Resprouters,assisted by somatic mutations,are as genetically diverse as nonsprouters in the world's fire-prone ecosystems

In fire-prone environments worldwide, resprouters mostly regenerate vegetatively after fire, whereas non (re)sprouters are killed by fire and rely entirely on stored seeds (soil or canopy storage) for regeneration. This dichotomy in post-fire regeneration strategies is a key mechanism for controlling the reproductive characteristics, demography, and population genetic structure of plant species in fire-prone ecosystems. Nonsprouters are considered to have higher within-population genetic variation than resprouters due to greater opportunities for recombination via their much greater seed production and frequent generation turnover. Empirical studies that explore this hypothesis are rare and the results are mixed. We collated published studies reporting genetic diversity measures of plant species in fire-prone ecosystems of four Mediterranean-climate regions. Ninety-two species were identified with unambiguous information on postfire regeneration type and with adequate sample sizes. We found no significant differences in population genetic diversity and structure between resprouters and nonresprouters across diversity parameters and genetic marker types, taxon groups or geographic regions. We conclude that resprouters are at least as genetically diverse as nonsprouters. We tested the hypothesis that accumulation of somatic mutations is a possible mechanism for maintaining genetic diversity among resprouters, by screening for microsatellite DNA mutations in the resprouter, Banksia attenuata. Genotyping of leaf material collected from disparate parts of individual plants demonstrated the existence of 2–5 somatic mutations among eleven microsatellite loci. The buildup of somatic mutations in meristematic buds during a resprouter's long lifespan and the considerable potential for interpopulation gene dispersal among resprouters may be key factors that enable their genetic diversity to keep pace with that of nonsprouters. Thus, resprouters and nonsprouters are equally capable of responding to natural selection, and therefore possess a similar potential to adapt to changing environmental conditions.     

Pollination,mating and reproductive fitness in a plant population with bimodal floral‐tube length

Mating patterns and natural selection play important roles in determining whether genetic polymorphisms are maintained or lost. Here, we document an atypical population of Lapeirousia anceps (Iridaceae) with a bimodal distribution of floral‐tube length and investigate the reproductive mechanisms associated with this pattern of variation. Flowers were visited exclusively by the long‐proboscid fly Moegistorhynchus longirostris (Nemestrinidae), which exhibited a unimodal distribution of proboscis length and displayed a preference for long‐tubed phenotypes. Despite being visited by a single pollinator species, allozyme markers revealed significant genetic differentiation between open‐pollinated progeny of long‐ and short‐tubed phenotypes suggesting mating barriers between them. We obtained direct evidence for mating barriers between the floral‐tube phenotypes through observations of pollinator foraging, controlled hand pollinations and measurements of pollen competition and seed set. Intermediate tube‐length phenotypes produced fewer seeds in the field than either long‐ or short‐tubed phenotypes. Although floral‐tube length bimodality may not be a stable state over long timescales, reproductive barriers to mating and low ‘hybrid’ fitness have the potential to contribute to the maintenance of this state in the short term.     

Sink strength manipulation in branches of a Mediterranean woody plant suggests sink-driven allocation of biomass in fruits but not of nutrients in seeds

The relative autonomy of branches within an individual plant may favor different resource allocation responses after reproductive losses. The assessment of these reproductive strategies at the branch level before their integration at the plant level would provide more insight into how plants deal with reproductive losses. Here, we present a field experiment to assess changes in the allocation strategies at the branch level after sink strength manipulation in a Mediterranean woody plant (Cistus ladanifer). We applied three levels of removal of developing fruits (75, 25 and 0 %) on branches of the same plants, and measured their effects on resource allocation (biomass, nitrogen and phosphorus) and seed production after controlling for the effects of branch diameter and leaf weight per branch. Our results suggest that after experimental fruit thinning, C. ladanifer branches presented a sink-driven allocation of biomass to fruits but this was not the case for the allocation of nutrients to seeds, which could be driven by competition with leaf biomass. Reductions in biomass per fruit resulted in a reduction in seed output since the average weight per seed remained constant. From these results, it could be suggested that an heterogeneous distribution of fruit losses among the branches within a crown would produce a higher impact on reproductive output than a more equitable distribution.     

