Heredity 63 (1989) 181—189 The Genetical Society of Great Britain Received 1 February 1989

Spatial autocorrelation of genotypes in populations of Impatiens pallida and Impatiens capensis

Daniel J. Schoen and Department of Biology, McGill University, Robert G. Latta 1205 Avenue Docteur Penfield, Montreal, Québec, Canada H3A IBI.

The genetic control and small scale spatial distributions of three conspicuous genetic polymorphisms—presence or absence of corolla spotting, yellow or white corolla colour, and presence or absence of stem waxiness—were studied in populations of the species Impati ens pallida and I. capensis occurring in Québec, Canada. The corolla spotting and colour morphs in I. pallida each appear to be determined by different pairs of unlinked loci. Dominant alleles at each locus pair are required for the expression of corolla spotting or yellow corolla colouration. Stem waxiness in I. capensis is also shown to be under genetic control. Significant clustering of identical flower and stem at small interplant distances was observed in most population samples, suggesting substructuring of genotypes in space. The sizes of patches where morph frequencies occurred in excess of random expectations were found to be consistent among populations and polymorphisms within the species, lending support to the interpretation that substructuring of genetic variation in these populations is due to restricted gene dispersal. The population genetical consequences of limited gene flow in these species is discussed.

INTRODUCTION discovery of population differentiation associated with discrete variation in the habitat has been Investigationsof population substructure in interpreted as due to spatially-varying selection have relied upon the study of the spatial distribu- (Antonovics and Bradshaw, 1970; Linhart, 1974; tion of conspicuous polymorphisms (Epling and Silander and Antonovics, 1979; Antlfinger, 1981), Dobzhansky, 1942; Wright, 1943, 1978; Epperson whereas the discovery of population substructure and Clegg, 1986), as well as that of electrophoreti- in the absence of any clear microenvironmental cally-detectable variation (Hamrick and Allard, heterogeneity has been interpreted as due to local 1972; Schaal, 1975; Ennos, 1985; Waser, 1987; gene dispersal (Epperson and Clegg, 1986). Dewey and Heywood, 1988; Epperson and Allard, Moreover, the absence of population substructure 1989). Such investigations have often, but not is sometimes taken as evidence that gene flow always, revealed significant departures from ran- distances in the study population is not highly dom spatial distribution of genotypes. For restricted (Epperson and Allard, 1989; Dewey and example, Schaal (1975) and Epperson and Clegg Heywood, 1988), though other explanations for (1986) detected significant spatial association of lack of spatial structure are also possible. For genotypes in Liatris cylindracea and Ipomoea pur- example, Epperson and Clegg (1986) suggested purea, but Ennos (1985), Epperson and Allard that a high mutation rate at the W/w locus in L (1989) and Dewey and Heywood (1988) failed to purpurea masked spatial substructure when this find strong evidence of non-random spatial distri- locus alone was studied, and Dewey and Heywood bution of genotypes in populations of the grass (1988) and Epperson and Allard (1989) note that Cynosurus cristatus, the lodgepole pine (Pinus con- spatial substructure can go undetected if it exists torta) and the tropical tree Psychotria nervosci, as a scale smaller than that used for sampling respectively. Significant departure from random between plants. spatial distribution of genotypes can be due to The existence of population genetic substruc- spatially-varying selection and/or to local disper- ture is important to be aware of, as it may influence sal of genes (Turner, Stephens, and Anderson, the pattern of genetic transmission within the 1982; Bradshaw, 1984). Thus, for example, the population, and determine how the population 182 D J SCHOEN AND ft G LATTA responds to global selection (Epperson, 1989). In Genetic studies of the flower and addition, the assumption of a lack of population stem polymorphisms substructure in cases where substructure is in fact present, can lead to biased estimates of mating Parentplants used in crosses for analysing the system parameters (Elistrand, Torres, and Levin, genetic control of corolla spotting and corolla 1978; Ennos and Clegg, 