Repeated Geographic Variation at Three Enzyme Loci in the Amphipod Platorchestia Platensis
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An improveddirect RNA sequence method:Its ap- CorrespondingEditor: D. R. Cavener Evolution,41(2), 1987, pp. 438-441 REPEATED GEOGRAPHIC VARIATION AT THREE ENZYME LOCI IN THE AMPHIPOD PLATORCHESTIA PLATENSIS JOHN H. McDoNALD Departmentof Ecology and Evolution,State UniversityofNew Yorkat StonyBrook, StonyBrook, NY 11794 Received September8, 1986. Accepted November 10, 1986 A correlationof allelefrequency with an environ- notransferase(ALAT, E.C. 2.6.1.2), glucose-6-phos- mentalfactor is one of the mostcommon forms of phateisomerase (GPI, E.C. 5.3.1.9),and mannose-6- evidencefor natural selection on allozymeloci. How- phosphateisomerase (MPI, E.C. 5.3.1.8). ever,an apparentassociation of allelefrequency with Horizontalstarch gels containing 10.5% starch were a singleenvironmental gradient, such as a latitudinal usedfor electrophoresis. The pH 8.6 Tris-borate-EDTA clinecorrelated with temperature, may also result from buffersystem of Nichols and Ruddle(1973) was used randomdrift in isolatedpopulations followed by mi- forall threeenzymes. The stains used are those of Shaw gration.One wayto testthe drift hypothesis is to ex- andPrasad (1970) for GPI, Nicholsand Ruddle (1973) amineareas of repeatedenvironmental variation for forMPI, andthe fluorescent glutamate pyruvate trans- repeatedpatterns of genetic differentiation, since ran- aminasestain of Harrisand Hopkinson(1976) for dom processeswould not producea consistentasso- ALAT. Eachamphipod was ground whole in an equal ciationof a particularallele with a particularenviron- volumeof 0.05 M Tris-HCl,pH 8. Amphipodswere ment.Examples of such repeated patterns include six capturedby hand from under rocks and seaweed in the enzymeloci in Drosophila melanogaster,in whichlat- upperintertidal area of beaches.The averagesample itudinalclines of allele frequency are present on three size was 49 individuals. continents(Oakeshott et al., 1981,1982, 1983, 1984). Eachlocus had two common alleles. There were also At six loci in Fundulus heteroclitus,latitudinal clines two rareGpi alleles,with mean frequenciesover all arepresent in both Chesapeake Bay and on theAtlantic locationsof 0.031 and 0.011, and one rareMpi allele coast(Powers et al., 1986).In Mytilusedulis, clines at witha meanfrequency of 0.0005. The commonallo- theLap locusare present in Long Island Sound (Koehn zymewhich migrated furthest toward the anode was et al., 1976) and twosmaller estuaries (Boyer, 1974). designated100. HereI reportrepeated differentiation between different For each locus,several pairs of locations were sur- habitatsin the amphipod Platorchestiaplatensis at loci veyedinitially. The locationswithin each pair differed codingfor three polymorphic enzymes: alanine ami- fromeach otherin a subjectiveassessment of habitat NOTES AND COMMENTS 439 CONNECTICUT 1 b Im 2e*,. le k Ns NEW YORK 3 m~~~~~~3 A6 Nevv l seYlRA6 4m4m~~~~~~~~4 X 0 30k~~~~~~~~~~~ so~~9 .. 8x 7x 20p~~~~~~~~~4 0 30 km ~ ~ ~ ~ ~ ~ ~ ~~ 1k FIG. 1. Locationsof P. platensis samples.Both insets use thesame scale. Letters indicate the type of beach: e, estuanine,m, manine,b, bay;o, ocean;p, protected;x, exposed. type.A hypothesiswas formulatedto explainthe re- gestedthat the Gpi'00 allele might be selectedagainst sultsof this initial survey, and this new hypothesis was on theshores of shelteredbodies of water, where the thentested with a newset of pairs of locations. A sign climateis warmerin summerand cooler in winterthan test(Sokal and Rohlf,1981 pp. 449-450) was used on ocean beaches.To testthis hypothesis, eight bay- becausethe null hypothesisis thatany one pair of oceanpairs were surveyed (locations 5-12). Bay sam- locationsmight differ in allele frequency due to random ples werecollected on the shoresof bays and large drift,but the direction