GENETIC EFFECTS OF GERMINATION TIMING AND ENVIRONMENT: AN EXPERIMENTAL INVESTIGATION

Seeds of many species do not germinate immediately after dispersal, but instead may remain indefinitely in a dormant but viable state. Although it is well established that seeds often exhibit diversified patterns of dormancy and germination, the causes and consequences of this variation remain poorly understood. In this study, we investigate the extent to which seed genotypes of the desert mustard Lesquerella fendleri differentially germinate and establish under experimental conditions in a greenhouse. We used a two-way factorial design to compare genotypes of Lesquerella plants derived from seeds that germinated and established at different times and under different soil water regimes. Overall allozyme allele frequencies of Lesquerella plants varied significantly with both germination time and initial soil water availability. Single-locus heterozygosity analyses revealed that seeds sown into initially low water conditions produced plants that were significantly more heterozygous than plants derived from seeds experiencing constantly high water conditions, but heterozygosity did not differ significantly among plants originating from early- and late-germinating seeds. This is the first study to experimentally demonstrate that germination timing and environment can significantly affect the genetic structure of emerging plant populations. The study suggests that germination and survival behavior may (1) play an important role in generating and maintaining the genetic structure of natural plant populations and (2) set the stage for subsequent evolution.     

A LIFE-HISTORY BASED STUDY OF POPULATION GENETIC STRUCTURE: SEED BANK TO ADULTS IN PLANTAGO LANCEOLATA

We explored the extent to which the soil seed bank differed genetically and spatially in comparison to two actively growing stages in a natural population of Plantago lanceolata. All seed-bank seeds, seedlings, and adults of P. lanceolata within eight subunits in a larger population were mapped, subjected to starch gel electrophoresis, and allozyme analysis in 1988. Gel electrophoresis was also used to estimate the mating system in two years, 1986 and 1988. The spatial distributions of seeds, seedlings, and adults were highly coincident. Allele frequencies of the dormant seeds differed significantly from those of the adults for four of the five polymorphic loci. In addition, a comparison of the genotype frequencies of the three life-history stages indicated that the seed bank had an excess of homozygotes. Homozygosity, relative to Hardy-Weinberg expectations, decreased during the life cycle (for seed bank, seedlings, and adults respectively: F_it = 0.19, 0.09, 0.01; F_is = 0.14, 0.04, -0.12). Spatial genetic differentiation increased sixfold during the life cycle: (for seed bank, seedling and adults: F_s1??? = 0.02, 0.05, 0.12). The apparent selfing rate was 0.01 in 1986 and 0.09 in 1988. These selfing rates are not large enough to account for the elevated homozygosity of the seed bank. Inbreeding depression, overdominance for fitness, and a “temporal Wahlund's effect” are discussed as possible mechanisms that could generate high homozygosity in the seed bank, relative to later life-history stages. In Plantago lanceolata, the influence of the mating system and the “genetic memory” of the seed bank are obscured by the time plants reach the reproductive stage.     

MATING SYSTEM AND GENETIC STRUCTURE OF THE DISTYLOUS TROPICAL TREE PSYCHOTRIA FAXLUCENS (RUBIACEAE)

The population genetics and mating system of the understory tropical rain forest tree Psychotria faxlucens were studied at two plots in Los Tuxtlas, Veracruz, México, on the Gulf of México coast. This species is distylous and is pollinated by moths, mainly small hawkmoths. The seeds are dispersed by gravity and by frugivorous birds. Controlled pollinations indicate that the trees are self-incompatible and that intramorph pollinations have lower compatibilities than intermorph pollinations. The pollen: ovule ratio is high, suggesting obligate xenogamy. Using electrophoretic allozyme methods we found that eight out of 20 loci were polymorphic (P = 0.400), the observed heterozygosity (H) was 0.198, and the mean expected heterozygosity (H) was 0.495, both relatively high values compared with that reported for tropical trees. The genetic differentiation between the two plots is low, as shown by the heterogeneity in allele frequencies and the F_st (mean F_st seedlings = 0.031, mean F_st adults = 0.026), although for some loci the plot differentiation is statistically significant. The studied populations are near Hardy-Weinberg proportions, both for seedlings (mean F = 0.128) and adults (mean F = 0.075). From the fixation rate, an indirect estimate of the outcrossing rate at equilibrium gave a mean of t = 0.898 for plot 1 and 0.685 for plot 2. Direct single loci and multiloci outcrossing rate estimates were generally not statistically different from 1.0.     

