MAINTENANCE OF ECOLOGICALLY SIGNIFICANT GENETIC VARIATION IN THE TIGER SWALLOWTAIL BUTTERFLY THROUGH DIFFERENTIAL SELECTION AND GENE FLOW

Differential selection in a heterogeneous environment is thought to promote the maintenance of ecologically significant genetic variation. Variation is maintained when selection is counterbalanced by the homogenizing effects of gene flow and random mating. In this study, we examine the relative importance of differential selection and gene flow in maintaining genetic variation in Papilio glaucus. Differential selection on traits contributing to successful use of host plants (oviposition preference and larval performance) was assessed by comparing the responses of southern Ohio, north central Georgia, and southern Florida populations of P. glaucus to three hosts: Liriodendron tulipifera, Magnolia virginiana, and Prunus serotina. Gene flow among populations was estimated using allozyme frequencies from nine polymorphic loci. Significant genetic differentiation was observed among populations for both oviposition preference and larval performance. This differentiation was interpreted to be the result of selection acting on Florida P. glaucus for enhanced use of Magnolia, the prevalent host in Florida. In contrast, no evidence of population differentiation was revealed by allozyme frequencies. F_ST-values were very small and Nm, an estimate of the relative strengths of gene flow and genetic drift, was large, indicating that genetic exchange among P. glaucus populations is relatively unrestricted. The contrasting patterns of spatial differentiation for host-use traits and lack of differentiation for electrophoretically detectable variation implies that differential selection among populations will be counterbalanced by gene flow, thereby maintaining genetic variation for host-use traits.     

Capturing Genetic Variation during Ecological Restorations: An Example from Kankakee Sands in Indiana

Genetic variation in populations, both natural and restored, is usually considered crucial for response to short‐term environmental stresses and for long‐term evolutionary change. To have the best chance of successful long‐term survival, restored populations should reflect the extant variation found in remnants, but restored sites may suffer from genetic bottlenecks as a result of founder effects. Kankakee Sands is a large‐scale restoration being conducted by The Nature Conservancy (TNC) in northwestern Indiana. Our goal was to test for loss of genetic variation in restored plant populations by comparing them with TNC’s seed source nursery and with local remnant populations that were the source of nursery seed and of the first few restored sites. Allozyme analysis of Baptisia leucantha, Asclepias incarnata, Coreopsis tripteris, and Zizia aurea showed low levels of allozyme diversity within all species and reductions in polymorphism, alleles per locus, and expected heterozygosity between remnants and restorations for all species except A. incarnata. Almost all lost alleles were rare; restored populations contained almost 90% of alleles at polymorphic loci that occurred in remnants at frequencies greater than 1%. Allele frequencies for most loci did not differ between remnants and restored sites. Most species showed significant allele frequency differentiation among remnant populations and among restored sites. Our results indicate that seed collection techniques used at Kankakee Sands captured the great majority of allozyme variation present in seed source remnant populations.     

Allozyme variation in the diploid (A genome) populations ofScilla scilloides (Hyacinthaceae)

Allozyme variation at eleven loci encoding seven enzyme systems were examined in 20 populations of diploid (genome AA, 2n = 16)Scilla scilloides in China. In comparison with the average species of seed plants studied, populations of this species display a high amount of genetic variation (A = 2.0, P = 58.6%, H_o = 0.172, and H_e = 0.185). Allozyme variation pattern revealed predominant outcrossing within populations and considerable differentiation (F_ST = 0.314) among populations as well as between the subtropic and temperate regions. The wide distribution, long existence and outcrossing are presumably the main factors responsible for the high genetic diversity within populations. But the gravity dispersal of seeds and pollination by small insects set limits to the increase of genetic variation within populations and promote differentiation between populations and regions. In addition, allozyme variation does not distinguishS. scilloides var.albo-viridis and suggests that subtropic populations may be considered as a genetic entity.     

