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2Division of Science, Truman State University, Kirksville, Missouri 63501; and 3Department of Zoology, Miami University, Oxford, Ohio 45056
Received for publication April 27, 1998. Accepted for publication November 17, 1998.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: allozymes • Asteraceae • clonal • gene flow • genetic diversity • genetic structure • habitat fragmentation • Helianthus occidentalis • prairie
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, 1943
, 1946
; Hanski, 1991
; Rhodes and Chesser, 1994
). In addition, genetic variation within a patch may decrease if the remaining population is small because the effects of genetic drift and inbreeding will be greater. Both drift and inbreeding can ultimately lead to fixation of alleles in the patches. Maintaining high levels of genetic diversity is thought to be important for the long-term survival of species because it may provide the capacity to respond to environmental change by lowering susceptibility to stresses (Ledig, 1988
; Gillespie and Guttman, 1989
; Kopp, Guttman, and Wissing, 1992
).
However, a serious problem in efforts to conserve diversity or mitigate changes after disturbance is the lack of knowledge of the genetic structure of many species prior to disturbance. Data from empirical studies support theoretical models that habitat fragmentation affects plant population genetic diversity and structure but with more varied responses than predicted (Young, Boyle, and Brown, 1996
). For example, empirical studies of Acer saccharum suggested that habitat fragmentation actually increased gene flow (Foré et al., 1992
; Young, Merriam, and Warwick, 1993
; Ballal, Foré, and Guttman, 1994
). Life history characteristics of a species are likely to account for some of the variance in the response to habitat fragmentation.
Difference in dispersal ability is one of the primary characteristics that is likely to result in variation in response to habitat fragmentation (Bawa et al., 1991
; Vankat, Wu, and Foré, 1991
; Foré et al., 1992
). The ability of a plant species to disperse its genes is influenced by its mechanism of pollination, breeding system (Hamrick, Linhart, and Mitton, 1979
; Hamrick, 1983
, 1989
; Loveless and Hamrick, 1984
; Govindaraju, 1988
; Hamrick and Godt, 1989
), and of seed dispersal (Linhart et al., 1981
; Loveless and Hamrick, 1984
). In some species, the effect of limited dispersal can be modified by frugivores (Loveless and Hamrick, 1984
) or by persistence of seeds in the soil seed bank (Templeton and Levin, 1979
; Jefferson and Usher, 1987
). Several empirical studies examining the effect of habitat fragmentation on genetic structure have used species with long-distance dispersal potential. Acer saccharum (Foré et al., 1992
; Young, Merriam, and Warwick, 1993
; Ballal, Foré, and Guttman, 1994
) is wind and bee pollinated with wind-dispersed seed. Asclepias verticillata (Foré and Guttman, 1996
), although insect pollinated, has wind-dispersed seed. These studies have shown little genetic differentiation among remaining habitat patches.
We were interested in comparing the spatial genetic structure of species with different seed dispersal capabilities in a landscape with fragmented habitat. In this study, we examined the spatial genetic structure of Helianthus occidentalis Riddell ssp. occidentalis Riddell (western sunflower) population in the Edge of Appalachia Preserve System in southern Ohio. We compared the genetic structure of this species with data previously reported for Asclepias verticillata, a species with wind-dispersed seed, from the same prairie patches (Foré and Guttman, 1996
). Helianthus occidentalis was selected as a model species because it occurred in the same prairie patches as A. verticillata and was also insect pollinated. However, the seeds have no adaptations to enhance dispersal of seed from the maternal plant. Helianthus occidentalis is a self-incompatible, perennial forb (Heiser et al., 1969
) reported to produce numerous rhizomes and vegetative shoots (Rogers, Thompson, and Seiler, 1982
). We also examined the extent of clonal growth within the preserve as vegetative reproduction has been reported to be a factor in structuring genotypes in clonal species (Ellstrand and Roose, 1987
; Hermanutz, Innes, and Weis, 1989
; McClintock and Waterway, 1993
; Korpelainen and Kolkkala, 1996
). The status of this species in Ohio is threatened, however it is locally abundant. Although the status of this species was not a factor in selecting the model species for this study, data from this study will provide a baseline for monitoring genetic diversity and structure.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Because these communities harbor many rare and endangered species, the Ohio chapter of The Nature Conservancy and the Cincinnati Museum of Natural History have been actively acquiring land and establishing preserves in this area since 1959. Historical information on this area was recently reviewed by Foré and Guttman (1996)
. In this preserve system, there are more than 50 small prairie patches surrounded by forests. Helianthus occidentalis appears to be restricted to these prairie openings. In 1987 and 1989, 29 of the prairie patches in the preserve were inventoried and H. occidentalis was recorded in 22 (data provided by The Nature Conservancy).