POPULATION GENETIC STRUCTURE OF CECROPIA OBTUSIFOLIA,A TROPICAL PIONEER TREE SPECIES

Theoretical analyses of the genetic organization of pioneer species have postulated two very different scenarios. Some models have predicted that such species would show strong population substructuring, whereas other models have suggested that extinction and recolonization can augment gene flow and reduce interpopulation differentiation. We tested these alternative scenarios by analyzing the genetic structure of eight loci from populations of the pioneer dioecious tree, Cecropia obtusifolia, in the tropical rain forest region of Los Tuxtlas, México. The populations studied exhibit low overall F_ST values, no clear pattern of isolation by distance, and high estimates of gene flow. These results suggest either that the species is not at a genetic equilibrium under present levels of gene flow with populations derived from each other in the recent past, or that pollen and seed dispersal in this species occur over long distances (up to more than 100 km). Mating among relatives appears higher than expected by chance based on significantly positive fixation indices (F) and F_IS values at some loci. However, no direct evidence for biparental inbreeding was found. The multilocus and single-locus outcrossing rates for C. obtusifolia were estimated at t_m = 0.974 (SE = 0.024) and t_s = 0.980 (SE = 0.035), respectively. These are not significantly different from 1, and the difference, t_m — t_s = — 0.006 (SE = 0.018), is not significantly different from 0. These estimates, however, could be biased because in all enzymes, except PGM-1, we found statistically significant departures from the mixed-mating model used to estimate them. Two rare alleles were found only in seeds collected from the soil, and the greatest number of different alleles were found also in soil seeds. It is hypothesized that the seed bank may play an important role in the genetic buffering of C. obtusifolia. Significantly positive or negative fixation indices in adults at some loci and significantly different heterozygosities among different life stages (from seeds to adults) suggest the action of selection at some loci.     

Isozyme Variation in a Tropical Pioneer Tree Species (Cecropia obtusifolia,Moraceae) with High Contents of Secondary Compounds1

We define 48 allozyme loci for a tropical pioneer tree species, Cecropia obtusifolia Bertol, which has high contents of secondary compounds. Our goals were to find the effects of extraction procedures on artifacts and variation in resolution of enzyme banding patterns; to explore the relationship among the variation of the loci sampled and the enzymes' molecular structure, metabolic function and substrate; to obtain estimates of the genetic variation in this species at Los Tuxtlas rain forest (México) and to explore the variation of allelic frequencies in six successive life-history stages of the species. The resolution of the isozymes bands and the actual banding pattern varied with the type and age of tissue, the collection and storage procedures, the extraction buffer, and other loading and running procedural details. However, artifactual variation was eliminated with a new extraction buffer for species with high contents of secondary compounds. Of the 26 enzymes resolved for C. obtusifolia, we found that enzymes with a greater number of substrates and an oligomeric quaternary structure tended to be more variable than their counterparts, but the relationship was not statistically significant. The proportion of polymorphic systems varied significantly with the metabolic pathway and the function of the enzymes. Enzymes involved in starch synthesis are significantly more variable (p < 0.05) than all others, except those involved in amino acids metabolism and the proportion of polymorphic enzymes is also significantly associated with the hnction of the enzyme, the hydrolases and isomerases are significantly more variable than lyases and oxidoreductases enzymes. The percentage of polymorphic loci for C. obtusifolia was estimated at P = 27.1%. The effective number of alleles was estimated at ne = 1.3 and ne = 2.4 for all loci and only polymorphic ones respectively and the average heterozygosity (H) for all 48 loci was estimated at H = 0.05. Allele frequencies varied throughout the life-cycle of the species, with significant differences for some alleles and loci among some life-cycle stages. “Tree seeds” allele frequencies differ significantly (P < 0.05) from “rainy dispersed seeds” in 7 of 8 loci and from “soil seeds” in Six of eight loci. Allele frequencies of all three seed categories (“tree seeds”, “rainy dispersed seeds”, and “soil seeds”) differed strongly from established individuals (seedlings, juveniles and adults), while allele frequencies of established individuals are relatively similar to one another. Seedling allele frequencies at most loci were also significantly different from those found in seeds collected from trees, seed-rain, and soil. Two alleles (at GOT-2 and FE-2) were only found in soil seeds and one allele (at LAP-2) was only found in seedlings.     