Biochemical assessment of the taxonomic diversity of the operculate land snail, Cyclophorus fulguratus (Gastropoda: Cyclophoridae), from Thailand

Allozyme variation was examined in 223 samples of the operculate land snail Cyclophorus fulguratus from 13 localities across three regions of Thailand. Using horizontal starch gel electrophoresis, 13 allozyme presumed loci (12 polymorphic) were screened. Heterozygosity was moderate in C. fulguratus (H_exp = 0.008–0.127) with a high genetic heterogeneity among samples (F_st = 0.734). Populations showed a greater genetic differentiation in central Thailand (F_st = 0.380) than in northeastern Thailand (F_st = 0.108), suggesting frequent gene flow among populations in northeastern Thailand. C. fulguratus exhibits a strong pattern of isolation by distance over the entire tested species range in Thailand and may potentially have been involved in an extensive local fragmentation. Results of the distance analysis revealed that large genetic divergence has occurred among the central, northeastern and eastern Thailand groups [D = 0.361–0.701], strongly suggesting populations from these three geographical regions may actually represent or else be evolving into separate species.     

POPULATION VARIATION,OUTCROSSING, AND COLONIZATION OF DISTURBED AREAS BY CALAMAGROSTIS CANADENSIS: EVIDENCE FROM ALLOZYME ANALYSIS

Calamagrostis canadensis (a rhizomatous grass) exists in temperate forest sites of different successional age. It can rapidly colonize disturbed sites to form dense swards. We examined allozyme variation in: four populations (mature forest, intermediate aged forest, forest cutblock, wetland); nine small plots (2 m × 4 m) within the cutblock; and progeny of several families from three populations; in order to assess the mode of colonization of disturbed areas and the effect of successional changes on population genetic structure. All four populations showed equal and extensive genetic variation (1.5 to 1.7 alleles per locus [K], 41.7% to 50% polymorphic loci [PPL], H_st = 0.155 to 0.208) and were not genetically differentiated (G_st = 0.0193, 1 = 0.986 to 0.997). The cutblock subpopulations also showed considerable genetic variation (K = 1.6 to 1.8, PPL = 50% to 58.3%, H_st = 0.151 to 0.278) and no microdifferentiation (G_st = 0.034, I = 0.967 to 0.997). We found 14 different genotypes among the 30 individuals sampled from the cutblock as a whole (based on five polymorphic loci). The cutblock subpopulations had from nine to 14 different genotypes each (same five loci, 18 individuals per subpopulation). Seed produced was primarily outcrossed (multilocus estimate 0.888 to 0.900). We concluded that disturbed sites are colonized primarily by sexually produced seedlings. Potential genetic drift and natural selection, which occur during subsequent successional changes, do not result in reduced genetic variation or population differentiation.     

Evolutionary implications of allozyme and RAPD variation in diploid populations of dioecious buffalograss Buchloë dactyloides

Buffalograss, Buchloë dactyloides, is widely distributed throughout the Great Plains of North America, where it is an important species for rangeland forage and soil conservation. The species consists of two widespread polyploid races, with narrowly endemic diploid populations known from two regions: central Mexico and Gulf Coast Texas. We describe and compare the patterns of allozyme and RAPD variation in the two diploid races, using a set of 48 individuals from Texas and Mexico (four population samples of 12 individuals each). Twelve of 22 allozyme loci were polymorphic, exhibiting 35 alleles, while seven 10-mer RAPD primers revealed 98 polymorphic bands. Strong regional differences were detected in the extent of allozyme polymorphism: Mexican populations exhibited more internal gene diversity (H_e= 0.20, 0.19) than did the Texan populations (H_e= 0.08, 0.06), although the number of RAPD bands in Texas (n= 62) was only marginally smaller than in Mexico (n= 68). F-statistics for the allozyme data, averaged over loci, revealed strong regional differentiation (mean F_RT=+ 0.30), as well as some differentiation among populations within regions (mean F_PR=+ 0.09). In order to describe and compare the partitioning of genetic variation for multiple allozyme and RAPD loci, we performed an Analysis of Molecular Variance (AMOVA). AMOVA for both allozyme and RAPD data revealed similar qualitative patterns: large regional differences and smaller (but significant) population differences within regions. RAPDs revealed greater variation among regions (58.4% of total variance) than allozymes (45.2%), but less variation among individuals within populations (31.9% for RAPDs vs. 45.2% for allozymes); the proportion of genetic variance among populations within regions was similar (9.7% for RAPDs vs. 9.6% for allozymes). Despite this large-scale concordance of allozyme and RAPD variation patterns, multiple correlation Mantel techniques revealed that the correlations were low on an individual by individual basis. Our findings of strong regional differences among the diploid races will facilitate further study of polyploid evolution in buffalograss.     