Limited seed dispersal and microspatial population structure of an agamospermous grass of West African savannahs, Hyparrhenia diplandra (Poaceae)

We studied the microspatial population structure of the perennial tussock grass, Hyparrhenia diplandra (Poaceae), a facultative agamospermous species of West African savannahs. The microspatial population structure of H. diplandra was investigated by choosing two 100-m(2); quadrats at random from which all individuals were mapped. The genotype of every individual was determined using two highly polymorphic microsatellite loci. A chloroplast locus was also used to investigate the role of seed dispersal on the genetic structure of populations. The genetic diversity index (0.85) was high for a clonal species. Significant genetic differentiation over short distances was detected by F statistics, and spatial autocorrelation analyses within both quadrats showed significant isolation-by-distance patterns, both with the cytoplasmic locus and the nuclear loci. Some clones formed large patches (up to 5 m in diameter) whereas others were more scattered. However, the genetic differentiation between quadrats was much higher when studied with the cytoplasmic locus than with the nuclear loci, indicating that gene flow via pollen, but not seeds, may frequently occur between quadrats. The maintenance of genetic diversity in this facultative agamospermous species most likely results from several factors, such as low seed dispersal ability, nonnegligible gene flow through pollen, and selective pressures induced by regularly occurring fires in this ecosystem.     

Biometry and DNA fingerprinting detect limited genetic differentiation among populations of the apomictic blackberry Rubus nessensis (Rosaceae)

Clonal plant species have in many recent studies exhibited unexpectedly high levels of genetic differentiation. This seems to hold true also for several apomictic (agamospermous) species in spite of their potential ability to spread single genotypes over large areas through seed dispersal. By contrast, previous DNA fingerprint studies have demonstrated apomictic Rubus species to be extremely homogeneous. The present study was undertaken to estimate the extent and distribution of intraspecific genetic variation in R. nessensis . Populations differed significantly for many biometrical characters but this variation was evenly distributed, with few significant differences between single pairs of populations. Patterns indicating ecoclinal variation, which could be adaptive in origin, were not found. Populations also clustered close together in a two-dimensional CVA plot, with only one population being somewhat further removed. DNA analysis of later collected material from this population as well as from three others yielded identical DNA fingerprints with each of two different multi-locus DNA probes. Taken together with previous DNA fingerprint studies, these results indicate that a major part of the R. nessensis plants in Northern Europe have been derived from a single recombinational event, possibly making this genet the most widespread land-living organism in the world. The encountered variation in biometrical characters is probably instead caused by e.g. minor somatic mutations. Rubus species have relatively large, bird-dispersed seeds that probably travel further than the small, wind-dispersed seeds found in most previously investigated apomicts (e.g. in Asteraceae). The potential for single genotypes to disperse over large areas should therefore be greater in Rubus than in many other apomictic species groups.     

Defining the genetic architecture underlying female- and male-mediated nonrandom mating and seed yield traits in Arabidopsis

Postpollination nonrandom mating among compatible mates is a widespread phenomenon in plants and is genetically undefined. In this study, we used the recombinant inbred line (RIL) population between Landsberg erecta and Columbia (Col) accessions of Arabidopsis (Arabidopsis thaliana) to define the genetic architecture underlying both female- and male-mediated nonrandom mating traits. To map the genetic loci responsible for male-mediated nonrandom mating, we performed mixed pollinations with Col and RIL pollen on Col pistils. To map the genetic loci responsible for female-mediated nonrandom mating, we performed mixed pollinations with Col and Landsberg erecta pollen on RIL pistils. With these data, we performed composite interval mapping to identify two quantitative trait loci (QTLs) that control male-mediated nonrandom mating. We detected epistatic interactions between these two loci. We also explored female- and male-mediated traits involved in seed yield in mixed pollinations. We detected three female QTLs and one male QTL involved in directing seed number per fruit. To our knowledge, the results of these experiments represent the first time the female and male components of seed yield and nonrandom mating have been separately mapped.     

THE LEVEL OF AGAMOSPERMY IN A NEBRASKA POPULATION OF SPIRANTHES CERNUA (ORCHIDACEAE)

This study investigated reproduction in a prairie population of Spiranthes cernua in eastern Nebraska. Spiranthes cernua reproduces both sexually and asexually, through adventitious embryony. Agamospermous seeds are monoembryonic, polyembryonic, have extruded or free embryos, or lack embryos. These kinds of seeds are extremely rare in Spiranthes vernalis, a close relative that relies exclusively on sexual reproduction. Sexually derived seeds are monoembryonic or lack embryos. One objective was to quantify the amount of sexual reproduction in a natural population. In 1985 and 1986, maximum estimates of sexual seed production per capsule were 19.2% and 33.6%, respectively. Thus, every capsule examined contained a high proportion of agamospermous seeds. In addition, attempts were made to manipulate the reproductive system with experimental crosses performed in the field. The treatments examined agamospermy, autogamy, and outcrossing. In all treatments the majority of seeds in a capsule were agamospermous.     

