Analysis of a Hybrid Zone in Fundulus M%Ils in a Northeastern Florida Ecotone

Heredity74 (1995)117—128 Received 12 January 1994 Genetical Society of Great Britain

Analysis of a hybrid zone in Fundulus m%ils in a northeastern Florida ecotone

CHARLES F. DUGGINS JR*, ALVAN A. KARLINt, TIMOTHY A. MOUSSEAU & KENNETH G. RELYEA Department of Biological Sciences, University of South Carolina, Columbia, SC 29208, tDepartment of Biology, University of Arkansas at Little Rock, Little Rock, AR 72204 and Department of Biology, Armstrong State College, Savannah, GA 31419, U.S.A.

Fundulusmajalis and F. similis are small fish that inhabit the coastal waters of eastern North America. Fundulus majalis occurs from New Hampshire to northeastern Florida where it is replaced by F. sitnilis. Past morphological studies of these species have suggested the existence of a hybrid zone in the Flagler County area of northeast Florida; this location also corresponds to an ecological transition from northern Juncus—Spartina marsh to southern mangrove marsh. We used protein electrophoresis to test the hypothesis that there is a significant barrier to gene flow between the two species in the area of Flagler County, FL. Twenty populations were surveyed electrophore- tically for 26 protein producing loci. Significant allozyme variation was observed for five loci: adenosine deaminase, an esterase, isocitrate dehydrogenase, malate dehydrogenase and phosphoglucomutase. Allele frequencies at these loci vary clinally from a 'majalis'-like distribution in more northern localities to a 'similis'-like distribution in southern localities. The steepest portion of the dine occurs over a 30 km portion of intracoastal salt marsh bordered by the northern and southern boundaries of Flagler County, FL, which led us to examine eight populations from within this area. We speculate that the nominal F. majalis and F. similis were allopatric and in the process of speciation when they recontacted one another, and the process of differentiation broke down and secondary intergradation occurred. The width of the genotypic dines (only sMdh-A is fixed at both ends), Nm estimates, and the absence of significant linkage disequilibria support the hypothesis that the rapid shift in genotypes and morphology in Flagler County is a dispersal-independent dine maintained by selection across the rapid ecological transition of an ecotone.

Keywords:clime,Cyprinodontidae, ecotone, Fundulus, hybrid zone, linkage disequilibria.