Genetic Diversity and Structure in Austrocedrus chilensis Populations: Implications for Dryland Forest Restoration

In South America, 94% of dry‐temperate lands present some degree of environmental degradation, highlighting the need for ecological restoration. We analyzed geographic patterns of genetic variation in Austrocedrus chilensis, a dominant conifer of the steppe‐forest ecotone in the eastern Andes, to examine its potential for restoration. We sampled 67 locations in Argentina and estimated genetic parameters to determine the effects of historical factors affecting diversity, together with inbreeding and gene flow, using 12 allozyme loci. Genetic diversity decreased southwards in eastern populations, which are marginal for the range of the species and patchily distributed, while high genetic admixture was detected in continuous western populations, possibly reflecting postglacial migrations from northern and eastern sources. Higher inbreeding (F_IS > 0.14) was recorded in northern compared with southern populations, attributed to the impact of recent bottlenecks resulting from anthropogenic fires. Gene flow was found to be moderate overall (F_ST = 0.12). The implications of these results for restoration actions focusing on Austrocedrus were explored. Relatively small, inbred yet genetically diverse northern populations should be the subject of passive restoration efforts, while experimental common gardens should be established toward the south, to support active restoration approaches. This illustrates how ahead of time information on patterns of genetic variation can support restoration efforts for dryland tree species.     

Microsatellite polymorphism and genetic differentiation in three Norwegian populations of Elymus alaskanus (Poaceae)

 Variation at seven microsatellite loci was investigated in three local E. alaskanus populations from Norway and microsatellite variation was compared with allozyme variation. The percentage of polymorphic loci was 81%, the mean number of alleles per polymorphic locus was 5.7 and expected heterozygosity was 0.37. An F-statistic analysis revealed an overall 48% deficit of heterozygotes over Hardy-Weinberg expectations. Gene diversity is mainly explained by the within population component. The averaged between population differentiation coefficient, F _st , over 7 loci is only 0.13, which accounts for only 13% of the whole diversity and was contrary to allozyme analysis. The mean genetic distance between populations was 0.12. However, a χ² -test showed that allele frequencies were different (p < 0.05) among the populations at 5 of the 7 loci. In comparison with the genetic variation detected by allozymes, microsatellite loci showed higher levels of genetic variation. Microsatellite analysis revealed that population H10576 possesses the lowest genetic variation among the tested three populations, which concur with allozyme analysis. The dendrogram generated by microsatellites agreed very well with allozymic data. Our results suggest that natural selection may be an important factor in shaping the genetic diversity in these three local E. alaskanus populations. Possible explanations for deficit heterozygosity and incongruence between microsatellites and allozymes are discussed. Received November 6, 2001; accepted April 24, 2002 Published online: November 14, 2002 Addresses of the authors: Genlou Sun (e-mail: Genlou.sun@STMARYS.CA), Biology Department, Saint Mary's University, Halifax. Nova Scotia, B3H 3C3, Canada. B. Salomon, R. von Bothmer, Department of Crop Science, The Swedish University of Agricultural Sciences, P.O. Box 44, SE-230 53, Alnarp, Sweden.     

