese plants were searched on two dates at
the Pequop Mountains study area to determine the Valley, Jack Creek Campground and west of Maggie
presence ofeggmasses. Summit in the Independence Mountains) (Fig. 1).
Statisticalsignificance(consideredatp < 0.05through- The site in the Pequop Mountains supported large
out)was determinedusingchi-squarecomparisons. numbers of both red and black phenotypes during
1989 and 1990. Not surprisingly, the few locations of
BESULTS
sympatrybetween redandblackforms aretopograph-
Distribution of Euphydryas in the northern ically complex; these sites are canyons where sharply
Great Basin. Surveys in northern Nevada and adja- definedwarm andcoolslopeexposures supportingdis-
cent areas revealed a fairlyclear distribution ofpopu- tinctiveplantcommunities arejust meters apart.
lations of the E. chalcedona group (Fig. 1). In the In northwestern Nevada (and also adjacent north-
southern portion ofthe studyarea, the phenotypically eastern California and southern Oregon), populations
red Euphydryas anicia wheeleri (Hy. Edwards) is of E. colon are all far north of the Humboldt Biver
widespread and often common. Across much of (Fig. 1). These largelyblackEuphydryas colon walla-
Nevada, this subspecies is associated principallywith censis Gunder are sympatric in some locales on the
Castillejaangustifolia,butalsowithCastilleja linariae- Sheldon National Wildlife Befuge (Humboldt
folia Gray, Pedicularis centranthera Gray, and Penste- County), butapparentlydonothybridize,withthealso
mon speciosus (all Scrophulariaceae) (Murphy & largely black Euphydryas anicia veazieae Fender &
Ehrlich 1983, GTA unpublished data). The presently Jewitt. The twophenotypes have partiallyoverlapping
known northern distributional limit of E. anicia flight seasons, but the details oftheir microsympatry
wheeleri in Nevada occurs in the Toana Bange, require definition. Inthisregion, E. colon is associated
Windemere Hills, Snake Mountains, and Independ- with Symphoricarpos while E. anicia apparentlyuses
ence Mountains (all Elko County) westward generally both Penstemon and Castilleja (Bauer in Howe 1975,
south andeast ofthe Humboldt Biver (Fig. 1). Austin & Murphy 1998b). Just south and east ofthis
In the northeastern portion of the study area, the area E. anicia veazieae intergrades broadly with the
phenotypically black Euphydryas colon nevadensis redderEuphydryas anicia macyi Fender &Jewett.
Bauer predominates and is relatively widespread Outside these two regions, onlyone form ofthe E.
across sagebrush-dominated (Artemisia) slopes and chalcedona complexis presentat anygivenlocation in
alongripariancorridors (Fig. 1). Its knownlarvalhost- the Great Basin of Nevada, although in some areas
plants are Symphoricarpos oreophilus and possibly their distributions approach (Fig. 1). The affinity of
Penstemon (Bauer in Howe 1975, GTA unpublished each population is unequivocal and individuals are
data). The southern distributional limit of E. colon readily identifiable by superficial characters that are
nevadensis in eastern Nevadais in the Pequop Moun- consilientwithgenital morphology (see below).
tains. Wing phenotypes. As noted, the broad scale sur-
In the narrow geographic band ofoverlap between veys located sites in northeastern and northwestern
E. anicia wheeleri andE. colon nevadensis, three sites Nevada where two E. chalcedona complex taxa were
were foundwhereblackandredforms andintermedi- sympatric: the Pequop Mountains, the Windemere
ates flytogether (northern end ofthe Pequop Moun- Hills, the Snake Mountains, andthe Sheldon National
tains, Windemere Hills in the Thurston Spring area, Wildlife Befuge. The black form and intermediates
Tabor Creek in the Snake Mountains) and an addi- were found in the Owyhee Biver Valley and at two
tional three sites with one form and intermediates sitesinthe Independence Mountains (Table 3, Fig. 1).
(Wildhorse Crossing Campground in Owyhee Biver Although at six ofthese seven sites (except in Hum-
i .
180 Journalofthe Lepidopterists' Society
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Volume57, Number 3 181
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182 Journalofthe Lepidopterists' Society
Table 4. ComparisonofgenitalphenotypesofEuphydnjaschalcedonacomplextaxainNevada(onetaxonpresentinallopatricpopulations,
twotaxapresentinsympatricpopulations).