hybrid zone becoming more and more shallow with Introduction time (Hewitt, 1989). In cases in which species occur in abutting habitats, Hybridzones are regions where genetically different populations meet and interbreed to produce hybrids and if they hybridize where they come into contact, the (Barton & Hewitt, 1989). Hybrid zones appear to be resulting hybrid zone will be in the ecotone produced common; Hewitt (1989) noted that in many groups where the habitats abut. Hewitt (1989) observed that one-third to one-half of the species are subdivided by many hybrid zones occur along ecotones. In these hybrid zones. Hybrid zones may be wide or narrow, cases one may observe parallel dines in allele frequen- and may or may not be associated with ecotones. cies across many genetic loci. These dines in allele Hybrid zones fall into three principal categories: frequencies seemingly closely parallel dines in environ- those maintained by selection against hybrids (tension mental parameters (e.g. temperature, salinity). Other zones), those maintained by selection along an environ- loci may not vary clinally; presumably these loci are not mental gradient and those in which there is equivalent affected by selection by the parameters that vary across selection on the allelic differences between the two the ecotone. races resulting in the dine of allele frequencies in the Alternatively, a rapid environmental gradient may lead to a hybrid zone within a species. This could result from strong selection across many loci in response to *Con.espondence 118 C. F. DUGGINS ETAL. selection along an environmental gradient across a flow between the two nominal forms in this 30 km of short geographical distance. Brown & Chapman marsh, and therefore to test Relyea's (1983) hypothesis (1991) demonstrated a situation in two subspecies of that they are conspecific. The results should also Fundulus heteroclitus where two populations which provide insight into the nature of hybrid zones. had apparently diverged in allopatry maintained some of their differences after recontact with a dine in Materials and methods characters across an environmental gradient. The question is, therefore, how to differentiate Materials strong dines in morphology and allelic isozymes which result from hybridization and restricted gene flow Specimens were collected from the 20 localities listed across an ecotone vs. those dines which result entirely below, placed on dry ice and returned to the laboratory from selection on the populations of a species which where they were stored at —70°Cuntil used for happen to cross an ecotone, i.e. the presence of a electrophoresis. The localities were chosen based on hybrid zone even though there is no barrier to gene morphological and meristic evidence described in an flow. earlier study of the species complex (Relyea, 1983). The Fundulus majalis—Fundulus similis species Sixty individuals per locality were assayed except for complex includes three nominal species: F. majalis the Georgia locality (sample 2, 38 individuals) and the (Walbaum), F. similis (Girard) and F. persimilis Miller. Carrabelle, Florida locality (sample 20, 20 individuals). They are small fishes of the family Cyprinodontidae The sample numbers correspond to those of Fig. 1 and and typically occur in brackish waters of intracoastal in the tables. waterways, low energy coastlines and estuarine The geographical samples were collected at North lagoons. Fundulus majalis ranges continuously, or Carolina: (1) Brunswick Co., Wrightsville Beach; nearly so, along the coast from New Hampshire Georgia: (2) Chatham Co., Oaussau Island; Florida: (3) (Jackson, 1953; Brown, 1957) southward to north- St. Johns Co., Matanzas Inlet; (4) Flagler Co., 13 km S. eastern Florida. Except for the region in southeast of the Flagler Co. line; (5) Flagler Co., 14.5kmS. of the Florida between Lake Worth and Miami, Fundulus Flagler Co. line; (6) Flagler Co., 16 km S. of the Flagler similis is continuously distributed from northeastern Co. line; (7) Flagler Co., 17.5 km S. of the Flagler Co. Florida around the Florida peninsula and along the line; (8) Flagler Co., 18.5 km S. of the Flagler Co. line; coast of the Gulf of Mexico to Tampico, Mexico (9) Flagler Co., 19.3 km S. of the Flagler Co. line; (10) (Miller, 1955; Relyea, 1983). Flagler Co., 20 km S. of the Flagler Co. line; (11) Based on presumed sympatry of the two forms inthe Flagler Co., Rte 100 at intracoastal waterway, 22.25 Matanzas River in northeastern Florida, approximately km S. of the Flagler Co. line; (12) Flagler Co., 0.8 km N 25 km north of the Flagler Beach area, Miller (1955) of Flagler Beach State Park, 26km S. of the Flagler Co. concluded that F. majalis and F. similis were distinct line; (13) Brevard Co., Rte 40 at the intracoastal water- species. Brown (1957) also recognized both species, way; (14) Brevard Co., SR 404 at the intracoastal and in his key to the species of Fundulus used head waterway near Satellite Beach; (15) Monroe Co., length to distinguish them. However, Relyea (1983) Grassy Key; (16) Monroe Co., Big Pine Key; (17) concluded that F. majalis and F. similis were Collier Co., Marco Island; (18) Pinellas Co., Ft. DeSoto conspecific. He showed that from north to south along Co. Park; (19) Dixie Co., Horseshoe Beach; (20) the Atlantic coast head length, pigmentary patterns and Franklin Co., Carrabelle. other diagnostic characters varied continuously from F. majalis-Iike to F. similis-Iike. Relyea (1983) also suggested that gene flow from the nominal F. majalis Electrophoreticmethods into the disjunct Florida east coast population of the Proteinsamples for electrophoresis were obtained by nominal F. similis had distinguished this population homogenizing individual fish in an equal volume of from other F. similis populations in the Florida Keys chilled distilled water. The resultant slurry was centri- and Gulf of Mexico. fuged at 25000 g at 4°C for 60 mm. The supernatant When Relyea (1983) concluded that F. rnajalis and of water soluble proteins was decanted and stored at F. similis were conspecific, he reported that the 4°C overnight, a maximum of 18 h prior to electro- diagnostic features changed sharply as geographical phoretic separations. To determine the specific origin samples were measured along a north—south transect of each enzyme studied, organ extracts were analysed over a linear distance of only 30 km in a coastal marsh separately. in northeastern Florida. The purpose of this investiga- In a preliminary survey, 26 proteins were routinely tion was to determine the extent and nature of gene surveyed in 20 individuals from each population (Table HYBRID ZONES AND ECOTONES 119

84 82 80 35 35

30 30

Fig. 1 Collection localities and population numbers of Fundulus samples. Populations 4—12 are located from 25 25 north to south in numerical order and are each approximately 1—4 km apart. See Materials and methods for precise 84° 82° 80° localities.