1982; Ritland, 1985). Here coloure in I. pa/lida, and stem waxiness in I. we report evidence pertaining to the genetic control capensis, originated from seedlings that were har- and degree of population substructure of three vested in May of 1987 from natural populations conspicuous polymorphisms in the species I. pal- at Mont St. Hilaire. The I. pallida seedlings were lida and I. capensis.Previouswork with I. pallida brought into a screened insect exclosure located has revealed the existence of spatial heterogeneity near the McGill University Biology Field Station in the magnitude and direction of phenotypic at Mont St. Hilaire, while the 1. capensis seedlings selection of a number of quantitative traits (Stewart were grown in an adjacent garden plot. The plants and Schoen, 1987), as well as the presence of were grown for approximately four months, during within-population differentiation of a number of which time the crosses described below were car- quantitative traits (Schemske, 1984). We interpret ried out, and seed collections were made. To pro- the significance of the present findings in light of mote germination, harvested seeds were stored at these earlier results. 4°C between two layers of wet filter paper in petri dishes for 4-5 months. Germination occurred readily after this treatment (>80 per cent germina- tion), and the germinated seedlings were transfer- MATERIALSAND METHODS red to 7 cm (I. pallida) or 10cm (I. capensis) pots. These progeny plants were then grown to maturity Organisms and study site in greenhouses at the McGill University Phytotron, Impatienspal/ida and I. capensisarecommon, at which time the flower and stem phenotypes were annual plants which grow in forests throughout scored. eastern (Schemske, 1978; Waller The genetic control of corolla spotting in I. 1980). Plants of both species reproduce via cleis- pa/lida was analysed by examining the progeny of: togamous and chasmogamous flowers, with the (1) self-fertilized parents of known (S1 latter flower type typically being produced only on families); and (2) crosses between parents of larger individuals. The I. pal/ida populations known phenotype (F1 families). S1 families were studied occurred on relatively dry ground in two produced through the collection of seed from cleis- separate valleys in a beech-maple forest at Mont togamous flowers. These were assembled from St. Hilaire, Quebec, Canada (Maycock, 1961). The parents with unspotted corollas (n =10families), I. capensis populations occupied sites directly as well as from parents with spotted corollas (n = adjacent to streams or wet depressions at Mont St. 24families). Families ranged in size from 9—36 Hilaire. Populations of both species occurred as progeny each (=189, SD=8.3). F1 progeny dense, almost monospecific stands. Densities in families were produced through crosses made with the populations studied ranged from 80 to 300 chasmogamous flowers. These included crosses plants per square metre. between two parents each with spotted corollas Populations of I. pallida at Mont St. Hilaire (n =5 families),two parents each with unspotted are polymorphic for at least one of two distinct corollas (n=2 families), and two parents, one flower colour polymorphisms. These polymorph- having spotted corollas and the other having un- isms are the colouration of the chasmogamous spotted corollas (n =9 families). Family sizes for corolla, either yellow or white, and the presence these crosses ranged from 8-129 progeny each or absence of spotting in the corolla throat (fig. =25,SD =28•8). 1). Populations of I. capensis at Mont St. Hilaire Parent plants from the I. pa/lida population are polymorphic for the presence or absence of exhibiting the yellow-white corolla colour poly- a waxy bloom on the stem surface (fig. 1). We also morphism were less readily available, and thus noted variation in the degree of corolla spotting only S families were used to analyse the genetic in I. capensis, though this variation was continuous control of this . In collecting as opposed to the more discrete variation associ- material from parents in the more uniform lighting ated with corolla spotting in I. pallida, and could conditions of the enclosure, we noted the existence not, therefore, be easily applied to the analysis of of variation in the white corolla morph, with population substructure. expression ranging from pure white to cream. We POPULATION SUBSTRUCTURE IN IMPATIENS 183 ;y. —4---