Forest encroachment is a major concern in the management of the current prairie patches in the Edge of Appalachia Preserve System. Our interpretation of aerial photographs from 1938, 1958, and 1988 indicates that most of the patches included in this study have decreased in area and become more fragmented (Foré and Guttman, 1996
).
Patch selection and plant sampling
A total of eight patches was selected for this study (Fig. 1). Plant survey data of prairie patches were used to select four regions in which H. occidentalis was reported to occur in more than one patch. Within each region, an initial patch with at least 30 stems was arbitrarily selected. Neighboring patches within 150 m from this initial patch were surveyed for the presence of H. occidentalis. Distances between patches and regions in 1988 and area of the eight patches in 1988 were determined from unrectified, vertical aerial photography at a scale of 1:9600. At the time of sampling, H. occidentalis was only observed in one patch of the Shooting Star and Teakettle regions. (Helianthus occidentalis was observed only in Teakettle North, but Asclepias verticillata was observed in both Teakettle South and North.)
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Additional samples (maximum of 40 plants) were arbitrarily collected in three patches (Lynx 4 and 7 and Shooting Star). These data and data from mapped patches were used to address questions about genetic diversity within and among regions.
Leaves from each plant were individually bagged, labeled, and placed on ice. Leaf material was stored at -70° C in the laboratory. A voucher specimen has been deposited in the Cincinnati Museum of Natural History Herbarium and the Willard Sherman Turrell Herbarium (MU) of Miami University.
Genetic analyses
Starch-gel electrophoresis of allozymes was used to collect genetic data. Preliminary allozyme surveys were used to choose polymorphic enzymes for this study. In preliminary surveys, four individuals were electrophoresed on six buffer systems and stained for 20 enzyme systems. After determining the best buffer system to resolve each enzyme, a more intensive survey of 42 individuals from three different patches was conducted to determine which loci were polymorphic. The polymorphic loci that we were able to resolve and surveyed for all other samples were one locus of glucose-6-phosphate isomerase (GPI-2, E. C. [Enzyme Commission] 5.3.1.9), resolved with Tris-citrate pH 7.2 buffer (Kephart, 1990
) and two loci of phosphoglucomutase (PGM, E. C. 5.4.2.2), resolved with System 11 buffer (Soltis and Soltis, 1987
).
Leaf tissue was ground in microbuffer (Werth, 1985
) enhanced with 5% PVP-40 and 0.1% 2-mercaptoethanol. Horizontal starch (15% w/v, Sigma Chemical Co., St. Louis, Missouri) gel electrophoresis and histochemical staining followed standard procedures (Wendel and Weeden, 1989
; Kephart, 1990
). Genotypes were inferred from stained phenotypes and scored for each plant. Isozymes and alleles were interpreted based on subunit structure (Kephart, 1990
) and scored on the basis of relative mobility (Hickey, Guttman, and Eshbaugh, 1989
; Shaklee et al., 1990
). An internal standard was included so that relative migration values could be used for initial comparisons of alleles from different runs. These comparisons were later confirmed by compiling individuals from different runs onto a single gel.
Statistical analyses
To determine the extent of clonal growth, the total number of different multilocus genotypes and number of stems with the same genotype were determined. The proportion of the total number of observed genotypes with more than one stem and the mean number of samples with the same genotype were calculated for each patch. Least squares linear regression was used to model the relationship between the number of different multilocus genotypes and patch area.
Distance between mapped stems within a patch with the same multilocus genotype was calculated with coordinate data for each pair of stems with the distance formula 
The average distance between putative clones in a patch was calculated with these pairwise distance values.
For each patch, mean sample size per locus (N), mean number of alleles per locus (A), mean observed heterozygosity (Ho, direct-count estimate), and mean Hardy-Weinberg expected heterozygosity (He, unbiased estimate) were computed using BIOSYS-I (Swofford and Selander, 1981
). Significant differences between Ho and He within a patch were determined with t tests. To compare genetic diversity among patches, the proportion of heterozygous loci within individuals of a patch (HLWI) was calculated for each patch, and differences between patches were determined with one-way analysis of variance. Subsequent comparisons of HLWI for patches were evaluated using Fisher's least significant difference procedure (LSD).