1). Following the initial survey, five polymorphic (1980)and Crabtree & Buth (1981). When electro- systems (Ada-A, Est-3, slcdh-A, sMdh-A, and Pgm- morph (allelic) variation was observed, the electro- A) were evaluated for 40 additional individuals of morph with the greatest anodal migration was called a, most populations. However, as our survey proceeded the next b, and so on. and the pattern of genotypic variation became evident, we surveyed additional loci in an attempt to gain more Analyticalmethods information. This survey consisted of comparing five fish from a 'majalis' locality (population 2, near Savan- Individualgenotypes for each protein were assigned nah, Georgia) to five fish from a 'similis' locality (popu- based on standard methods (Brewer, 1970) of gel inter- lation 20, near Carrabelle, Florida). These loci were pretation. These presumed genotypes were entered acid phosphatase, adenylate kinase, alcohol dehy- into aiosys-i (Swofford and Selander, 1981) for drogenase, aldehyde oxidase, aldolase, creatine kinase, computer analysis. fumarase, beta galactosidase, glutamate dehydrogen- To determine general patterns of gene frequency ase, glycerol dehydrogenase, hexokinase, mannose distributions and genetic differentiation among popula- phosphate isomerase, nucleoside phosphorylase, octa- tions, Nei's (1978) genetic identity and distance nol dehydrogenase, sorbitol dehydrogenase, succinate estimates were calculated. These methods were used to dehydrogenase and xanthine dehydrogenase. No construct phenograms by the UPGMA method. important variation was found at these loci, and they Additional clustering algorithms were used and the were no longer assayed. method that yielded the greatest cophenetic correlation Our techniques of starch gel electrophoresis were was chosen. similar to those described by Brewer (1970) and As another measure of genetic differentiation we Selander et a!. (1971) (see Table 1 for electrophoretic computed Wright's (1978) F-statistics. In one popula- conditions). The stain for Gludh was modified from tion model that we used, the total population (T) was Buth & Berg (1979) to contain 9.0 g D-glucose, 0.005 g subdivided into two subpopulations (S) on the basis of each of NBT, MTT, PMS and 0.01 g of NADP. All gels species and the geographical samples (1) were con- were 13 per cent starch (Electrostarch lots 392, 307 sidered as individual units of their respective species. and 84, Otto Hiller Electrostarch Company, Madison, Because Relyea (1983) suggested that differentiation Wisconsin). Isozyme separation was essentially the may exist within local clusters of populations of F. same on different lots of Electrostarch, although lot 84 similis, we analysed another population model. In this required approximately 20 per cent longer running model, four subpopulations (S) of the total (T) popula- times. The locus nomenclature follows Fisher et a!. tion were defined based on a Nei's genetic identity of 120 C. F. DUGGINS ETAL.

Table 1 Enzymes, loci and electrophoretic conditions estimate the degree of gene flow through the area of transition (Flagler County) of the two nominal species. Electrophoretic We estimated linkage disequiibria within popula- Enzyme (E.C. number) Locus conditionsf tions using the maximum likelihood methods of Weir & Cockerham (1989). Disequilibria were estimated using Adenosine deammase Ada-A TC 8.0 the FORTRAN listing provided by Weir (1990). This (E.C. 3.5.4.4) method produces six disequilibrium coefficients from Aspartate aminotransferase M-Aat-ATC 8.0 S-A at-ATC 8.0 two-locus, two-allele data. Weir & Cockerham's (1989) (E.C. 2.6.1.1) model permits the simultaneous estimation of the two Catalase(E.C. 1.11.1.6) Cat TC 8.0 Esterase (nonspecific) Est-1 LiOH single-locus disequilibria, DA and DB, the composite Est-2 LiOH digenic disequilibrium iAB, the two trigenic disequili- Est-3 LiOH brium estimates DABB and DAAB, and the composite Glucose dehydrogenase Gludh TM 7.4 quadragenic disequilibrium estimate AAABB. Because (E.C. 1.1.1.47) five loci varied along the dine from F. ma/ailsinthe Glucose phosphate isomerase Gpi-A LiOH north to F. similis in the south, we calculated a total of (E.C. 5.3.1.9) Gpi-B LiOH 10 disequilibrium estimates of each type for each Gd/i TM 7.4 Glutamate dehydrogenase population. (E.C. 1.4.1.3) Glyceraldehyde-phosphate Gapdh-ATM 7.4 dehydrogenase (E.C. 1.2.1.9) Results Isocitrate dehydrogenase mlcdh-ATC 8.0 (E.C.1.1.1.42) slcdh-ATC8.0 Lactate dehydrogenase Ldh-A TM 7.4 Electrophoretic results (E.C. 1.1.1.27) Ldh-B TM 7.4 Of the 27 loci coding for proteins surveyed in this Ldh-C TM 7.4 study, 19 were monomorphic or nearly so (frequency Malate dehydrogenase sMdh-ATC 8.0 of the common allele in all populations >0.95). (E.C. 1.1 .1.37) sMdh-BTC 8.0 mMe-A TM 7.4 Although Est-2 and Cat were polymorphic, they were Malate dehydrogenase difficult to score consistently and were omitted from (E.C. 1.1.1.40) sMe-B TM 7.4 Phosphoglucomutase Pgm-A TC 8.0 analysis. The observed genotypes for the remaining (E.C. 2.7.5.1) Pgm-B TC 8.0 five polymorphic loci are reported in Table 2. Phosphogluconate Pgdh-A TC 8.0 When gene frequencies were used to estimate dehydrogenase (EC 1.1.1.44) genetic distance and similarities, Nei's (1978) identity Superoxide dismutase sSod-A TM 7.4 values varied from 0.832 to 0.999 and distance values varied from 0.184 to 0.001 (the full matrix is available tLiOH: lithium hydroxide (Selander eta!., 1971), 0.04 W/cc, from A.A.K. on request). The pattern of identity values 4.5 h; TC 8.0: Tris-citrate pH 8.0 (Selander eta!., 1971), 0.03 is apparent from the phenogram (Fig. 2) which clearly W/cc, 5 ii (lot nos. 392 and 307), 0.04 W/cc, 5 h (lot no, 84); clusters samples. The cluster of samples 1—6 represents TM 7.4: Tris-maleate-EDTA (Selander et al., 1971), 0.03 7 the nominal F. majalis and the northern one-quarter of W/cc, 5h. the transition zone (Flagler County), 7—10 represent the more central samples from the transition zone whereas samples 11—14 represent the southern one- 0.98, as evident on the phenogram (Fig. 2). For both quarter of the transition zone and samples from the population models, FST represents the variance in allopatric Florida east coast population of the nominal allele frequencies among subpopulations, F repre- F. simiils; 15 and 16 are Florida Keys samples of the sents the reduction in heterozygosity of an individual nominal F. similis and 17—20 are Florida Gulf coast relative to the total population, and F15 represents the samples. The clustering pattern is consistent with reduction in heterozygosity of an individual due to Relyea's (1983) suggestion that Florida east coast nonrandom mating within its subpopulation. samples of the nominal F. similis (7—14) can be To determine whether observed allele frequencies distinguished from Florida Keys and Florida Gulf and genotypic frequencies were in agreement with coast samples (15—20). Hardy—Weinberg predictions, we first tested each geographical sample for each locus individually. Geneticresults Following this analysis, we used x2to determine homogeneity within each nominal species and within the Ofthe 100 x2testsperformed (five loci in each of 20 entire sample. We used Slatkin's (1985) method to geographical samples), 92 showed no significant HYBRID ZONES AND ECOTONES 121