Maternal environmental effects of competition influence evolutionary potential in rapeseed (<Emphasis Type="Italic">Brassica</Emphasis><Emphasis Type="Italic">rapa</Emphasis>)

Maternal environmental effects reflect the contribution of the maternal environment to the offspring phenotype. Maternal effects are prevalent in plants and animals and may undergo adaptive evolution and affect patterns of natural selection within and across generations. Here, we raise two generations of a rapeseed (Brassica rapa) population derived from a cross between a rapid-cycling and an oilseed genotype in competitive and noncompetitive settings. Maternal environment had little effect on average offspring phenotypes. Maternal genotypes, however, differed in the sensitivity of almost all offspring phenotypes to the maternal environment, demonstrating genetic variation in maternal effects for traits expressed throughout ontogeny. Maternal environment did not significantly affect progeny seed production, and maternal genotypes were not variable for this trait, indicating no evidence for direct maternal effects on offspring fitness. Maternal environment influenced natural selection in the progeny generation; disruptive selection acted on seed mass among seeds matured in the noncompetitive maternal environment versus no significant selection on this trait for seeds matured in the competitive maternal environment. Although maternal effects did not directly increase fitness, they did affect evolutionary potential and selection in the progeny generation. These results suggest that diverse phenotypes of both wild and cultivated B. rapa genotypes will depend on the maternal environment in which the seeds are matured.     

How robust is nonrandom mating in wild radish: do small pollen loads coupled with more competing pollen donors lead to random mating?

In previous studies of the weedy annual Raphanus sativus we have demonstrated that mating is nonrandom in greenhouse plants, suggesting that sexual selection is possible. To investigate how these greenhouse results might translate to conditions more similar to the field, we manipulated both pollen load size and the number of competing pollen donors on stigmas. While the smallest pollen loads (22 grains per stigma) were small enough to reduce fruit and seed set, seed siring success was unaffected by pollen load size. When the number of competing donors in a mixed pollination was increased to four, the proportion of seeds sired by the pollen donors was the furthest from expectation, suggesting that nonrandom mating increases as the number of donors per pollination increases. There was no significant interaction between pollen load size and number of competitors per pollination. Overall, mating remained nonrandom across all treatments. Thus differential seed paternity is likely to occur in the field as well as in the greenhouse.     

Field performance of <Emphasis Type="Italic">Theobroma cacao</Emphasis> L. plants propagated via somatic embryogenesis

Somatic embryogenesis is an in vitro clonal propagation method with potential to contribute to the improvement of cacao varieties. Before using this technology for commercial production, it is essential that somatic embryogenesis-derived plants be tested in field conditions. Therefore, we established a field test at Union Vale Estate, Saint Lucia. Thirty- to 50-yr-old trees were selected for clonal propagation as potentially high yielding based on local farmers observations. Clonal plants were propagated in vitro from immature flowers by embryogenesis and micropropagation. Multiple plants from nine genotypes were acclimated to greenhouse conditions then returned to Saint Lucia and planted in a field. Orthotropic rooted cuttings and locally propagated open pollinated seedlings were also planted for a total of 214 trees. Growth data were collected every 4–6 mo. including: stem diameter, stem height, length of the longest jorquette branch, number of jorquette branches, and dates of first flowering and fruiting. At 4.5 yr after planting in the field there were no major differences in all growth parameters among the propagation methods evaluated with exception of the orthotropic rooted cuttings. Trees grown from seeds were slightly taller then trees propagated by the other methods. Trees propagated as orthotropic rooted cuttings exhibited smaller average stem diameters, shorter stem heights to the jorquette, and shorter jorquette branches. We concluded that somatic embryo-derived plants demonstrated normal phenotypes in field conditions and have growth parameters similar to plants propagated by traditional methods.     