Exact tests were used to determine whether observed genotypic frequencies at each locus were significantly different from Hardy-Weinberg predictions. Genotypes were pooled into three classes: (1) homozygotes for the most common allele, (2) common/rare heterozygotes, and (3) rare homozygotes and other heterozygotes. Sequential Bonferroni adjustment of Type I errors (Rice, 1989
) was used to determine significant differences over all genotype tests. The tests for significant differences from Hardy-Weinberg predictions were conducted on the entire data set and a subsample that included only one copy of each different multilocus genotype within a patch. The inbreeding coefficient at each locus (F; Wright, 1965
) was used as a second means of examining biases. A positive F value indicates heterozygote deficiency and a negative value indicates heterozygote excess. To determine whether mean fixation over all populations (FIS) was significantly different from zero, the computation N(FIS)2 was used where N is the total number of individuals, with one degree of freedom in a 
2 distribution (Baker, 1981
). In addition, an inbreeding coefficient was calculated for each patch [F* = (He - Ho)/He, where He and Ho are expected and observed heterozygosities]. Least squares linear regression was used to model the relationship between the number of stems sampled and F*.
Hierarchical analysis of genetic differentiation (Fxy) among patches within a region and among regions was performed using Wright's (1978)
formulation. Genetic distance among patches was calculated using Rogers' (1972)
genetic distance. Unweighted pair-group method with arithmetic averaging (UPGMA) was used to cluster the sites based on Rogers' genetic distance.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Genetic diversity
The mean number of alleles per locus, A, ranged from 3.3 to 4 (Table 4). Mean observed heterozygosity, Ho, ranged from 0.39 to 0.55, and mean expected heterozygosity, He, ranged from 0.49 to 0.60 (Table 4). There was no significant difference between Ho and He within a patch (t 
1.7, df = 4). The proportion of heterozygous loci within individuals in a patch (HLWI) was significantly different among patches (F = 2.69, P = 0.01, df = 7, 392). Catseye 3 has significantly more heterozygous individuals than Teakettle. No patches within the regions Lynx or Catseye have a significantly different proportion of heterozygous individuals than other patches of that region.
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2 = 10.8, df = 1). Fixation indices by locus indicate that Pgm-1 is more homozygous than expected in all patches (Table 5). However, there was no such trend across patches for Gpi or Pgm-2. Within a patch there was no clear trend for fixed homozygosity at all loci. The inbreeding coefficient for each patch, F*, ranged from 0.038 to 0.348 (Table 5), indicating that the level of inbreeding was relatively low to moderate, but there was no linear relationship between F* and the number of individuals sampled [predicted F* = 0.139 + 0.0012 (number of individuals sampled); r2 = 0.063, P = 0.54, df = 1, 6].
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Raw allele frequency data indicated that no major allele at a locus was common over all patches (Table 6). No alleles were unique to a region. However, there were alleles unique to a patch within a region; Gpi-2a was observed only in Lynx 7 of the Lynx region, and Pgm-1e and Pgm-2c were observed only in Catseye 2 of the Catseye region. Alleles that were missing in some patches were generally rare alleles. Within the Lynx and Catseye regions, allele frequencies of at least one locus were significantly different among patches within a region (all pairwise values of P < 0.0001). Allele frequencies of at least two loci were significantly different among regions (all pairwise values of P < 0.003).
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) indicated that most (>90 %) of the variation of Helianthus occidentalis in the total sampling range occurred within patches with little variation among all patches (Fxy = 0.098). There was more genetic differentiation among patches within a region (Fxy = 0.084) than among regions (Fxy = 0.016). This distribution of genetic differences is further supported by Rogers' (1972)
genetic distance measures (Table 7). These measures indicated that genetic differentiation between patches in different regions was not greater than between patches within a region even though geographic distance is substantially less. Lynx 3, 4, and 5 were the least differentiated patches (Fig. 2). Lynx 7 was highly differentiated from other patches in the Lynx region and was most similar to Catseye 3. Patches in the Catseye region were also more similar to patches in other regions than to each other.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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).
Our data further indicate that the interpretation of clone size is scale dependent. Sampling at 1-m intervals along transects 5 m apart suggested that clones could have been as much as 5 m in diameter. However, smaller scale sampling identified a minimum of four different genotypes in a 0.6-m2 area. Sipes and Wolf (1997)
also found that samples of Cyclandenia humilis var. jonesii collected 5 m apart often shared the same nine-locus genotype, but when sampled at a smaller scale data suggested that clones can be large but are interdigitated. The intermingling of genets has been reported for other species such as Fragaria chiloensis (Alpert, Lumaret, and Di Guisto, 1993
).