Figure1 Polymorphisms in Impatienspallida and I. capensis. A. Spotted corolla, I. pallida. B. Unspotted corolla, I. pallida. C. Waxless stem, I. capensis. D. Waxy stem, I. capensis. coded all individuals lacking deep yellow corollas, capensis (table 1). These transects were positioned as having white corollas, a practice which is most so as to sample a large, uninterrupted section of likely to correspond to that used in the field surveys each population. Transects 'were 1 or 2 m wide by of natural population samples. S1 progeny were up to 32 m long. Within a given transect, sampling collected both from parents with yellow corollas was carried out at all points in a rectangular lattice (n =12families), and from parents with white of sampling points each separated by 10 cm within corollas (n =6families). These families ranged in the rows of the lattice, and by 20 cm between the size from 4 to 22progenyeach (= 86,SD =5.3). rows (fig. 2). The corolla or stem morph of the Only small S1 families of 5-15 progeny each plant closest to (but no farther than 5 cm from) were available for the study of stem waxiness in each lattice point was scored. Plants were removed I. capensis. These family sizes were too small to from the ground after scoring to ensure that each allow formal Mendelian analysis, and so instead plant was scored only once. When no plant (and we conducted a preliminary genetic analysis based in the case of I. pallida, when no chasmogamously on the detection of segregation in the progenies of ) was present at a lattice point, a parents differing in the expression of this trait. missing value was entered in the data record for that spatial position (fig. 2). This process of samp- ling resulted in several hundred plants being scored Fieldsampling methods and data analysis for all polymorphisms in each population transect. Forthe study of spatial distribution of genotypes We employed spatial autocorrelation analysis we sampled plants by laying out two belt transects (Sokal and Oden, 1978) in lieu of possible alterna- in each of two large populations of I. pallida, and tive analyses based on Wright's FST or Nei's GST one belt transect in each of four populations of I. statistics (Wright, 1951; Nei, 1973), because these 184 D. J. SCHOEN AND A. G. LATTA

TableI Populationsandtransects of Impatiens pal/ida and I. capensis studied

Frequencies (standard error) of:

Unspotted White Waxy Population/ Transect corolla corolla stem Species Transect dimensions morph morph morph

I. pal/ida 1/A 276mx2Om0.78(0.01) 049(001) —t Ipal/ida 1/B 324 mx 20 m079 (001) 040 (0.01) —t I. pal/ida 2/A 186 mx 20 m080 (0.02) 0 —'4 Ipal/ida 2/B 190 mx 20 m034 (0.02) 0 —t Icapensis 1/A 16'OmxIOm—I —i 087(001) 1.capensis 2/A 220 mx 10 m— — 052(0.02) I. capensis 3/A 240 mx 10 m —1 — 056 (0.01) I.capensis 4/A 12OmxlOm— — 083 (0.02)

§ scored only in Impatiens pal/ida. I' Scored only in hnpatiens capensis.

0 x X XO XXX investigation, the total number of joins between 0 XOXOx XX

ized normal deviate (SND) (Sokal and Oden, Table 3 Segregation of corolla spotting in F1 progeny of 1978). Correlograms (graphs of SND versus dis- crosses of spotted x spotted parents in Impatiens pallida tance class) are used below to summarize the results of these analyses. Corolla spotting F1 family Spotted Unspotted x2(1:1)