POPULATION GENETIC STRUCTURE AND GENE FLOW ACROSS ARID VERSUS MESIC ENVIRONMENTS: A COMPARATIVE STUDY OF TWO PARAPATRIC SENECIO SPECIES FROM THE NEAR EAST

To shed light on the potential effects of xeric/arid versus mesic environments on plant population genetic structure and patterns of gene flow, we have compared allozyme and cpDNA haplotype variation in populations of two closely related, highly outcrossed, and largely wind-dispersed winter annuals of Senecio (Asteraceae). The species form a distinctive zone of parapatric distribution in the Near East by differing in their ecogeographical regimes. Senecio vernalis mainly thrives in the mesic Mediterranean life zone of Israel, whereas S. glaucus inhabits either xeric maritime or arid (semi-) desert sites. Significant differences in allozymic population subdivision among S. vernalis (θ_n = 0.04; Nm_n = 5.85) and S. glaucus (θ_n = 0.12; Nm_n = 1.85) largely resulted from topogeographical substructuring present within the latter species. Because of the similarity of within-region estimates of population structure for S. glaucus with those measured among populations of S. vernalis, it appears unlikely that ecological “aridity” factors per se are important in influencing levels of population differentiation in these species. Based on hierarchical F-statistics and tests of isolation by distance, we further conclude that geographical topologies influence the level and mode of nuclear gene flow (via pollen and/or seed) among and within subsets of S. glaucus populations, although without providing a complete barrier to interregional dispersal (dNm_reg = 2.16) and without promoting allopatric differentiation via drift. The allozymic data further suggested that S. vernalis and S. glaucus form a zone of secondary contact in the Near East, accompanied by an almost complete interspecific barrier to nuclear gene flow (dnNm_sp = 0.253). However, to account for the considerable sharing of cpDNA haplotypes, both at the intra- and interspecific level, it is necessary to invoke either (1) selection acting against alien nuclear but not cytoplasmic DNA; or (2) the sporadic immigration of cpDNA via seed with large homogenizing effects on cytoplasmic population structure over time.     

Behavioral responses of Ceratitis capitata flies to bait spray droplets and natural food

We usedF-statistics to quantify the population structure of two sympatric species of leaf beetles,Oreina cacaliae andO. speciosissima (Chrysomelinae, Coleoptera), which share the same microhabitat since they feed on the same herbaceous host plants. We measured genetic differentiation at six allozyme loci 1) among populations separated by relatively small distances (40 to 250 kilometers), 2) within each population, and 3) between sexes within populations. For both species, the populations studied are not panmictic. For each population, heterozygosities are relatively high, but the observed heterozygosities are generally lower than the expected values. Overall, within-population differentiation is high and similar for both species (F _is=0.326 forO. cacaliae and 0.332 forO. speciosissima). Additionally, populations of both species are highly differentiated (F _st=0.234 versus 0.051 forO. speciosissima). ForO. cacaliae,F _is andF _st are greater among females than among males, while forO. speciosissima,F _st is sustantially greater among the males whileF _is is slightly greater among males. Differences in gene frequency among the sexes were statistically tested using a modifiedF _st with sex as the defining category, and the sexes differed significantly with the exception of one population inO. cacaliae. Possible explanations for this difference are discussed.     

Do seed transfer zones for ecological restoration reflect the spatial genetic variation of the common grassland species Lathyrus pratensis?

A common ecological restoration approach is the reestablishment of vegetation using seed mixtures. To preserve the natural genetic pattern of plant species local seed material should be used. Consequently, seed transfer zones (seed production areas and seed provenance regions) have been delineated for ecological restoration in Germany. Although it is assumed that these transfer zones represent genetic variation, there remains a lack of empirical data. In this study, we analyzed whether seed transfer zones reflect the genetic variation of the common grassland species Lathyrus pratensis. We sampled 706 individuals from 37 populations in Bavaria, Germany and analyzed genetic variation using amplified fragment length polymorphisms. In our study, we observed higher levels of genetic variation and fragment rarity in the southern Bavarian populations compared to northern populations. Our analyses revealed a strong genetic differentiation between southern and northern Bavarian populations delineated along the Danube River. However, seed production areas and seed provenance regions reflected genetic variation of L. pratensis only to a limited degree. Our study illustrates that the level of genetic variation within populations strongly depends on population history. Furthermore, the geomorphological and climatic attributes, which have been used to delineate seed provenance regions, do not reduce gene flow among populations. Seed collections for gene banks and seed production should comprise seeds from populations in southern and northern Bavaria representing the strong genetic variation between both regions, but prioritize southern populations due to higher levels of variation.     