A Genetic Assessment of Ecological Restoration Success in Banksia attenuata

Rarely assessed in the success of ecological restoration projects is the maintenance of genetic variation in restored populations and, critically, their offspring. A founding population sourced from a limited genetic pool of nonlocal provenance seed can result in genetic bottlenecking and inbreeding, potentially reducing future population resilience and restoration success. We used microsatellite markers to assess the genetic variation of natural and restored populations, and their offspring, in Banksia attenuata R.Br. (Proteaceae), a keystone species of Banksia woodlands in south‐west Australia. Both natural and restored populations, and their offspring, displayed similarly high levels of heterozygosity (H_e range = 0.57–0.62) and allelic diversity (N_e range = 6.67–8.86) across 7 microsatellite loci. There was very weak population divergence (F_ST = 0.006) between the restored population and the adjacent natural population, indicating local provenance sourcing of seed. Genetic structuring within the natural population was weak, but detectable at 10 m and more strongly genetically structured than the restored population (Sp = 0.006 and 0.002, respectively). Complete outcrossing, low‐correlated paternity, and very low bi‐parental inbreeding were observed in both populations. Extensive pollen dispersal was observed within and among populations, with >50% of paternity assigned to sires beyond the local population. In a greenhouse experiment, differences in the overall performance of seedlings from natural and restored populations were negligible. Results indicate the successful genetic management of B. attenuata in this restoration project, from which general principles emphasizing the use of diverse local provenance seeds, genetic integration, and delivery of pollinator services are supported.     

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.     

The relative contributions of seed and pollen dispersal to gene flow and genetic diversity in seedlings of a tropical palm

Seed and pollen dispersal shape patterns of gene flow and genetic diversity in plants. Pollen is generally thought to travel longer distances than seeds, but seeds determine the ultimate location of gametes. Resolving how interactions between these two dispersal processes shape microevolutionary processes is a long‐standing research priority. We unambiguously isolated the separate and combined contributions of these two dispersal processes in seedlings of the animal‐dispersed palm Oenocarpus bataua to address two questions. First, what is the spatial extent of pollen versus seed movement in a system characterized by long‐distance seed dispersal? Second, how does seed dispersal mediate seedling genetic diversity? Despite evidence of frequent long‐distance seed dispersal, we found that pollen moves much further than seeds. Nonetheless, seed dispersal ultimately mediates genetic diversity and fine‐scale spatial genetic structure. Compared to undispersed seedlings, seedlings dispersed by vertebrates were characterized by higher female gametic and diploid seedling diversity and weaker fine‐scale spatial genetic structure for female gametes, male gametes and diploid seedlings. Interestingly, the diversity of maternal seed sources at seed deposition sites (N _em) was associated with higher effective number of pollen sources (N _ep), higher effective number of parents (N _e) and weaker spatial genetic structure, whereas seed dispersal distance had little impact on these or other parameters we measured. These findings highlight the importance maternal seed source diversity (N _em) at frugivore seed deposition sites in driving emergent patterns of fine‐scale genetic diversity and structure.     