Genitalphenotype
Taxon C D Mean1 Chi-;quare-
E. aniciawheeleri — —
allopatric — 42 120 5.73 10.30
sympatric 12 7 5.36
E. aniciaveazieae/macyi — — —
allopatric 49 71 30 4.87 15.62
sympatric 18 24 5.32
E. colonnevadensis
allopatric 1 26 9 2.27 5.58
sympatric 112 34 2.24
E. colon wallacensis
allopatric 7 44 7 2.00 2.60
sympatric 2 8 4 2.14
E. chalcedonakingstonensis —
so. Nevada 19 5 2.20
E. chalcedonamacglashanii
westernNevada 4 75 17 2.14
E. aniciahennosa — — —
so. Nevada 19 5.79
E. aniciamorandi
— — —
so. Nevada 21
1 Derivedbysubstituting1-6forgenitaltypesA-F,respectively(seetext).
2 Significantvaluesinbold.
boldt County) intermediate phenotypes were present, thetotalmales,Table2). Individualsofboththe RI and
nopopulationswerecomposedprimarilyofintermedi- BI classes, however, were sufficiently divergent from
ate butterflies. the pure phenotypes that theywould have been con-
Museum specimens from the six sites in Elko sidered outliers in single-form populations elsewhere
County with two forms and intermediates (n = 222), in Nevada. Thephenotypicdistributionoffemaleswas
redandintermediateindividualswerethesmallerpro- significantlydifferentfrom thatofmales (chi-square =
portion ofthe populations, ranging from 5 to 43% of 40.09; df= 4), due largelyto proportionally fewerfe-
field caught samples (Table 1). No true intermediate male intermediates (21%) than males (35%). This dif-
females were found. The variation of E. colon ference may have been even greater ifsampling con-
nevadensis noted by Bauer (in Howe 1975) probably tinued through the end ofthe flight season. No true
refers largely to intermediate phenotypes resulting intermediate females were foundin eitheryear.
from hybridization (e.g., see commentby Scott 1978). Genital morphology. The distribution of male
In the Pequop Mountains, most butterflies (68% in genitaltypes followedgenerallyacceptedspecies clas-
1989, 62% in 1990) were either red orblack (Table 2) sifications and biogeography as did wing phenotypes
with essentiallythe samewingcolorpatterns foundon (Tables 3, 4). As previously indicated (Gunder 1929,
butterflies found at single-form sites. The remaining Scott 1978), E. chalcedona and E. colon had genital
individuals had intermediate wing color (RI, I, BI). morphologyoftypes A, B, and C, whereas those ofE.
The number ofblack individuals probablywas under- anicia were largelytypes D, E, and F. Some intrasub-
estimatedbecause surveyswereterminatedbeforethe specific variation was noted. Most notably, E. anicia
end ofthe flight season in bothyears. Therewere sig- veazieae and possiblyE. anicia macyi hadproportion-
nificant differences in phenotypic distributions for ally more D and E genital configurations than E. ani-
males and for the total sample for both years (chi- ciawheeleri,Euphydryasanicia morandi Gunder, and
square = 10.55 and 10.87, respectively; df= 4), butnot Euphydnjas anicia hennosa (W. G. Wright), all of
forfemales (chi-square = 3.79; df= 3). Most interme- which had mostlytype F.
diates were either RI or BI; only a few, two in 1989 There were, however, some differences in the fre-
(out of387butterflies) and eight in 1990 (out of194), quency of genitalia types between areas supporting
were classified as true intermediates (overall <2% of one versus two members ofthe species group (Table
Volume 57, Number3 183
4). In northeastern Nevada, E. anicia from locations Table5. Numbers ofmales andfemales in samples oftheEu-
also inhabited byE. colon had significantly more type phydryaschalcedonacomplexfromtheGreatBasin.
E and fewer type F genitalia than non-sympatric E, Taxonorphenotype Male Females % Females
anicia wheeleri. On the Sheldon National Wildlife
Refuge,whereE. aniciaveazieaeflieswithE. colon,die Pequop Mountains(mark-release-recapturestudy)
former had significantly fewer type D and E genitalia Pure 330 56 14.5
Intermediate 180 15 7.7
than in areaswidioutE. colon. Therewas no overlap in Total 510 71 12.2
diegenital morphologyofbutterflies scoredaspure for Hybridzone(museumspecimens)
anyareawheretwotaxaweresympatric (Table3). Pure 169 26 13.3
Butterflies with intermediate wing color showed a Intermediate 23 3 11,5
range of genital types (Table 3). BI individuals had Total 192 29 13.1
genital types B, C, and D and RI phenotypes had D, OtherGreatBasin(museumspecimens)
E, and F. True intermediates in wing color (I) exhib- EE.. ccohlaolncedona 213830 5747 2234..18
itedgenital types B, C, E, and F. E. anicia 1097 430 28.3
Sex ratio. Apparent male female sex ratios among Total 1510 561 27.1
:
Euphydryas from the Great Basin ranged from 2.6:1
forE. anicia tojust over 3:1 forE. chalcedona and E.
