Björn Widén

GENOTYPIC DIVERSITY, MOLECULAR MARKERS AND SPATIAL DISTRIBUTION OF GENETS IN CLONAL PLANTS, A LITERATURE SURVEY

We present a literature survey of studies using molecular markers to investigate genet diversity and structure in clonal plants. The data from 40 studies comprised 45 species of which only two were studied by DNA methods, the rest by isozyme markers. Less than one third of the studies provided information about the spatial distribution of individual genets within populations, and only 12.5% of the studies used mapping of all ramets within plots or part of the population in combination with identification of multilocus genotypes. We also present two case studies. InGlechoma hederacea morphological criteria were used to select clones. Multi-samples of ramets within these “clones” turned out to be variable using isozymes indicating that these “clones” in most cases consisted of several genets. One individual multilocus genotype covered tens of square metres. InHylocomium splendens samples from 10×10 cm plots usually consisted of a mixture of multilocus genotypes, but occasionally the whole plot consisted of one genet.

Pollen limitation and distance-dependent fecundity in females of the clonal gynodioecious herb Glechoma hederacea (Lamiaceae)

SummaryPollen movement is often restricted in natural populations, and insufficient pollination is a potential constraint on sexual reproduction in outcrossing species. Seed-set should decrease with increased distance from the pollen source in outcrossing plants. This prediction was tested using females of the clonal, gynodioecious herb Glechoma hederacea in three natural populations. In controlled pollinations, both hermaphrodites and females had similar high percentages of fruit-set and seed-set. In a natural population where a female clone was isolated from the nearest hermaphroditic clone by c. 100 m, fruit-set was low (1%). In another population where hemaphroditic clones were rare and female clones had a patchy distribution, fruit-and seed-set in females were pollen-limited and decreased with increased distance from the nearest pollen source. The estimated mean pollen dispersal distance was 5.9 m when calculated on fruit-set and 5.3 m when calculated on seed-set. The most frequent pollinators were bumblebees. The mean and median distances moved by pollinators between ramets were 0.13 m and 0.05 m. In a third population where female clones were isolated from the nearest hermaphrodites by more than 200 m, fruit-set was 0%. After introduction of 16 hermaphroditic ramets in the center of the female clone, fruit-set varied between 0% and 100% in individual female ramets. Fruit-set decreased with increased distance from the pollen source. The mean and median pollen movement distances were 1.06 m and 0.54 m.

Pollinator-mediated selection on floral traits in a synthetic population of Senecio integrifolius (Asteraceae)

We measured pollinator-mediated selection on quantitative floral traits through female reproductive success in a phenotypically variable population of Senecio integrifolius, consisting of plants derived from two natural populations, including genotypes with extreme trait values (head width) and plants on which all rays had been removed. In addition to significant heritabilities for all traits, we also documented genetic variation in phenotypic plasticity for head width. A phenotypic selection analysis demonstrated (1) a strong positive relationship between the percentage seed-set and the overall cross-fertility determined in a previous greenhouse study

Nuclear and chloroplast microsatellites reveal extreme population differentiation and limited gene flow in the Aegean endemic Brassica cretica (Brassicaceae)

Nuclear and chloroplast microsatellite markers were used to study population structure and gene flow among seven Cretan populations of the Aegean endemic plant species Brassica cretica (Brassicaceae). Both nuclear and chloroplast markers revealed exceptionally high levels of population differentiation (overall FST = 0.628 and 1.000, respectively) and relatively little within‐population diversity (overall HS = 0.211 and 0.000, respectively). Maximum‐likelihood estimates of directional migration rates were low among all pairs of populations (average Nm = 0.286). There was no evidence that differences in flower colour between populations had any influence on historical levels of gene flow. In addition, a haplotype network showed that all five chloroplast haplotypes found in the sample were closely related. Together, these results suggest that current patterns of diversification in B. cretica are mainly a result of genetic drift during the last half million years. The main conclusions from the present study are consistent with the prevailing hypothesis that plant diversification in the Aegean region is driven by random rather than adaptive differentiation among isolated populations.

Allozyme diversity and genetic structure of marginal and central populations of Corylus avellana L. (Betulaceae) in Europe

Abstract.Corylus avellana L. (hazel, Betulaceae) is a long-lived, widespread shrub in Europe, having its northern range margin in Fennoscandia and a postglacial history involving range-expansion from refugial areas in southern Europe. In this study, we tested for a relationship between marginality and low within-population genetic diversity by assessing patterns of variation at 14 putatively neutral allozyme loci (comprising 43 putative alleles) within and between 40 natural populations of C. avellana along a north-south transect in Europe. Geographically marginal populations (central Sweden) showed lower levels of within-population diversity than populations in more central regions, as indicated by significant negative correlations between latitude and the percentage of polymorphic loci (rS=–0.47, P < 0.001), the average number of alleles per locus (rS=–0.65, P < 0.001), the expected heterozygosity (rS=–0.19, P < 0.05), and the proportion of distinguishable genotypes (rS=–0.56, P < 0.001). These patterns, combined with the unusually high between-population component of gene diversity (GST=19.7%) and allelic richness (AST=24%) in the marginal region, can be attributed to historical bottlenecks during the species’ postglacial range-expansion, but may also reflect a history of genetic drift in the small, isolated populations occupying the marginal region. Information on the spatial distribution of genotypes provide further support for a role of vegetative reproduction (layering) in the structuring of genetic variation within populations.