DEMOGRAPHIC GENETICS OF A PIONEER TROPICAL TREE SPECIES: PATCH DYNAMICS,SEED DISPERSAL,AND SEED BANKS

We consider whether changes in population-genetic structure through the life cycle of Cecropia obtusifolia, a tropical pioneer tree, reflect its gap-dependent demography and the role of evolutionary processes that are important for this species. We asked whether the spatial scale at which population-genetic subdivision occurs corresponds to the scale of habitat patchiness created by gap dynamics; whether patterns of seed dispersal and storage in the soil affect spatial genetic patterns; and whether spatial genetic patterns change through the species life cycle. We estimated Wright's F-statistics for six successive life-history stages for individuals grouped into subpopulations according to occurrence in natural gaps, physical proximity, or occurrence within large quadrats. For each life stage, F_ST-statistics were significantly higher when individuals were grouped by gaps, although concordant patterns across life stages for the three grouping methods were obtained. This supports the hypothesis that patchy recruitment in gaps or among-gap heterogeneity influences the species' genetic structure. F-statistics of seeds collected from females before dispersal (tree seeds), seed-rain seeds, soil seeds, seedlings, juveniles, and adults grouped by gaps, were, respectively: F_IT = 0.004, 0.160, 0.121, 0.091, –0.0002, –0.081; F_IS = –0.032, 0.124, 0.118, 0.029, –0.016, –0.083; and F_ST = 0.035, 0.041, 0.003, 0.063, 0.015, 0.002. Spatial genetic differentiation in rain seeds was not significantly lower than that of tree seeds. The loss of genetic structure in the soil seed bank, relative to that found in the seed rain may be due to sampling artifacts, but alternative explanations, such as microsite selection or temporal Wahlund effect are also discussed. If structure among soil seeds is unbiased, the peak in seedling F_ST may be due to microsite selection. F_IS of seeds in the rain and soil were significantly greater than zero. A Wahlund effect is the most likely cause of these positive F_IS values. Such fine-scale substructuring could be caused by correlated seed deposition by frugivores. The decrease in F_IS from seedlings to adults could result from loss of fine-scale genetic structure during stand thinning or from selection.     

How does population genetic diversity change over time? An experimental seed bank study of Atriplex tatarica (Chenopodiaceae)

Atriplex tatarica is an annual, early successional, facultative halophilic species of frequently disturbed human-made habitats in Central and Eastern Europe. We investigated to what extent the plants grown from seeds extracted from soil seed bank differed genetically to mature aboveground plants in experimental populations of A. tatarica over two successive years. At each of five plots 50 aboveground plants and 50 plants extracted from seeds stored in soil were assayed for allozyme analysis in 2003 and 2004. At the start of experiment, we introduced 1000 seeds of the study species into each of five experimental plots. While the species dominated in all of the experimental plots in the first year, the second year A. tatarica coverage decreased dramatically. Overall allele frequencies of soil seeds and mature plants showed significant differences between life history stages in both years, but not within years in soil seeds as well as mature plants stages. While mature plants showed a significantly greater amount of single and multilocus heterozygosity in both consecutive years, comparison between years did not yield any significant differences. In the same way, despite a relatively large seed bank the species population genetic parameters, i.e. allelic richness (A), observed heterozygosity (H_o), gene diversity (H_s), inbreeding coefficient (F_IS) and fixation index (F_ST), did not change over years between as well as within life history stages. The soil seeds and mature plants significantly differed in H_o, H_s and F_IS, while the A and F_ST were not significantly different between life history stages.     

Do surface plant and soil seed bank populations differ genetically? a multipopulation study of the desert mustard Lesquerella fendleri (Brassicaceae)

Seed banks are an important component of many plant populations, but few empirical studies have investigated the genetic relationship between soil seeds and surface plants. We compared the genetic structure of soil seeds and surface plants of the desert mustard Lesquerella fendleri within and among five ecologically diverse populations at the Sevilleta National Wildlife Refuge in Central New Mexico. At each site, 40 Lesquerella surface plants and 40 samples of soil seeds were mapped and genetically analyzed using starch gel electrophoresis. Overall allele frequencies of soil seeds and surface plants showed significant differences across the five populations and within three of the five individual populations. Surface plants had significantly greater amounts of single and multilocus heterozygosity, and mean surface plant heterozygosity was also greater at the total population level and in four of the five individual populations. Overall soil seed (bot not surface plant) homozygosity was significantly greater than predicted by Hardy-Weinberg expectations at the total and individual population levels. Although F-alpha estimates revealed similarly small but significant genetic divergence within each life-history stage, estimates of coancestry showed that fine-scale (0.5-2 m) genetic correlations among the surface plant genotypes were roughly twice those of soil seed genotypes. An unweighted pair group method with arithrnetic mean cluster analysis indicated that in the two geographically closest sites, the surface plants were slightly more genetically similar to each other than to their own respective seed banks. We also found weak and/or negative demographic associations between Lesquerella soil seed and surface plant densities within each of the five sites. We discuss the difficulties involved with sampling and genetically comparing these two life-history stages.     