Genetic resources of wild barley in the Near East: structure, evolution and application in breeding

Genetic diversity and structure of populations of the wild progenitor of barley, Hordeum spontaneum, from three countries, Israel, Turkey and Iran, in the Near East Fertile Crescent, are compared and contrasted. The analysis is based on electrophoretically discernible allozymic variation in proteins encoded by 27 shared loci in 2125 individuals representing 52 populations of wild barley. The results indicate that: (a) H. spontaneum in the Near East Fertile Crescent is very variable genetically; (b) genetic differentiation of populations includes some clinal but primarily regional and local patterns often displaying sharp geographic differentiation over short distances; (c) the average relative genetic differentiation (G_st) was 54% within populations, 39% among populations, and 8% between the three countries; (d) allele distribution is characterized by a high proportion of unique alleles (51%), and a high proportion of common alleles that are distributed either locally or sporadically; (e) discriminant analysis by allele frequencies successfully clustered wild barley of each of the three countries (96% correct classification); (f) a substantial portion of the patterns of allozyme variation in the wild gene pool is significantly correlated with the environment and is predictable ecologically, chiefly by a combination of humidity and temperature variables; (g) natural populations of wild barley are, on the average, more variable than two composite crosses and land races of cultivated barley. The spatial patterns and environmental correlates and predictors of genetic variation of H, spontaneum in the Fertile Crescent indicate that genetic variation in wild barley populations is not only rich but at least partly adaptive and predictable by ecology and allozyme markers. Consequently, conservation and utilization programmes should optimize sampling strategies by following the ecological-genetic factors and allozyme markers as effectively predictive guidelines.     

Amplified fragment length polymorphism (AFLP) analysis of the genetic structure of the zebra mussel, Dreissena polymorpha, in the Mississippi River

1. We predicted that zebra mussel, Dreissena polymorpha (Pallas), genetic structure in the Mississippi River would follow a model of invasive species genetics, which predicts low genetic structure among populations of recently established species. This prediction was upheld in our previous genetic study using allozymes, however, one locus yielded anomalous results. 2. We employed amplified fragment length polymorphism (AFLP) analysis as a neutral marker to assess the amount of genetic structure within and among populations, and as a test of expected population structure from both invasion genetic theory, and the results from our previous study. 3. There was greater spatial differentiation, as measured by F_st, observed using AFLP's than for allozymes (P < 0.001). There was no evidence that AFLP variation conformed to an isolation by distance model, and genetic relationships of populations, as measured by AFLP markers, were not similar to those detected in our allozyme survey. 4. The lack of concordance between these two genetic marker systems probably reflects their differential responses to drift, migration, and selection occurring during this rapid invasion. Strong population structure is counter to predictions that populations of invasive species will not be differentiated, as with observations based on allozyme markers. Therefore, newly established species may require genetic surveys using multiple marker systems to evaluate population structure.     

Genetic variation and differentiation in Nordic populations of Elymus alaskanus (Scrib. ex Merr.) Löve (Poaceae)

 To gain information on the extent and nature of genetic variation in Elymus alaskanus, levels and distribution of genetic variation were assessed within and among 13 populations originating from Iceland, Norway, Sweden and Russia using allozymes. The results showed that four (30.7%) of the 13 loci were polymorphic within the species, while the mean percentage of polymorphic loci within the populations was 1.9%. The mean number of alleles per locus for the species was 1.8 and 1.02 across the populations. Genetic diversity at the species level was low (H _es=0.135), and mean population diversity was notably lower (H _ep=0.005). A high degree of genetic differentiation was observed among populations. The salient points emerging from this study are: (1) statistically significant differences were found in allele frequencies among populations for every polymorphic locus (P<0.001), (2) the high mean coefficient of gene differentiation (G _ST) showed that 95% of the total allozyme variation was attributable to differences among populations, and (3) relatively high genetic distances between the populations were obtained (mean D=0.16). The Norwegian populations had the highest genetic diversity as compared with the other populations. Geographical comparisons revealed three different groups of populations clearly differentiated, i.e. Scandinavia (Norway and Sweden), Iceland and Russia. Cluster and principal coordinates analyses revealed the same genetic patterns of relationships among populations. Generally, this study indicates that E. alaskanus contains low allozymic variation in its populations. The implications of these results for the conservation of the species are discussed. Received: 23 October 1998 / Accepted: 19 December 1998     