Population divergence of genetic (co)variance matrices in a subdivided plant species, Brassica cretica

Abstract The present study of Brassica cretica had two objectives. First, we compared estimates of population structure (Qst) for seven phenotypic characters with the corresponding measures for allozyme markers (Fst) to evaluate the supposition that genetic drift is a major determinant of the evolutionary history of this species. Secondly, we compared the genetic (co)variance (G) matrices of five populations to examine whether a long history of population isolation is associated with large, consistent differences in the genetic (co)variance structure. Differences between estimates of Fst and Qst were too small to be declared significant, indicating that stochastic processes have played a major role in the structuring of quantitative variation in this species. Comparison of populations using the common principal component (CPC) method rejected the hypothesis that the G matrices differed by a simple constant of proportionality: most of the variation involved principal component structure rather than the eigenvalues. However, there was strong evidence for proportionality in comparisons using the method of percentage reduction in mean‐square error (MSE), at least when characters with unusually high (co)variance estimates were included in the analyses. Although the CPC and MSE methods provide different, but complementary, views of G matrix variation, we urge caution in the use of proportionality as an indicator of whether genetic drift is responsible for divergence in the G matrix.

Conservation Biology in Agricultural Habitat Islands

Modern agriculture in temperate areas has a twofold impact on natural populations of plants and animals: it causes fragmentation and decreases the quality of habitats. Economy forces modern farmers to establish large fields that are easily worked. Ditches, habitat islands (piles of stone or shallow soils covered with vegetation) and irregularities in the fields are considered as obstacles and are therefore often removed. From a conservation point of view this development is often detrimental, because populations are fragmented, reducing migration between them.

Allozyme diversity in relation to geographic distribution and population size in Lathyrus vernus (L.) Bernh. (Fabaceae).

Lathyrus vernus (L.) Bernh. is a diploid, long-lived perennial and insect-pollinated herb with no special adaptation to long-distance dispersal. It occurs on neutral soil in deciduous forests throughout western Eurasia. Due to specific habitat preferences,L. vernus has a fragmented distribution with isolated populations. We investigated allozyme variation at eleven loci in 20 populations ofL. vernus from one geographically central region (the Czech Republic and the Slovak Republic) and two geographically marginal regions (southern and central Sweden) in the species present-day distribution. There was a clear differentiation between the three regions and the genetic distance between the populations was highly correlated with geographic distance. The total genetic diversity (HT) was 0.354. The proportion of genetic diversity due to differentiation between regions, and to differentiation between populations within regions, accounted for 10% each. There was no difference in level of genetic diversity between the three regions. No significant difference in level of genetic diversity was found between small and large populations. The genetic diversity inL. vernus may either be a result of the long generation-time of the species or peculiarities in the post-glacial migration species, e.g. survival only in refugia far east of the sampled populations and/or migration as a continuous process not involving founder-events.

Geographic variation and dispersal history in Fennoscandian populations of two forest herbs

Abstract. Carex digitata and Melica nutans are forest understorey herbs with wide European distributions and their northern range margins in Fennoscandia. The species have closely similar habitat requirements, occur in small populations in old forest stands on base-rich to neutral soils and have restricted dispersal abilities at the present day. This study investigates the structure of allozyme variation (12 and 8 loci, respectively) in material of both species (38 and 37 populations, respectively) from throughout southern Sweden and southern Finland. Both species show a relatively low overall genetic diversity (HT excluding monomorphic loci=0.17 and 0.18, respectively). The hierarchic structuring of allelic diversity in the species is similar, with a relatively high between-population component of diversity (GST=0.36 and 0.37, respectively). Neither of the species shows a clear intraspecific pattern of geographic differentiation. The lack of large-scale patterns of geographic differentiation is not consistent with a simple scenario of discrete and independent waves of immigration into Fennoscandia. However, particularly in M. nutans, a group of populations from a lowland belt across southwestern Finland and southern central Sweden is somewhat differentiated from populations to the north and south. A number of rare alleles in both species are widely, but patchily distributed in low frequencies. Hybridization may account for the scattered occurence of some of the rare alleles in Carex digitata, but cannot explain the distribution of rare alleles in Melica nutans.

Flower and cotyledon asymmetry in Brassica cretica: Genetic variation and relationships with fitness

Abstract Plants of the partially self-incompatible perennial herb Brassica cretica, derived from controlled cross- and self-pollinations within each of seven populations, were raised under uniform conditions and scored for two measures of developmental stability, flower asymmetry (quantified as the difference in length and width between opposite petals) and cotyledon asymmetry (quantified as the difference in the area of the two lobes of each cotyledon). The primary goals were to assess the level of heritable variation in asymmetry, the effect of selfing on mean asymmetry, and the relationship between asymmetry and components of fitness. A paternal half-sibling analysis of data on flower asymmetry failed to detect significant levels of genetic variation at the within-population level, whereas the between-population component reached significance for all measures of asymmetry. Analysis of family-structured data from another crossing experiment revealed significant between-population variation in cotyledon asymmetry and a tendency for inbred progeny to produce more asymmetric cotyledons than outbred progeny. However, the response to inbreeding was weak and differed in magnitude between populations. Judging from the ranking of populations, we found no support for the hypothesis that the mean expression of developmental stability is controlled by genomewide characteristics such as the level of inbreeding. Correlations between measures of asymmetry and fitness were too low to be declared statistically or biologically significant. The present study provides little evidence that flower and cotyledon asymmetry serve as more appropriate predictors of genetic health than conventional (direct) measures of fitness. Corresponding Editor: M. Dudash