Cophenetic Correlation =0.705

2 3 S 4 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

I I I I I I [1 Fig. 2 Phenogram for Fundulus 0.90 0.91 0.92 0.93 0.94 0.95 0.960.97 0.98 0.99 1.00 obtained with Nei's (1978) unbiased genetic identity values. Neis (1978) Genetic Identity

Table 2Genotypic distributions for five loci for 20 populations of Fundulus

Population Ada-A Est-3 skdh-A sMdh-A Pgm-A

(1) Wrightsville Beach bb(60) dd(2) bb(38) aa(60) aa(60) de(17) bc(18) df(1) cc(4) ee(38) ef (2)

(2)Oaussaulsland ab(1) ce(2) bb(18) aa(38) a+a(2) bb(36) dd(2) bc(17) a+b(1) bc(1) de(11) cc(3) aa(35) ee(23)

(3)Matanzaslnlet bb(54) bc(1) ab(1) aa(58) aa(58) bc(6) bd(1) ac(1) ab(2) ab(2) be(4) bb(24) cc(2) bc(28) ce(5) cc(6) dd (1) de(9) ee(37)

(4)l3kmS.FlaglerCo.line bb(50) cc(1) ab(1) aa(57) aa(58) bc(9) ce(4) ac(1) ab(3) ab(2) cc(1) de(10) bb(20) ee(45) bc(27) cc (11)

(5)14.5kmS.FlaglerCo.line bb(49) bc(2) ab(2) aa(55) aa(57) bc(11) bd(1) bb(30) ab(5) ab(3) be(5) bc(21) ce(3) cc(7) dd (1) de (10) ee (38) 122 C. F. DUGGINS ETAL.

Table 2 Cone!inued

Population Ada-A Est-3 slcdh-A sMdh-A Pgm-A

(6)l6kmS.FlaglerCo.line bb(31) bb(1) aa(1) aa(49) aa(45) bc(19) bc(2) ab(9) ab(11) ab(11) cc(1O) be(6) ac(3) ac(1) cc(1) bb(20) bb(3) cd(4) bc(22) ce(14) cc(5) dd (1) de(7) ee(24)

(7)17.5kmS.FlaglerCo.line bb(26) bb(1) aa(4) a+a(1) aa(40) bc(27) bc(5) ab(23) aa(26) ab(18) cc(7) bd(1) ac(4) ab(23) bb(2) be(5) bb(16) bb(1O) cc(7) bc(1O) cd(3) cc(3) ce(17) de(4) ee (17)