RESULTS SpllxSpl2 75 54 341 Sp27xSp88 12 9 043 Genetic studies of the flower and Sp36xSp48 10 18 2-29 stem polymorphisms Sp9lxSplO 10 6 100 Sp65 x Sp89 23 4 13.37***, Noneof the I. pallida parents with unspotted corol- P<0005. las produced segregating S1 progenies, while 8 of § x2 against anull hypothesis of 3:1 for this family= 1-49 the 24 S1 families from parents with spotted corol- (P> 005). las showed segregation (table 2). Segregation ratios in each of the individual families, as well as the pooled data, conformed to a 3: 1 (spotted:un- plementary control of corolla spotting (Grant, spotted) ratio, though some individual families 1975), i.e., corolla spotting in I. pal/ida appears to conformed better to a 9:7 (spotted:unspotted) be determined by two unlinked loci, each with ratio (table 2). dominant alleles, and with one dominant allele at each locus required for the spotting trait to be expressed. This hypothesis provides the most par- Table 2 Segregation of corolla spotting in the S1 progeny of simonious explanation of the probable parental spotted parents in Impatiens pallida genotypes used in the crosses and the observed Corolla spotting segregation ratios in their progenies. SI The results from the analysis of the yellow-. family Spotted Unspotted X2(3: 1) x2(9 :7) white corolla colour polymorphism in I. pal/ida revealed no segregation in S families from parents J5 7 5 177 002 J9 25 6 0-53 750 ** with white corollas. Six of the families from parents J30 17 9 128 0-88 with yellow corollas showed segregation (table 4). I-1464a 15 3 066 5.37* Segregation ratios in S progeny families conform H464b 6 3 033 040 either to 3: 1 or 9:7 (yellow: white) ratio. As in H471a 13 4 002 2-82 the case of corolla spotting, it seems likely that H482 13 9 2-97 007 H484 16 3 086 6.04* two unlinked loci determine expression of the trait, Sum 112 42 0-421 with one dominant allele at each locus required for the yellow corolla trait to be expressed. * P<0.05. ** P<0.01. § Deviation 2[7]= 844, P>0-05; pooling of family data Table 4 Segregation of corolla colour in the S progeny of justified. yellow-flowered parents in Impatiens pallida tDeviation2[7] =2309,P <001; pooling of family data not justified. Corolla colour St —______family Yellow White x2 (3:1) x2 (9:7) The F1 families produced by intercrossing two spotted parents produced only spotted progeny, H82 4 4 267 0-13 H464a 9 9 600* 029 while the F1 families produced by intercrossing H464b 6 4 120 006 two unspotted parents produced only unspotted H471a 15 7 054 127 progenies. Five F1 families from the crosses of H482 14 3 0-49 4.71* spotted x unspotted parents, however, showed B22 7 2 0-04 1-69 segregating progenies. In four of these families, * P<0.05. individual and pooled segregation ratios con- formed to a 1: 1 (spotted: unspotted) ratio (table 3). The segregation ratio of the fifth segregating With regard to linkage among the loci coding family conformed poorly to a 1: 1 ratio, but confor- for corolla spotting and corolla colour, in the four med well to a 3: 1 (spotted:unspotted) ratio (x2= S1 families which segregated for both the spotted— 149,P> 005). Together the S1 and F1 cross results unspotted and yellow-white polymorphisms (e.g., support the genetic hypothesis of two-factor, corn- families H464a, H464b, H482, and H484), there 186 D. J. SCHOEN AND R. G. LATTA was nostrong evidence of non-independent segre- 10, gation of the two polymorphisms as evidenced by 75 Chi-square analysis of morph frequencies in 2 x 2 5 S 25 tables ofjoint segregation (X2[1] = 060, 213, O68, N 0 039, respectively, for the four families listed D above). -5- For the waxiess-waxy stem polymorphism in 75 I. capensis,segregationwas observed only in -10- families of waxless-stemmed parents. From this 10 B result, we hypothesize that at least one gene locus 7.5000,u.a.0-n.n codes for stem waxiness, with the gene(s) coding 5/ S 2 5 ".-it. ..•••••_o• for waxiess stems being dominant to the gene(s) . _.u_t...,0.Q 0000 N 0 0 for waxy stems. 0 0- D 25 0- 0-o/ o -5 Spatialautocorrelation of flower and stem -7.5 morphs in I. pallida and I. capensis populations -10 4 Belowwe describe separately the results from the 3 spatial autocorrelation analysis of each poly- 2 morphism, beginning with the spotted-unspotted Si corolla polymorphism. At short distance classes in NO both populations of I. pallida, there were sig- D-1 nificant excesses of at least one type of homotypic -2 -3- join, and significant deficits of heterotypic joins .4 involving the spotted-unspotted corolla poly- morphism. At larger distance classes there were 4 significant excesses of heterotypic joins, and sig- 3 nificant deficits of at least one class of homotypic 2 join (fig. 3). In two of the samples, excess Si homotypic joins at short diatances were found only NO for the unspotted corolla morph. In one sample, D -1 -2- excess homotypic joins at short distances were 3. found for both morphs, and in the remaining case, .4 excess homotypic joins at short distances were ii 21 31 found only for the spotted corolla morph (fig. 3). Distance c'ass(x20 cm) Excesses of homotypic joins at short distances Figure 3 Correlograms of standard normal deviates (SND) are interpreted here and below as patches of the for unspotted and spotted corollas in Impatiens pal/ida. morph in question (Sokal and Oden, 1978). In A. Population 1, transect A. B. Population 1, transect B. substructured populations, excesses of heterotypic C. Population 2, transect A. D. Population 2, transect B. joins are expected at larger distances when the Closed squares (unspotted—unspotted joins), open squares interpoint sampling distance spans patches of two (spotted—spotted joins), circles (unspotted—spotted joins). different morphs. Instances of multiple excesses of homotypic joins occurring at different spatial The yellow-white corolla colour polymorphism scales, such as seen in with the spotted—unspotted was present only in population 1 of I. pallida. Each corolla polymorphism in population 2 of I. pallida, of the transect samples in that population showed are not unexpected since with large interpoint significant excesses of homotypic joins involving sampling distances, two separate patches of the the white corolla morph at short distance classes same type may be sampled. Using the first x-inter- (fig. 4). In one of the samples, both classes of cept of the correlogram as an indicator of the homotypic joins were in excess at short distance average length of a patch, and focussing on the classes. Patch length as estimated using the morph which is true-breeding when self-fertilized procedure described above for the true-breeding (i.e. the unspotted corolla morph; see above) (i.e., white corolla) morph was between 2 and 3 m (Sokal, 1979; Epperson and Clegg, 1986), it can (fig. 4). be seen that patch length ranged from approxi- For the waxiess-waxy stem polymorphism in mately 2 to 4 m. 1. capensis,allfour populations studied showed POPULATION SUBSTRUCTURE IN IMPATIENS 187