colon (Table5). Females representedjustover 12% of Distribution of females may reflect habitat prefer-
the MRR sample from the Pequop Mountains and ences foroviposition, but this would be nearlyimpos-
13% ofmuseum specimens from the siteswith known sible to determine in an areawith aninterdigitation or
hybridization (Table 5), these not statistically distin- close proximity ofcontrasting vegetative associations
guishable (chi-square = 0.14; df= 1). The MRR sam- as in the Pequops. Similar habitat preferences have
ple, however, may be biased against females since, as been notedat othersiteswhere both forms occur.
noted above, the studywas terminatedbefore the end The temporal distribution ofred and black individ-
ofthe flightseason. Both the sample from the Pequop uals,however,was different. In 1989, 77% (n = 139) of
Mountains and the museum sample from the hybrid redindividualswerefirstcapturedonorbefore 1 June,
zone have a significantly different sex ratio than sam- but only 11% (n = 246) ofblack individuals were ini-
ples from the remainder of the Great Basin (chi- tiallyhandled before that date. In 1990, the temporal
square = 55.25, 20.12, respectively; df = 1). Further, subdivision was somewhat less pronounced, but still
within the Pequop sample itself, there is a significant evident. In both years, the ratio ofred to black indi-
difference between the sex ratio ofindividuals scored viduals decreased steadily throughout the study pe-
as pure (R and B) and all intermediates (chi-square = riod. Although both forms were present for most of
5.61; df= 1,butnotepotentialsamplingproblem),but each study period, substantial numbers of red and
notwithin the museum sample (chi-square = 0.03; df black individuals flew synchronously for less than a
= 1). Even the sex ratio ofindividuals scored as pure week duringeachyear.
was significantlydifferent from other Great BasinEu- In both years, red individuals were consistently
phydryas (chi-square = 27.38 in the Pequops, 17.57 morewornthanblackbutterfliescapturedonthe same
formuseum sample, df= 1). day. Based on these data and on correlation ofwing
Spatial and temporal distributions. The spatial weartoage, itisestimatedthatemergenceofredindi-
distributionsofredandblackbutterfliesinthe Pequop viduals peaked two to three weeks before the peak
Mountains did not appearto be distinct. In both 1989 emergence ofblack individuals. This is reinforced by
and 1990, individuals ofbothformswerefoundinsim- the phenology seen generally overthe broad expanse
ilar proportions in all subareas occupied. Virtually all ofthe northern Great Basinin Nevada. Although these
ofthe E. chalcedona group butterflies were encoun- datawere obtained over several decades andwide lat-
tered along the lower half of the study area, being itudinal and elevational ranges that tend to blur indi-
found throughout the wash (including areas along the vidualsiteandyearpatterns, dieyindicateapeakflight
dirt road and in areas of riparian vegetation). Given ofE. colon occurringdiree weeks afterthat ofE. ani-
the topography ofthe site and the logistical limits im- cia (Fig. 2).
posedonthe MRRefforts, itisdoubtfulthatthis study During both 1989 and 1990, red individuals were
couldhave detected differences in distribution ofthe much less numerous than blackindividuals. While the
twoforms thatwereless thanseveralhundreds ofme- data do notpermit exact population size estimates, in
ters. The males of both forms perch in and patrol both years the number ofblack individuals appeared
alongwashes searching for females andwould not be to be at least four times greater than the number of
expected to exhibit perceptible habitat segregation. redindividuals.
184 Journalofthe Lepidopterists' Society
Mating within and between forms. The results 150
of dye transfer experiments indicated that red and
black individuals at least attempted to interbreed. In
1989, three often recorded matings (virgin females at
time of first capture determined after Labine 1964,
mated at time of recapture) appeared to have been
red-black matings; no dye transferwas noted in 1990.