(8)18SkmS.FlaglerCo.line bb(14) bb(1) aa(9) aa(23) aa(41) bc(29) bc(7) ab(20) ab(20) ab(13) cc(16) bd(1) ac(5) bb(16) bb(4) be(3) bb(7) bc(1) cc(8) bc(14) cd(4) cc(4) ce (19) dd (1) de(4) ee (11)

(9)19.3kmS.FlaglerCo.line bb(18) ac(1) aa(8) aa(17) aa(31) bc(32) bb(1) ab(26) ab(28) ab(24) cc(1O) bc(4) ac(9) bb(15) bb(5) bd(2) bb(14) be(3) bc(2) cc(15) cc(1) cd(2) ce(25) cf(1) de(2) ee(4)

(1O)2OkmS.FlaglerCo.line bb(20) bc(11) aa(12) aa(15) aa(34) bc(25) be(9) ab(16) ab(27) ab(21) cc(15) cc(12) ac(1O) bb(18) bb(3) cd(1) bb(17) ce(18) bc(5) de(2) ee(7) (11)22.25kmS.FlaglerCo.line bb(12) bb(1) aa(12) aa(3) aa(34) bc(31) bc(12) ab(25) ab(27) ab(20) cc(17) be(3) ac(2) bb(30) ac(1) cc(23) bb(15) bb(4) cd(2) bc(6) bc(1) ce (15) de (1) ee(3)

(12)26kmS.FlaglerCo.Iine bb(9) bb(3) aa(11) aa(2) aa(24) bc(26) bc(1) ab(37) ab(26) ab(30) cc(25) bd(1) ac(3) bb(32) bb(6) be(3) bb(7) cc(22) bc(2) cd(4) ce(20) de (1) ee (5) HYBRID ZONES AND ECOTONES 123

Table 2 Continued

Population Ada-A Est-3 slcdh-A sMdh-A Pgm-A

(13)Rte4O bb(1) ab(1) aa(6) aa(1) aa(35) bc(18) bb(5) ab(22) ab(1O) ab(20) cc(41) bc(18) ac(5) bb(49) bb(5) be(2) bb(15) cc(32) bc(11) cd(1) cc(1) ce(1)

(14)SatelliteBeach bb(4) bc(19) aa(23) ab(8) aa(22) bc(20) bd(1) ab(22) bb(52) ab(28) cc(36) cc(34) ac(6) bb(1O) ed(2) bb(6) ce(3) bc(3) ee (1)

(15)GrassyKey bc(8) aa(5) aa(60) bb(60) aa(42) cc(51) ab(1) ab(13) cd(1) ac(7) bb(5) bb(4) be (22) cc(21)

(16)BigPineKey bc(8) aa(13) aa(60) bb(60) aa(27) cc(45) ab(9) ab(23) cd(4) ac(3) bb(1O) ce(2) bb(12) dd(1) bc(14) bd(2) cc(7)

(17)Mareolsland bb(2) bb(1) aa(60) bb(60) aa(4) bc(12) bc(17) ab(26) cc(46) cc(31) bb(30) cd(11)

(18)DeSotoCo.Park bc(8) bb(3) aa(60) ab(2) aa(1) cc(52) bc(14) bb(58) ab(25) bd(1) bb(34) cc(36) cd (5) dd(1)

(19)HorseshoeBeach bb(2) ab(2) aa(60) bb(60) aa(3) bc(11) bb(7) ab(21) cc(47) bc(17) bb(34) bd(2) be(2) cc(26) cd(6)

(20)Carrabelle bc(1) ac(1) aa(20) bb(20) aa(3) cc(19) bb(9) ab(6) bc(9) bb(11) cc (1)