of difference would be random. estuaries,as faras possiblefrom the ocean, while ocean A one-tailedtest was used because a specificaltemative sampleswere collectedon or near AtlanticOcean hypothesis,predicting which allele would be associated beaches.At sevenof these eight pairs of beaches,the witheach habitat,was formulatedbefore the second Gpi100allele was indeed less common at the bay site setof samples was collected.Because subjective, qual- (Fig.2b), which is significantlydifferent from the null itativehabitat classifications were used, it is important expectation(one-tailed sign test, P = 0.035). to emphasizethat the pairs of locationswere chosen At theAlat locus, the initial survey consisted of the and theirhabitat types were determined before the al- fourestuarine-marine pairs and the first five bay-ocean lozymedata were collected. pairs.At thatpoint the Alat'00 allele was less common The initialsurvey of variation at theGpi locus con- at theestuarine and baybeaches in sevenof the nine sistedof fourestuarine-marine pairs (locations1-4, pairs(Fig. 2c). The moststriking exception was Del- Fig. 1).At each pair, an estuarinesample was collected awareBay (sites9o and 9b). The beachwithin Dela- at a beachon theestuarine portion of a river,and a wareBay, like other bay beaches, is probablywarmer marinesample was collectedon a beach outsidethe in summer,cooler in winter,and lowerin salinitythan mouthof the river. A consistentgeographic pattern of the nearbyocean beach. However,unlike other bay Gpi allele frequencieswas not evidentwithin these beaches,where the P. platensishabitat is generallymud pairsof locations (Fig. 2a). However,the beach most andfine sand, the Delaware Bay beach consists of coarse exposedto the moderatingclimatic influence of the sand,gravel, and largerocks. In thisrespect the Del- AtlanticOcean, site 2m, had a higherfrequency of the awareBay beachis similarto P. platensishabitat on Gpi'00allele thanbeaches inside Long Island Sound ocean beaches.This suggestedthat the Alat'00 allele (site4m) and Fishers Island Sound (site I m). Thissug- maybe selectedagainst in protectedareas, which gen- 440 NOTES AND COMMENTS .35 _ .30 - a b .25- .20- ~.15- .10- .05- 0 1 2 3 4 5 6 7 8 9 10 11 12 .50 c d .40- .30- .20- .10 0_ 1 2 3 4 5 6 7 8 9 13 14 15 16 17 18 19 20 .90 e f .80- .70- .60 .50 .40 .30- 1 2 3 4 5 6 7 8 9 13 14 15 16 17 18 19 20 location location FIG. 2. Allelefrequencies atthe locations shown in Figure 1. For each pair of locations, the solid bar represents themarine, ocean, or exposedbeach; and thestriped bar representsthe estuarine, bay, or protectedbeach. a) Gpi,initial survey. b) Gpi,bay-ocean pairs. * Indicatesthat the frequency of Gpi'00is 0. c) Alat,initial survey. d) Alat,protected-exposed pairs. e) Mpi,initial survey. f) Mpi, protected-exposedpairs. erallyhave muddiersediment than beaches exposed againston protectedbeaches was testedat the eight to morewave action.To testthis hypothesis, eight smallbays on LongIsland Sound, and at sevenof these protected-exposedpairs on the northshore of Long theMpi'00 allele was less commonon the protected Islandwere surveyed (locations 13-20, Fig. 1). Exposed beach(Fig. 2f; P = 0.035,one-tailed sign test). beacheson LongIsland Sound consist of coarsesand One explanationfor the unusual Alat and Mpi allele and gravel,while protected beaches on thesmall bays frequenciesat thesite inside Delaware Bay could be are muchmuddier. At sevenof theeight pairs of lo- inadvertentsampling of a secondspecies. The amphi- cations,the Alat'00 allele was lesscommon in thepro- pods fromsites 9e and 9p wererun for an additional tectedsite (Fig. 2d), a significantdifference (one-tailed fiveenzyme systems (aspartate aminotransferase, sign test,P = 0.035). malatedehydrogenase, peptidase, phosphoglucomu- The patternat theMpi locuswas similarto thatat tase,and phosphogluconatedehydrogenase), yielding Alat.At six of the