Microsatellite variation of cassava (<Emphasis Type="Italic">Manihot esculenta</Emphasis> Crantz) in home gardens of Chibchan Amerindians from Costa Rica

We used 13 microsatellite marker loci to determine the genetic diversity of cassava (Manihot esculenta Crantz) grown in home gardens in two Chibchan Amerindian reserves in Costa Rica. We compared the levels of genetic diversity in the reserves with that of commercial varieties typically cultivated in Costa Rica. We found high levels of genetic diversity among cassava plants. Overall, 12 of the 13 loci examined were polymorphic in each Amerindian reserve (P = 92.3). Moreover, we found 36 alleles in the Coto Brus Reserve and 33 in the Talamanca Reserve. In the commercial varieties only nine loci were polymorphic (P = 69.2), and we only found 23 alleles. Heterozygosity was high for all groups of cassava (Coto Brus, Talamanca, and commercial varieties), but it was higher among the commercial varieties. The levels of heterozygosity and allele diversity indicate that there is significant genetic diversity in the home gardens that we examined. Another indication of the high diversity found in these gardens is the number of distinct multilocus genotypes, 28 at Coto Brus and 19 at Talamanca. There was also more than one distinct multilocus genotype found within the commercial varieties, as three were found in Valencia and four in Manyi. Our data also revealed low levels of genetic differentiation between the three groups of cassava (F_st = 0.03), and Nei’s genetic distances ranged from 0.0167 to 0.0343. In addition, F estimates (F_is and F_it) indicate excess heterozygotes, both at the subpopulation and the population level. A hierarchical analysis of the genetic variation revealed that variation between sampling locations within each of the three groups of cassava was larger than that between groups (Theta S = 0.0775 and Theta P = 0.0204, respectively). The variety Manyi was the group genetically most distant from all others. We discuss the consequences of these findings for in situ conservation of genetic resources.     

Genetic diversity in a seed production population vs. natural populations of Sitka Spruce

Isozyme analysis was applied to estimate the level of variation and the genetic structure of a seed-production population (i.e., seed orchard) and 10 range-wide natural populations of Sitka spruce (Picea sitchensis (Bong.) Carr.). Gene diversity and heterozygosity estimates were comparatively high in both the seed orchard and the natural populations studied. The seed orchard population showed a significantly higher number of alleles per locus and percentage of polymorphic loci. Though not significant, mean heterozygosity of the seed orchard was higher than that observed for all natural populations. Genetic distance analysis indicated that the seed-orchard population was genetically similar to three natural populations from which the parent trees were selected. Parent trees sampling breadth has been identified as the major cause for the observed increased level. The impact of recurrent selection and seed orchard biology and management on maintaining the genetic diversity is discussed.     

Genetic Structure and Gene Flow in Elymus glaucus (blue wildrye): Implications for Native Grassland Restoration