The number of individuals of each genotype is in parentheses. departure in genotypedistribution from the Mdh-B and Pgm-A; sample 12: Est-3; sample 15: Est-3 Hardy—Weinberg model (P> 0.05; adjusted for the and Pgm-A; sample 16: Est-3). As in most of these number of tests performed; Harris, 1985). Of the cases the departure could be explained by one or a few remaining eight cases, which is close to what one would individuals, we suggest that no systematic error in scor- expect by chance alone, there was one (sample 14: Est- ing affected our results. 3) in which an unexpected homozygote (ee) was Wahlund's Effect (Wahlund, 1928) can result when a observed whereas the other seven cases each involved population consists of a group of heterogeneous sub- a homozygote excess (sample 6: Ada-A; sample 8: S- samples. If there is heterogeneity in allele frequencies 124 C. F. DUGGINS ETAL. among subsamples, there will be a net deficiency of Table 3 Summary of Wright's F-statistics in Fundulus heterozygotes and an excess of homozygotes even if Hardy—Weinberg proportions are maintained within Locus F1 FIT FST Nm each subsample. When we pooled all samples and used Ada-A 0.046 0.048 0.45 5 0.799 BIOSYS-1 to compute x2of homogeneity considering sMdh-A 0.054 0.672 0.654 0.632 each sample as a population, we found statistical signi- Pgm-A 0.063 0.351 0.307 1.064 ficance (P4 0.05) for each locus. Similar results were slcdh-A —0.016 0.382 0.391 0.889 obtained when we used groups of populations as Est-3 0.046 0.330 0.298 1.088 defined by the dendrogram (samples 1—6, 7—20) for Mean 0.036 0.435 0.414 0.894 groups of samples representing each nominal species. These results suggest that allele frequency heterogeneity exists between samples of the nominal species. As another measure of population differentiation, total of 1200 disequilibria was estimated (20 popula- hierarchial F-statistics (Nei, 1978) were calculated for tions x 10 pairwise comparisions X 6 types of disequili- two slightly different single-level hierarchial models. In bria). Of these, most were not significant. A total of one model, we considered samples of each nominal 50 (about 4 per cent) of the 1200 exceeded the critical species to represent colonies of a subpopulation of the x2'whichis what one would expect by chance alone. total. In this case, samples 1—6 represented F. majalis From this we conclude that there are no significant dis- andsamples 7—20 represented F. similis. For this equilibria. Figure 3 shows the pattern of variation of model 25.3 per cent of the total genetic variation arose disequilibria from north to south across the hybrid from differences between the nominal species (FST) zone, Visual inspection of the figure clearly indicates whereas most of the genetic variation (59.2 per cent) higher variation of disequilibrium estimates within the was contained within samples. In a second model, we hybrid zone than either north or south of the zone. Of subdivided the total population as described by the the 50 disequilibrium estimates that were significantly phenogram into four subpopulations (samples 1—6, larger than zero, 36 were within the hybrid zone. 7—10, 11—14, and 15—20). As we would expect, the Inspection of Fig. 3 indicates that disequilibria are proportion of gene differentiation between subpopula- likely to be both highest and lowest within the hybrid tions increased to 36.8 per cent whereas the genetic zone. To test this hypothesis we have transformed all variation contained within samples remained at 59.2 disequilibrium estimates to their absolute values and per cent. These results suggest that samples included in used the Kruskal—Wallis nonparametric method (SAS, clusters in the phenogram (Fig. 2) are homogeneous 1988) for the absolute value of D across the hybrid and differentiated from other clusters. zone. Using this approach we found highly significant Finally, as measures of gene flow between samples, effects of hybrid zone (x2values,P< 0.00 1 for all we estimated Nm from Wright's F-statistics (Table 3) values: DA= 98, DR =103,AAB =84,DAAB =80, calculated for the entire sample, and the number of DABB =98,AAABB =92).The results are the same if migrant individuals per generation by Slatkin's (1985) one uses ANOVA. and Lewontin's (1974) methods. Our Slatkin estimate was based only on samples through the zone of transition between the two nominal species. In this zone, we Discussion estimate that in excess of 100 individuals per genera- Foreach of the five polymorphic loci examined, we tion migrate between populations. As an Nm greater could choose a particular allele that represents the than 0.5 is generally considered sufficient to counteract most common in each presumed species and follow its genetic divergence due to genetic drift (Wright, 1931), change in frequency through the study area. In Fig. 4 there is no apparent impediment to gene flow between we chose the most common allele for each locus in F. species through the zone of transition. Other estima- majalis and plotted its frequency in successive samples tors of Nm (Lewontin, 1974) based on all 20 samples around the Florida peninsula. From this representa- suggest estimates ranging from 0.632—1.088. Again, tion, it is clear that the allele frequencies vary dinally even when all 20 samples are considered, we can find throughout the study area. The genotypic dines are no evidence to suggest any impediment to gene flow. wide; only sMdh-A is fixed at both ends. This interpre- The use of disequilibrium estimates has been tation is consistent with the results of gene flow recently promoted as a particularly insightful means to analyses which suggest that there is no impediment to investigate 'disturbing forces' within populations (Weir gene flow. & Cockerham, 1989), especially within hybrid zones The 30 km bounded by the north and south borders (Barton & Hewitt, 1989; Szymura & Barton, 1991). A of Flagler County, FL, is of particular interest because North Hybrid South 0.08 8 H DA 1J 0.04

B 0.00 — -- —-

B -0.04

R -,.JD• -...... 0.08. E H IJBr H E Cl) w 0.04 B wCl) E I::I 8 8 E 1. -Q a- :: a) a- Cl) a) U) -0.08 - . E: . . . a 0.02 0.08 LAe C BB 8 £D 0.04 8C I1= C 0.01 =8 :C CC H8 = C C B -< = P C C — . 0.00• C 0.00 N C BC C C D 0 C m2 -0.04 C C/) -0.01 D2 -0.08 ______m 2 4 6 8 10 12 14 16 18 20 2 4 6 8 10 12 14 16 18 20 0 Population Population Fig. 3 Disequilibrium estimates for the 20 study populations of Fundulus, ordered from north to south. There are ten disequilibria estimates of each type (DA, D8, LAB, DABB, DAAB and AAB8) for each population reflecting the ten possible pair-wisecomparisons for the five loci that vary significantly along the clime (Fig. 4). Disequilibria were generated using the methods of Weir & Cockerham (1989). 126 C. F. D(JGGINS ETAL.