Because of the small sample size, no estimate was
made ofthe relative frequencyofmatingbetween in-
dividuals ofdifferent forms. The data indicate, how-
ever, that such matings (orattempts) do occur. Apr May Jun Jul Aug
Hostplant utilization. First-day hostplant accep-
tance datafor25 redand27blackfemales aresumma-
rized in Table 6. Red and black females showed no-
tably different hostplant acceptance patterns. All red
females acceptedCastilleja as anovipositionhostplant.
Twenty-two (88%) ofthese red females also accepted
Penstemon as an ovipositionhostplant. Noredfemales
accepted Symphoricarpos.
In contrast, blackfemales exhibitedawiderbreadth
ofoviposition hostplant acceptance. Twenty-one of27
(78%) black females accepted Castilleja during the
first day ofoviposition trials, 10 (37%) accepted Pen-
stemon, and 24 (89%) accepted Symphoricarpos.
There were statistically significant associations be-
Months
tween phenotype and acceptance ofSymphoricarpos
and Penstemon, but not in acceptance of Castilleja. Fig.2. PhenologyoftheEiiphydryasanicia{wheeleri,veazieae,
macyi; above) and E. colon (nevadensis, wallacensis; below) in
Red females accepted Penstemon significantly more northern Nevada. Records are dividedinto ten dayintervals from
often and Symphoricai~pos significantlyless often than AprilthroughAugustinclusively.
didblack females (Table 6).
a decline ofboth taxain zones ofsympatry, and/oreco-
Discussion
logicalandphenotypiccharacterdisplacements.
Post-Pleistocene climatic vicissitudes in the Great Euphydryas hybridization. Although there is
Basin had a profound bearing on distributions of considerable allopatry amongthe various phenotypes
plants and animals (e.g., Wells 1983, van Devender et of the E. chalcedona complex in Nevada and else-
al. 1987, Harris 1990, Grayson 1993, Elias 1994, but where, large areas exist where they are potentially
see Riddle 1995). While extirpations undoubtedlyoc- sympatric at the gross landscape level. Few instances,
curred (e.g., Grayson 1987), distributional shifts were however, of actual sympatry have been identified
perhaps a more widespread response to climatic (Dornfeld 1980, Ferris 1988, Austin & Murphy 1987,
change (Reveal 1979, Harris 1990, Thompson 1990, Guppy& Shepard2001). At these locations, the inter-
Grayson 1993, Elias 1994), species adapted to more actionsvaiyfrom no apparentintergradationtoatleast
mesic habitats retreated, those adapted to more xeric some hybridization (see also Scott 1978). Such vari-
environments extended their ranges, and all moved form responses arenotunique amonghybridizingtaxa
farthernorth, higherinelevation, ortocoolerslopeex- (e.g., Short 1965, Rising 1983, Sperling 1987, 1990,
posures (e.g., Bernabo & Webb 1977, Peters & Dar- Collins 1991).
ling 1985). The time scale ofdistributional permuta- The studies ofthe E. chalcedona complex in north-
tions varied among species (e.g., Webb 1986, Huntley eastern Nevadaindicate existence ofanarrowzone of
1991), but movements are thought to have been rela- sympatry between butterflies traditionally considered
tively rapid for insects (Elias 1994, Hewitt 1996). asthe specificentitiesE. anicia andE. colon. Somehy-
When closely related taxa enter into a changing biotic bridizationoccurs, althoughthereis noevidenceofin-
landscape, the potential consequences include not only trogressionoutsidethiszone. Severallines ofevidence
thefullgamutofinterspecificinteractions,butalsoapo- supportthis conclusion: (1) thepresenceofindividuals
tentialforgeneticreorganization,thefusionoflineages. withintermediatewingphenotype atsitesofsympatry,
Volume 57, Number 3 185
(2) a shift in male genital morphology across the hy- Table6. First dayacceptance ofthree potential hostplants by
bridzone, (3) anapparentlyskewedsexratio, (4) direct blackandredphenotypesoftheEuphydrijaschalcedonacomplexin
northeasternNevada.
evidence ofmating between forms, and (5) the exis-
tence ofdifferent hostplant preferences. In addition, Number Number Chi-
although there is an overlap in spatial distribution at Phenotype accepting rejecting square1
sites ofsympatry, thesephenotypes are effectivelydis- Castillejalinariaefolia
joined from one another locally via phenological dif- Black 21 6 4.3
ferences. Red 25
The observed level ofphenotypically intermediate Penstemonspeciosus
individuals inthe Pequop Mountains implies afairde- Black 10 17 12.1
Red 22 3
gree ofhybridization between the two forms. This is
what Short (1969) termed a "zone ofoverlap and hy- Sijmphoricaq^osoreoph lus
Black 24 3 38.7
bridization"inwhichsubstantialnumbers ofpurephe-
Red 25
notypes co-occur with hybrids leading to a bimodal
pattern ofgenotypes and/or phenotypes (e.g., Jiggins 1 Significantchi-squarevaluesinbold.