Interest in using native grass species for restoration is increasing, yet little is known about the ecology and genetics of native grass populations or the spatial scales over which seed can be transferred and successfully grown. The purpose of this study was to investigate the genetic structure within and among populations of Elymus glaucus in order to make some preliminary recommendations for the transfer and use of this species in revegetation and restoration projects. Twenty populations from California, Oregon, and Washington were analyzed for allozyme genotype at 20 loci, and patterns of variation within and among populations were determined. Allozyme variation at the species level was high, with 80% of the loci polymorphic and an average expected heterozygosity (an index of genetic diversity) of 0.194. All but two of the populations showed some level of polymorphism. A high degree of population differentiation was found, with 54.9% of the variation at allozyme loci partitioned among populations (F_st= 0.549). A lesser degree of genetic differentiation among closely spaced subpopulations within one of the populations was also demonstrated (F_st= 0.124). Self-pollination and the patchy natural distribution of the species both likely contribute to the low level of gene flow (N_m= 0.205) that was estimated. Zones developed for the transfer of seed of commercial conifer species may be inappropriate for transfer of E. glaucus germplasm because conifer species are characterized by high levels of gene flow. Limited gene flow in E. glaucus can facilitate the divergence of populations over relatively small spatial scales. This genetic differentiation can be due to random genetic drift, localized selective pressures, or both. In order to minimize the chances of planting poorly adapted germplasm, seed of E. glaucus may need to be collected in close proximity to the proposed restoration site.     

Far Eastern Mullet Mugil soiuy Basilewsky (Mugilidae,Mugiliformes): The Genetic Structure of Populations and Its Change under Acclimatization

The introduction of Far Eastern mullet (pilengas) in the Azov Sea in the 1970s–1980s has resulted in the formation of a self-reproducing commercial population. We have carried out a comparative population-genetic analysis of the mullet from the native (Primorye, the Sea of Japan basin) and the new (The Azov Sea basin) ranges. Genetic characteristics of three Primorye and three Azov local samples were studied using electrophoretic analysis of 15 enzymes encoded by 21 gene loci. In the Azov mullet, the initial heterozygosity characteristic of the donor population was preserved while the genotype and the allele compositions changed; the changes included a 1.9-fold reduction in the percentage of polymorphic loci and 1.5-fold reduction in the mean number of alleles per locus. The genetic differences between the Azov and the Primorye sample groups were highly significant. In the native range, no genetic differentiation among the mullet samples from different areas was found (G _st = 0.42%), whereas in the Azov Sea basin, the samples from spatially isolated populations (ecological groups) exhibited genetic differences (G _st = 1.38%). The genetic divergence of the subpopulations and the excess of heterozygotes at some loci in the Azov mullet suggest selection processes that formed genetically divergent groups associated with the areas of different salinity in the new range. The salinity level is assumed to be the most probable factor of local differentiating selection during fast adaptation and naturalization of the introduced mullet.     

The effects of heterozygosity at the af,st and tl loci in peas

Summary Eight near-isogenic lines of pea representing all the homozygous combinations of three genes af, st and tl, which modify leaf shape and size, were crossed in all possible ways excepting reciprocals. An analysis of the resulting 36 families has shown that homozygous mutant alleles at the tl locus acting with homozygous mutant alleles at the af and st loci increase both seed weight and plant haulm weight. The mutant alleles at the af and st loci seem, when homozygous, to have little effect by themselves upon seed weight but they do increase or decrease haulm weight, respectively. There is clear evidence of heterotic effects resulting from heterozygosity at each one of the three loci which modify seed weight, haulm weight and basal branching. The implications of such heterotic effects in pea breeding programmes are discussed.     

Conservation genetics of Butte County meadowfoam (Limnanthes floccosa ssp. californica Arroyo), an endangered vernal pool endemic

The endangered annual plant Limnanthes floccosa ssp. californica Arroyo is restricted to vernal pools in Butte County, California. To identify populations with unique genetic resources, guide reintroduction efforts, and design seed collection scenarios for long-term ex situ seed storage we determined extant genetic diversity and structure by surveying 457 individuals from 21 distinct populations using nine polymorphic microsatellite markers. We found low within population genetic diversity: low allelic diversity (1.9 [0.06 SE] alleles/locus); low heterozygosity (H _obs = 0.10 ± 0.018, H _exp = 0.19 ± 0.015), and a high fixation index (0.556 ± 0.044). The number of polymorphic loci ranged between 11 and 89%. Bayesian ordination determined 20 distinct populations and we found high genetic structure among these (F _st = 0.65, P < 0.0001). We identified notable gene flow barriers across populations, confirming regional structuring between three previously defined population density centers and two outlying populations (F _st = 0.21, P < 0.0001). Population size estimates ranged between ~50 and >5,000 extant plants per site. Our study confirms previous isozyme-based results and suggests that the loss of any population would represent a significant loss in the species’ genetic diversity. Recovery requires active restoration of existing populations and permanent habitat protection. We recommend close comparison of microhabitats of declining populations with genetically similar populations, to determine the potential for human assisted gene flow via seed movement to recover declining populations.     