1.0 -.- Est-3e slcdh-Ab 0.9 -•u-- sMdh-Aa 0.8 --E1--- Pgm-Aa —*-— Ada-Ab >. 0.7 4::: 9 0.4 . 0.3 a. 0.2 4 O---_-' 0,1 0.0 I I - 4 Graph of allele frequencies for Fig. 0 1 00 200 300 400 500 600 700 five loci in Fundulus against distance Distance from Population 1 (km) from population 1 (in km).

it is within this region that we observe the sharpest same chromosome. We tested for this possibility by change in allele frequencies (Fig. 4, samples 4—12). This estimating 'linkage' disequilibria. Most of the signifi- area represents an ecotone where the Juncus—Spartina cant disequiibria are in the Flagler County area, and marsh characteristic of the northern Atlantic Coast is the effect is enhanced if one uses the absolute value of replaced by mangrove marsh, characteristic of the estimates. This analysis was intended to illuminate southern peninsular Florida. Thus the maintenance of the pattern of gene flow found with the FST analysis. this zone could be attributed to selection along an That variance in disequilibria increases suggests environmental gradient (Endler, 1977; Hewitt, 1989). dispersal of parental genotypes through the zone. That Several studies, including Karl & Avise (1992), most disequilibria are not significant suggests that these DiMichele et al. (1991) and Brown and Chapman parental genotypes are freely mixing within their new (1991) have shown that allelic isozymes are often settings. The absence of significant disequilibria does adaptive and thus subject to selection. not support the hypothesis that a tension zone exists in With only minor exceptions, genotypic frequencies the Flagler County area. Instead, the dine seems to be in the Flagler County area and elsewhere throughout of the dispersal-independent type. Thus, our data the range of sample collections are in Hardy—Weinberg support Relyea's (1983) hypothesis that F. majalis and equilibrium. It is possible for genotypic frequencies to F. si,nilis are synonyms. be in Hardy-Weinberg equilibrium in a hybrid zone, Historically, there have probably been one or more especially if the hybrid zone is maintained by selection. secondary contacts between two allopatrically differen- Barton & Hewitt (1985) point out that in the bulk of tiated forms, F. majalis, a northern Juncus—Spartina the literature, a hybrid zone is synonymous with a dine. marsh form and the nominal F. similis, a southern They describe two types of dine: dispersal-indepen- mangrove marsh form (although F. similis does occur dent and tension zones. Dispersal-independent dines in Juncus-Spartina marsh in the northern Gulf of show a stable equilibrium of allele frequencies (or other Mexico). We speculate that during a previous inter- characters) at each locality along the dine. The parti- glacial epoch when southern Florida became too warm cular equilibrium point at each location along the dine to support a common ancestor to these forms, popula- parallels the change of selection coefficient along an tions became isolated from each other, one in the environmental gradient. Tension zones are where the northern Gulf of Mexico and the other along the homogenizing effects of dispersal are counteracted by eastern seaboard. Differentiation of these two allo- selection against intermediate genotypes (hetero- patric populations ensued with the nominal F. similis zygotes or recombinants). If the chine in the Flagler adapting to the environment of the Gulf of Mexico and County area were a tension zone, one would still expect the nominal F. majalis adapting to the environment of alleles characteristic of each species to occur in sets the eastern seaboard. Subsequent glaciation and asso- inherited from the parental forms, in a sense conjoined ciated climatic cooling displaced the distributions of by inheritance, not linked by physically being on the each southward. Avise (1992) summarizes the effects HYBRID ZONES AND ECOTONES 127 of Pleistocene glaciations on the biota of the southeastern USA). Populations of F. similis then dispersed Acknowledgements around the southern tip of the Florida peninsula and Theauthors acknowledge the financial support for this up the Florida east coast, and eventually recontacted project from the University of South Carolina and the more northern Atlantic form, the nominal F. Cameron University, Lawton, Oklahoma. Wesleyan majalis, somewhere on the east coast of Florida. College, Macon, Georgia provided some travel funds. Evidence of that contact is seen now in the Flagler D. Buth and S. Guttman critically commented on early County area. Apparently the two populations were drafts of the manuscript. We also thank the Office of incompletely differentiated and hybridization between Research in Science