8 101 A A 1. 6 .0-0/0 5 °.•°,0 4 S 2 S 2 5 ..•_\ 1D=,,..-il••,•• .,•..S_,S...a 0 N 0 N 0 D-2.5 O/00 i 0/ 0 0 D-2 / \/0 -4 so oI 7t 'o 0 -6' - 1 0 a -8

4- B 4 B 3 3 2i 2 S S1 N NO D—1 D -1 -2 -2 -3- -3 -4-

1 11 21 31 8 Distance class )x20 Cm) r_0 C 6- Figure 4 Correlograms of standard normal deviates (SND) 4 for white and yellow corollas in Impatiens pallida. A. Popu- S2 lation 1, transect A. B. Population 1, transect B. Closed N 0 squares (white-white joins) open squares (yellow—yellow D-2 •• 0 joins), circles (white—yellow joins) -4 /00/ -6/0 substructuring, with excesses of at least one class of homotypic join, and deficits of heterotypic joins 4 D at short distance classes (fig. 5). Patch length for 3 the true-breeding (i.e., waxy) morph was approxi- 2 mately 1 m or less. Si DiNO -2 DISCUSSION 3 4 Two-factor, complementary control of corolla 1 11 21 spotting and corolla colour has also been reported Distance class (xl 0 Cm) in other species (Clausen, 1951; Grant, 1975). Our hypothesis with regard to the genetic control of Figure 5 Correlograms of standard normal deviates (SND) these traits should, however, be viewed as pre- for waxy and waxless stems in Impatiens caoensis. A. Popu- liminary as it is based on small sample sizes and lation 1. B. Population 2.C.Population 3. D. Population 4.Closedsquares (waxy-waxy joins), open squares (wax- few types of crosses. More complex genetic less—waxless joins), circles (waxy—waxless joins). hypotheses cannot be excluded with certainty. Additional genetic studies will also be required before a precise genetic hypothesis can be put white corolla morphs of I. pallida (Schoen, unpub- forward for the control of stem waxiness, though lished data). It is notable that both the spotted— it seems likely from the data presented above that unspotted and yellow-white polymorphisms in I. a genetic polymorphism underlies the variation pallida are apparently widespread in eastern North observed. America (Fernald, 1950). At present we have no detailed information to The type of patch structure observed within the suggest either the presence or absence of selection populations of I. pallida and I. capensis at Mont at the loci responsible for the flower and stem St. Hilaire could, in theory, have its basis in polymorphisms observed in these populations. spatially-varying selection, limited gene flow, or Casual investigation of the distribution of seed set both. Small scale differentiation within popula- per fruit and of plant biomass have revealed no tions that is correlated with microenvironmental significant differences between the yellow and variation has been reported for a number of plant 188 D. J. SCHOEN AND R. G. LATTA species (Antonovics and Bradshaw, 1970; Linhart, the more genetically homogeneous phenotypic 1974; Silander and Antonovics, 1979; Antlfinger, class within each polymorphism. These patches 1981). Such correlation suggests the past and con- can be equated to patches of the more homozygous tinuing action of selection in creating and main- morph. If spatially-varying selection were respon- taining small-scale differentiation. On the other sible for the existence of patches, this would hand, Turner, Stephens, and Anderson (1982), require the existence of areas within each popula- Sokal and Wartenburg (1983), and Bos and van tion where one morph or the other was selectively der Haring (1988) have deomonstrated that patch favoured. Moreover, the relative constancy of structure develops rapidly under isolation-by-dis- patch size within and among polymorphisms sug- tance models, and does not require spatially- gests that in order for the observed spatial patterns heterogeneous selection. After 50 generations, to be maintained primarily by selection, the scale patch length approaches an equilibrium which is of spatially-heterogeneous selection, or grain of functionally related to Wright's neighbourhood the environmental variation (sensu Levins, 1968), size (Wright, 1969; Epperson and Clegg, 1986). would have to be roughly equivalent across poly- When selection