High population differentiation and genetic variation in the endangered Mexican pine Pinus Rzedowskii (Pinaceae)

Pinus rzedowskii is an endangered pine species from Michoaca´n (central Me´xico), which has been previously reported from only three localities. Classified within the subgenus Strobus, it exhibits intermediate morphological characters between subgenera Strobus and Pinus. We analyzed genetic aspects that could shed light on the evolution and conservation of this species. The genetic structure of nine populations was examined using 14 isozyme loci. Pinus rzedowskii has a relatively high level of genetic variation with 46.8% of the loci assayed being polymorphic, a total of 35 alleles, and a mean heterozygosity per population of 0.219. We calculated Wright's F_ST statistic to estimate gene flow indirectly and to evaluate whether or not there was genetic structuring among populations. We found a marked differentiation among populations (F_ST = 0.175) and significant inbreeding (F_IS = 0.247). No pattern of isolation by distance was found. We also constructed a dendrogram based on a genetic distance matrix to obtain an overview of the possible historical relationships among populations. Finally, we found a convex relationship between the genetic distance among populations and the number of ancestral lineages, suggesting that demographically this species has not been at risk recently. Although endangered, with small and fragmented populations, P. rzedowskii shows higher levels of genetic variation than other conifer species with larger populations or similar conservation status.     

Analysis of F ST outliers at allozyme loci in Pacific salmon: implications for natural selection

Natural selection has been invoked to explain the observed geographic distribution of allozyme allele frequencies for a number of teleost species. The effects of selection on allozyme loci in three species of Pacific salmon were tested. A simulation-based approach to estimate the null distribution of population differentiation (F _ST) and test for F _ST outliers was used. This approach showed that a majority of allozyme loci conform to neutral expectations predicted by the simulation model, with relatively few F _ST outliers found. No consistent F _ST outlier loci were found across species. Analysis of population sub-groups based on geography and genetic identity reduced the number of outlier loci for some species, indicating that large geographic groups may include genetically divergent populations and/or that there is geographic heterogeneity in selection pressure upon allozyme loci. Two outlier allozyme loci found in this analysis, lactate dehydrogenase-B and malic enzyme, have been shown to be influenced by selection in other teleost species. This approach is also useful in identifying allozyme loci (or other genetic markers) that meet assumptions for population genetic study.     

Genetic diversity of Typha latifolia (Typhaceae) and the impact of pollutants examined with tandem-repetitive DNA probes