and Technology (ORST) at the the two previously allopatric forms occurred. The University of Arkansas at Little Rock for providing selection pressures that resulted in the original differ- laboratory facilities and computer time. entiation apparently persist today as evidenced by the dlinal variation in allele frequencies through the contact zone (Fig. 4). Clinal variation in allele frequencies in estuarine cyprinodonts may be quite common. Within the distribution of F. heteroclitus, Powers et al. (1986) References found sharp dines in gene frequency, a complete shift AVISE,.i. c. 1992. Molecular population structure and the bio- from a northern allele to a southern allele within one geographic history of a regional fauna: a case history with degree of latitude. This is not an isolated case of lessons for conservation biology. Oikos, 63, 62—76. secondary contact through eastern peninsular Florida. BARTON, N. H. AND HEWITF G. M. 1985. Analysis of hybrid zones. Similar cases have been documented for: Fundulus Ann. Rev. Ecol. Syst., 16, 113—148. heteroclitus-F. grandis (Duggins et a!., 1989), Menidia BARTON, N. H.AND HEWFVF, G. M. 1989.Adaptation, speciation menidia and M. peninsulae (Duggins et a!., 1986), and hybrid zones, Nature, 341,497—503. Syngnathus (Herald, 1965), Brevoortia (Dahlberg, BERT, T. M. ANDHARRISON,R. o. 1988. Hybridization in Western Atlantic stone crabs (genus Menippe): evolutionary history 1970), Chasmodes (Springer, 1959), Spheroides (Shipp and ecological context influence species interactions. & Yerger, 1969) and the stone crab, Menippe (Bert & Evolution, 42, 528—544. Harrison, 1988). BREWER, G. .i. 1970. An Introduction to Isozyme Techniques. The case of the stone crab also parallels this case in Academic Press, New York. terms of morphological characteristics. In each case, BROWN, J. L. 1957. A key to the species and subspecies of the coloration and pattern change through the contact cyprinodont genus Fundulus in the United States and zone. In our case, females of the nominal F. majalis Canada, east of the continental divide. J. Wash. Acad. Sci., have vertical bars as juveniles and as the fish matures 47, 69—77. the bars become longitudinal. Juvenile and adult F. BROWN, B. L ANDCHAPMAN,R. w. 1991. Gene flow and mito- similis females have vertical bars. Females in Flagler chondrial DNA variation in the killifish, Fundulus hetero- County display variable phenotypes. Some have close clitus. Evolution, 45,1147—1161. to a typical 'majalis' or 'similis' pattern, others have BUTH, a a AND BERG, W. i. 1979. Glucose dehydrogenase vertical bars as juveniles, and a series of dots, dashes expression in some marine teleost fishes. Isozyme Bull., 12,71. and irregular markings as adults (Relyea, 1983). CRABTREE, C. B. AND Bum, i. G. 1981. Gene duplication and Although we collected mostly juvenile individuals, in diploidization in tetraploid catastomid fishes Catastomus the few female adults sampled we found no obvious fumeiventris and C. santaanae. Copeia, 1981, 705—708. correlation between pigmentation pattern and geno- DAHLBERG, M. D. 1970. Atlantic and Gulf of Mexico type. Our sample was too small to allow statistical tests. Menhadens, genus Brevoortia (Pisces: Clupeidae). Bull. Fl. In conclusion, the clinal pattern of genotypic State Mus., 15,91—162. frequencies, width of the genotypic dines, Nm values DIMICHELE, L., PAYNTER, K. T. AND POWERS, D. A. 1991. Evidence and the absence of significant disequilibria indicate that of lactate dehydrogenase-B allozyme effects in the teleost, there are no barriers to gene flow throughout the range Fundulus heteroclitus. Science, 253,898—900. of this species complex. The hybrid zone between DUGGINS, C. F., KARLIN, A. A. AND RELYEA, K. G. 1986. Systematics Fundulus majalis and F. similis in the Flagler County of the Key Silverside, Menidia conchorum, with comments on other Menidia species (pisces: Atherinidae). Tulane area is in an area of rapid ecological transition which Studies in Zoology and Botany, 25, 133—150. results in rapid shifts in genotypes and morphology. DUGGINS, C. F., KARLIN, A. A. AND RELYEA, K. G. 1989. Biochemi- The dispersal-independent dine of this hybrid zone is cal systematics in southeastern populations of Fundulus probably maintained by selection against the morpho- heteroclitus and Fundulus grandis. Northeast Gulf Science, logical and physiological characters associated with the 10, 95—102. different habitats. This hybrid zone could perhaps be ENDLER, J. A. 1977. Geographic Variation, Speciation and best termed an ecological hybrid zone. Clines. Princeton University Press, Princeton, NJ. 128 C.F. DUGGINS ETAL.

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