& Mallet 2000). The data, however, do not indicate
that the two E. chalcedona entities found in the Pe- be partially related to de facto assortative matingim-
quop Mountains will merge in the nearfuture. In hy- posedbydifferences inphenology. Bedpopulations in
brid zones generally, fusion is a long-term process warmeranddrierhabitattothesoudi (oron south fac-
(Barton & Hewitt 1983, Zink & McKitrick 1995). ingslopes) reach peak abundance considerablyearlier
While the separation between red and black forms is than blackpopulations incoolerandmore mesichabi-
not complete as previously thought (Ehrlich & Mur- tats farther north (Fig. 2). While climatic conditions
phy 1982, Austin & Murphy 1987, 1998b, Brussard et are a primary determinant ofdevelopmental phenol-
al. 1989), pure phenotypes outnumber intermediates ogy ofthese butterflies, synchronization with primary
and nopopulations dominated byhybridswarms have larval hostplants is also a contributing factor (Mooney
been found. The partial temporal segregation ofthe et al. 1980, 1981, Holdren & Ehrlich 1982). It seems
red andblackforms, reinforcedbybroad scale allopa- likelythatthe difference inemergencetimes serves as
try, has constrainedthe extent ofhybridization. the major barrier to matings between the two forms
Hybridizationwithin Euphydrijas is known to exist that might otherwise freely mate. It has been noted
in only highly restricted areas despite considerable that hybridization often takes place when one taxon is
overlap in their overall distributions. Narrow hybrid considerablyrarerthan the other (e.g., Sibley & Short
zones are typical for many taxa and usually are main- 1959, Ficken & Ficken 1968, Taylor 1973, Silberglied
tained by some strong selective factor (Sibley 1961, &Taylor 1978). Itis ofinterestin this contextthat the
Saino & Villa 1992, Scriber 1994, Scriber & Gage phenologies are switched in central and northern
1995, Jiggins et al. 1996, Harrison & Bogdanowicz Idaho with E. colon flying early and overlapping the
1997, Malletet al. 1998). This is furthercurbedin the laterflyingE. anicia (Ferris 1988).
Great Basin by the highly insular nature of suitable Other instances of reported intergradation among
habitat forEuphydrijas enforcedbyahighlydissected Euphydrijas in Nevada (Ehrlich & Murphy 1982,
topography. Amongseveral modelsusedtoexplainhy- Murphy & Ehrlich 1983, Austin & Murphy 1987,
bridzones (Endler 1977, Moore 1977, Moore & Price 1998b, Brussard et al. 1989) nowappear, with further
1993, van den Bussche et al. 1993), the best explana- data, tobeonlylocalvariation. Variabilitywithinpopu-
tion forthe hybridization ofEuphijdryas in northeast- lations does not necessarily indicate hybrid origin
ern Nevada appears to be the "tension-zone" or "dy- (Brown &Wilson 1956, Sibley&West 1958, Schueler
&
namic equilibrium" model in which reduced fitness of Rising 1976).
hybridsis offsetbycontinuedintrogressionofparental Scott (1978, 1986) points to a gradual phenotypic
genes (Bigelow 1965, Barton& Hewitt 1989, Barton & change ofEuphydrijas occurringin some areas andan
Gale 1993). The high relative abundance ofpure phe- abrupt change in others. Species would be expected
notypes in the Pequop Mountains, an indicator ofre- mostlyto change abruptlywhereas subspecies mayei-
productive isolation and species-level differentiation ther change abruptly or gradually, especially depend-
(Patton 1973, Tucker & Schmidly 1981, Benedict ing upon the absence or presence ofpast or present
1999), may suggest that matings between phenotypi- ecological or other barriers. Wing color and pattern
cally similar individuals are more frequent than be- appearto be evolutionarilylabile among Euphydnjas
tween phenotypically dissimilar individuals. This may and potentially relate to thermoregulatory considera-