Genetic diversity at variable-number-tandem-repeat (VNTR) loci was examined in the common cattail, Typha latifolia (Typhaceae), using three synthetic DNA probes composed of tandemly repeated “core” sequences (GACA, GATA, and GCAC). The principal objectives of this investigation were to determine whether: (1) the previously reported almost complete lack of polymorphism at allozyme loci in this species was indicative of a reduced amount of genetic diversity at VNTR loci as well; (2) VNTR markers were informative about possible clonal propagation; and (3) significant differences in genetic structure of sampling sites were associated with differences in environmental levels of pollutants at those sites. Previously, widespread sampling across the eastern United States, surveying across ten allozyme loci, has detected only two genotypes, involving a difference at a single locus, among 104 populations. In this study, the amount of genetic diversity detected at VNTR loci: (1) among ramets (N = 40; 40 genotypes detected) collected at ∼8-km intervals along a 320-km transect; (2) among ramets (N = 220; 117 genotypes detected) from five study sites separated by 50–3000 m; and (3) even among ramets within each study site [N = 44 per site; from 13 to 34 genotypes detected per site (270 m²)] exceeds that previously found in those more geographically widespread allozyme surveys. Among the 260 ramets analyzed here, the mean number of bands scored per individual was 48.61 (SD = 2.80). Mean genetic similarity among ramets collected along the 320-km transect was 0.91, which was within the range of mean genetic similarity within the five study sites (range: 0.89–0.95). Among the five study sites, 61% of the samples analyzed appeared to be clonal ramets, with up to 12 clones detected for 44 ramets sampled within a site. Clones grew intermingled and ranged up to 39 m in extent. Permutation tests of genetic similarity revealed significant genetic differentiation between each of the five study sites. Consistent with the previous allozyme studies, T. latifolia was characterized by extremely low genetic variation relative to levels of polymorphism detected at VNTR loci in other plant species. Estimated heterozygosity among ramets along the 320-km transect ranged from 0.11 to 0.13, while that within the five study sites ranged from 0.05 to 0.12. Estimates of F_st (0.32–0.41) also indicated considerable genetic subdivision among these stands. Significantly higher genetic diversity was detected at the two study sites that chemistry and toxicity data indicate to be the most severely impacted by pollutants. Although this correlation does not establish cause and effect, the results of this study indicate that the analysis of genetic diversity at VNTR loci may be a useful tool for monitoring anthropogenic-induced changes in the genetic structure of natural populations of plants.     

THE RELATIVE IMPORTANCE OF HISTORICAL EVENTS AND GENE FLOW ON THE POPULATION STRUCTURE OF A MEDITERRANEAN RAGWORT,SENECIO GALLICUS (ASTERACEAE)

Comparisons of cytoplasmic and nuclear diversity within and among natural plant populations have the potential to distinguish the relative influences of seed and pollen dispersal on contemporary gene flow, or alternatively, may permit inferences of the colonization history of a species via seed. We examined patterns of cpDNA and allozyme variation in Senecio gallicus, a diploid, annual plant that occurs in both coastal and ruderal inland areas of the Iberian Peninsula and southern France. The species appears to have a strong propensity for long-distance seed dispersal. Five cpDNA haplotypes were found by RFLP analysis among a sample of 111 individuals derived from 11 populations. Differences in haplotype frequencies across populations were most evident with respect to a dramatic increase in the frequency of a derived haplotype from coastal to inland localities. The level of cpDNA differentiation among populations within the inland group (θ₀ = 0.07) was significantly less than that seen within the coastal group (θ₀ = 0.41). In contrast, for allozymes, no significant difference in population structure was evident between collections from coastal and inland habitats. At the rangewide geographic scale, there was only a very weak association between inferred levels of gene flow and geographic distance for cpDNA, and no such association was found for allozymes. It appears that while seed movement in the species might be sufficiently great to disturb the pattern of isolation by distance for cpDNA, it cannot fully account for the nearly randomized spatial structure at polymorphic allozyme loci. It is suggested that isolation of populations in Atlantic-Mediterranean coastal refugia during previous glacial maxima, and the effects of subsequent colonization events in inland areas, have had an important effect on molding the present genetic structure of the species.     

Geographic Influence on Genetic Structure in the Widespread Neotropical Tree Simarouba amara (Simaroubaceae)

This study aimed to assess the population genetic structure of a widespread Neotropical tree species, Simarouba amara, at local, regional and continental spatial scales. We used five microsatellite loci to examine genetic variation in 14 natural populations (N?=?478 individuals) of this vertebrate-dispersed rain forest tree species in Panama, Ecuador, and French Guiana. Estimates of genetic differentiation (F _st and R _st) were significant among all but one population pair and global differentiation was moderate (F _st?=?0.25, R _st?=?0.33) with 94% of genetic variation ascribed to differences among three main geographic regions (Central America, Western Ecuador, Amazon basin). There was no evidence of isolation by distance within regions. Allele-size mutations contributed significantly (R _ST > F _ST) to the divergences between cis- and trans-Andean populations, highlighting the role of the northern Andean cordilleras as an important geographic barrier for this species.