Heterozygote excess through life history stages in Cestrum miradorense Francey (Solanaceae), an endemic shrub in a fragmented cloud forest habitat

Comparisons of genetic diversity and population genetic structure among different life history stages provide important information on the effect of the different forces and micro‐evolutionary processes that mould diversity and genetic structure after fragmentation. Here we assessed genetic diversity and population genetic structure using 32 allozymic loci in adults, seeds, seedlings and juveniles of eight populations of the micro‐endemic shrub Cestrum miradorense in a highly fragmented cloud forest in central–eastern Mexico. We expected that due to its long history or rarity, this species may have endured the negative effects of fragmentation and would show moderate to high levels of genetic diversity. High genetic diversity (H_e = 0.445 ± 0.03), heterozygote excess (F_IT = ?0.478 ± 0.034, F_IS = ?0.578 ± 0.023) and low population differentiation (F_ST = 0.064 ± 0.011) were found. Seeds had higher genetic diversity (H_e = 0.467 ± 0.05) than the later stages (overall mean for adults, seedlings and juveniles H_e = 0.438 ± 0.08). High gene flow was observed despite the fact that the fragmentation process began more than 100 years ago. We conclude that the high genetic diversity was the result of natural selection, which favours heterozygote excess in all stages, coupled with a combination of a reproductive system and seed/pollen dispersal mechanisms that favour gene flow.     

Quantitative genetic analysis of embryo heterosis in faba bean (Vicia faba L.)

Seeds, i.e. embryos, may be genetically different from either of their parents and moreover may express their own heterosis. The objective was to genetically analyse embryo heterosis for their own weight (i.e. seed weight) in comparison with their seedlings’ heterosis, taking the large-seeded crop (Vicia faba L.) as model. A specific diallel mating scheme was used, based on four parental lines, creating 76 seed genotypes in generations P, F₁, F₂ and BC. Mature seed weight was assessed for these embryo genotypes in 3 years at one German location, and young plant biomass yield of seedlings emerging from these seeds in two greenhouse experiments. The quantitative genetic analysis showed an average of 10.6% mid-parent heterosis for mature seed weight and 14.5% mid-parent heterosis for juvenile biomass. In both traits, the embryos contributed markedly and significantly via their own genes to the genetic variation. For mature embryo weight heterosis, apparently the parental difference in seed weight was decisive, whereas for juvenile biomass heterosis, genetic unrelatedness of parents had priority.     

Microsatellite DNA evidence for genetic drift and philopatry in Svalbard reindeer

Mainland populations of Arctic reindeer and caribou Rangifer tarandus often undergo extensive movements, whereas populations on islands tend to be isolated and sedentary. To characterize the genetic consequences of this difference, levels of genetic diversity and subdivision of Svalbard reindeer (R. t. platyrhynchus) from two adjacent areas on Nordenskjiöldland, Spitsbergen were estimated using data from up to 14 microsatellites. The mean number of alleles per locus in Svalbard reindeer was 2.4 and mean expected heterozygosity per locus was 0.36. The latter value was significantly lower than in Canadian caribou and Norwegian reindeer but higher than in some other cervid species. Large samples of females (n = 743) and small samples of males (n = 38) from two sites ≈ 45 km apart showed genetic subdivision, which could be due to local population fluctuations or limited gene flow. To our knowledge, this is the first study to report significant differentiation at microsatellite loci in Rangifer at such short geographical distances. Neither population showed genetic evidence for recent population bottlenecks when loci unbiased with respect to heterozygosity were analysed. In contrast, false signals of a recent bottleneck were detected when loci upwardly biased with respect to heterozygosity were analysed. Thus, Svalbard reindeer appeared to conform to the paradigm of island populations made genetically depauperate by genetic drift.