interacts with local gene dispersal, morphisms. There is no evidence from the physical the result may be either enhancement of diminu- nature of the habitats studied that this type of tion of patch structure. In particular, when selec- spatially-structured selection could operate. A tion acts on a polymorphism independently of tentative conclusion at present, is that patch struc- location, patch structure can be eroded (Epperson, ture in the case of these polymorphisms is due 1989). On the other hand, when fitnesses at a primarily to limited gene dispersal. polymorphic locus are dependent on location, In earlier studies of some of these same popula- limited gene dispersal can enhance patch structure tion samples, we have discovered significant spatial (Maynard Smith, 1966; Dickinson and heterogeneity among plots (separated by several Antonovics, 1973). metres) in both the direction and magnitude of Gene dispersal distances in the Impatiens phenotypic selection coefficients associated with species studied here appear to be quite limited, as quantitative traits such as cotyledon area, timing judged by observations of seed dispersal (Schmitt, of production, and characteristics related to Ehrhardt and Swartz, 1985; G. Bell, personal com- growth rate (Stewart and Schoen 1987). Such munication) and pollinator movements among spatial heterogeneity of selection when coupled chasmogamous flowers (Dubé, 1988). A more with limited gene dispersal and the type of popula- important factor contributing to reduced pollen tion substructure observed here could lead to local dispersal distance in these species is the large pro- adaptation and population differentiation over portion of seeds that are produced as a result of small distances (Via and Lande, 1985). Schemske cleistogamous self-pollination. This proportion (1984) in fact has reported that samples collected can comprise virtually all seeds that are produced from 30-50 m apart in two populations of 1. pallida in some of the populations studied. We have no from Illinois, U.S.A. were differentiated for a num- information at present on possible assortative mat- ber of traits, and he suggests that heterogeneity of ing mediated through pollinator preference for one selection and limited gene flow may be responsible flower morph, as reported by Brown and Clegg for this differentiation. While the results reported (1984) in I. purpurea, though if such assortative by Stewart and Schoen (1987) together with the mating were to occur in conjunction with limited present study are not sufficiently complete to gene dispersal, this would accentuate any already demonstrate that within-population differentiation existing patch structure. in I. pal/ida is indeed the result of local adaptation Given the above observational and anecdotal and restricted gene flow, they suggest that this evidence pertaining to gene flow distances, scenario is consistent with the population and together with the theoretical expectation that reproductive biology of the species. limited gene flow can promote population sub- structure, one might predict a priori that popula- Acknowledgements We thank Cremilda Dias, Steven Stewart tions of I. pallida and I. capensis should exhibit and Jonathan Brassard for assistance, and the 1988 class of patch structure for the polymorphisms. The results Biology 331A at McGill University for help in censusing the of this study indicate that there is, indeed, substruc- populations of 1. capensis. We also thank Dr Graham Bell for sharing withus his data on the genetic control of stem waxiness turing of genotypes coding for these polymorph- in I. capensis. This project was supported by a research grant isms. Such substructuring is most apparent and from the Natural Sciences and Engineering Council (NSERC) consistently observed for the morph types which of Canada to DJS and by a NSERC undergraduate research are true-breeding and which, therefore, comprise student award to RGL. POPULATION SUBSTRUCTURE IN IMPATIENS 189

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