Further examination of clonal structure at a smaller geographic scale and of possible effects of fire are needed to better understand the clonal growth strategy and genetic structure of H. occidentalis within a patch. Additional markers would provide greater confidence in identifying genets, as increasing the number of loci sampled increases the probability of identifying separate genotypes.
Spatial genetic structure
Genetic differentiation of Helianthus occidentalis in the Edge of Appalachia Preserve System is greater among patches within a region (separated by 11–143 m) than among regions (1.8–7.8 km). These findings are consistent with data from a study at a similar scale of Clarkia springvillensis, a bee-pollinated species with no obvious seed dispersal mechanism (McCue, Buckler, and Loltsford, 1996
). Genetic data from Asclepias verticillata occurring in the same study patches as Helianthus occidentalis indicated a different genetic structure (Foré and Guttman, 1996
). Asclepias verticillata showed less total genetic differentiation among patches (Fxy = 0.033) with most of the differentiation among region (Fxy = 0.026) and much less of this variation occurring between patches within a region (Fxy = 0.007).
The genetic, environmental, and life history data support the hypothesis that genetic differences between patches within regions could be attributed to different seed dispersal capabilities. Asclepias verticillata, a species with wind-dispersed seeds, has greater dispersal capabilities than H. occidentalis, and gene exchange is less likely to be restricted between patches within a region. In addition, the number and density of A. verticillata stems are small (50 stems/patch). Although these two factors are predicted to increase divergence among populations due to drift (Loveless and Hamrick, 1984
), low population size and density may increase pollinator foraging distance (Levin and Kerster, 1969
; Handel, 1983
; Loveless and Hamrick, 1984
), and in small populations any migration is likely to have a larger effect (Handel, 1983
; Loveless and Hamrick, 1984
). Helianthus occidentalis has no seed dispersal adaptations, and the number and density of stems within each patch are large (>100 stems/patch). These features likely restrict gene flow even over a short distance and contribute to the high levels of genetic differentiation among patches within a region.
We hypothesize that the habitat fragmentation that has occurred within the preserve has increased genetic differentiation of H. occidentalis among patches on a small geographic scale. We observed large genetic differences between patches in the Catseye region even though these two patches have the smallest geographical separation in our study. However, these two patches have been fragmented since at least 1938. The three patches with the greatest genetic similarity, Lynx 3, 4, and 5, were the only patches in this study that were continuous in 1938 (but fragmented by 1958). Genetic differences between the patches in this study probably resulted from a combination of reduced gene flow among patches remaining after fragmentation and founder effects.
Founder effect is another possible factor contributing to the observed genetic differences in H. occidentalis among patches as it is likely that some of these populations have re-established after release from human use. The data show that there is an absence of a single common allele throughout the study area and in most cases missing alleles within a patch are relatively rare alleles. Marked genetic differences among populations have been attributed to founder effects in several studies (e.g., Linhart and Premoli, 1994
; Wang, Wendel, and Dekker, 1995
; Hansen and Mensberg, 1996
). Although low levels of genetic variability have been reported in some founding populations (e.g., Baker and Moeed, 1987
; Linhart and Premoli, 1994
; Wang, Wendel, and Dekker, 1995
), founder events do not necessarily reduce genetic variability (Barrett and Kohn, 1991
). For example, descendent populations of a rare butterfly, Plebejus argus, had a lower number of alleles than the source population, but these losses were of rare alleles and the effects on genetic diversity measures were relatively mild (Brookes et al., 1997
). Although H. occidentalis has a high degree of genetic differentiation among patches, there is still a high level of variation within a patch and little evidence of inbreeding within patches. Genetic diversity (Ho) and the mean number of alleles per locus (A) were greater in H. occidentalis than in Asclepias verticillata from the same study sites (Foré and Guttman, 1996
).
Although H. occidentalis appears to be doing well in the preserve system, the data from this study provides us with a baseline from which to monitor genetic diversity and structure of H. occidentalis. If genetic diversity decreases and genetic differentiation increases among patches within a region, this would signal that gene flow has been altered. Management plans would then want to more actively enact strategies to increase gene flow, such as creating corridors between patches to prevent further loss of genetic variation, which may decrease the viability of the species. In addition, the effect of burning on reproductive strategies used by the species needs to be explored further if fire continues to be a management tool.
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FOOTNOTES |
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4 Author for correspondence (sfore{at}truman.edu
).
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LITERATURE CITED |
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