DIFFERENTIAL POLLEN-TUBE GROWTH RATES AND NONRANDOM FERTILIZATION IN HIBISCUS MOSCHEUTOS (MALVACEAE)

The prevalence of nonrandom fertilization due to postpollination events has rarely been studied in natural populations, despite important implications for outcrossing rates, mate choice, and plant fitness. Nonrandom paternity within fruits can be caused by both unequal fertilization and unequal embryo abortion. Using self-compatible Hibiscus moscheutos, we studied the potential for nonrandom fertilization by comparing growth rates of pollen-tubes from different donors. The branched style of Hibiscus allowed within-flower comparisons between pollen donors. Relative pollen-tube growth rates were determined by applying pollen from pairs of donors to different stigmas on adjacent stylar branches. We then measured the number of callose plugs per tube in cross-sectional transects across the style after 3 hr. We demonstrate that rates of callose plug formation can be used as a sensitive indicator of relative pollen-tube growth rate. Differences between pollen donors were common and repeatable. Self-pollen-tubes grew slower than outcross pollen-tubes in some crosses and faster in others. Allozyme variation in glucose phosphate isomerase was used to show that individuals with fast-growing pollen-tubes sired a disproportionate number of seeds following mixed pollinations (up to 72%). Since seed abortion was negligible, we conclude that variation in pollen-tube growth rates leads to nonrandom paternity within fruits.     

Genetic evaluation of Jatropha curcas: an important oilseed for biodiesel production

Jatropha curcas, internationally and locally known, respectively, as physic nut and pinh?o manso, is a highly promising species for biodiesel production in Brazil and other countries in the tropics. It is rustic, grows in warm regions and is easily cultivated. These characteristics and high-quality oil yields from the seeds have made this plant a priority for biodiesel programs in Brazil. Consequently, this species merits genetic investigations aimed at improving yields. Some studies have detected genetic variability in accessions in Africa and Asia. We have made the first genetic evaluation of J. curcas collected from Brazil. Our objective was to quantify genetic diversity and to estimate genetic parameters for growth and production traits and seed oil content. We evaluated 75 J. curcas progenies collected from Brazil and three from Cambodia. The mean oil content in the seeds was 31%, ranging from 16 to 45%. No genetic correlation between growth traits and seed oil content was found. However, high coefficients of genetic variation were found for plant height, number of branches, height of branches, and stem diameter. The highest individual narrow-sense heritabilities were found for leaf length (0.35) and width (0.34), stem diameter (0.24) and height of branches (0.21). We used a clustering algorithm to genetically identify the closest and most distant progenies, to assist in the development of new cultivars. Geographical diversity did not necessarily represent the genetic diversity among the accessions collected. These results are important for the continuity of breeding programs, aimed at obtaining cultivars with high grain yield and high oil content in seeds.     

短枝木麻黄种群苗期表型多样性评价

以来源于大洋洲原生种源区、亚洲原生种源区、亚洲引种次生区以及非洲引种次生区4个区域的20个种源短枝木麻黄种子和当年生幼苗为材料,通过种子千粒重以及幼苗苗高、地径、一级侧枝粗度、一级侧枝长度等7个性状对短枝木麻黄表型多样性进行了研究,以探讨种群苗期表型遗传差异,为短枝木麻黄早期遗传选择和遗传改良提供基本资料。结果表明:(1)短枝木麻黄种子千粒重在区域间和区域内种源间差异极显著,且千粒重具有显著的地理变异模式,随经度的增大而降低。(2)当年生幼苗苗高、地径在不同区域间及区域内种源间均存在极显著差异,其中泰国干东港种源幼苗生长最好(苗高76.6cm,地径4.64mm),而种源汤加的幼苗生长最差(苗高28.3cm,地径2.58mm)。(3)当年生幼苗一级侧枝粗度、一级侧枝长度、二级侧枝长度、每小枝节数和齿叶数在不同区域间及区域内种源间均存在极显著差异,其中齿叶数在区域间的变异系数最大(82.15%)。(4)通径分析表明,一级侧枝长度对苗高具有显著的正向影响作用,而一级侧枝粗度和二级侧枝长度对地径具有显著正向影响作用,它们可作为短枝木麻黄优良新品种筛选的参考因子。     

GENETIC COMPARISONS OF SEED BANK AND SEEDLING POPULATIONS OF A PERENNIAL DESERT MUSTARD,LESQUERELLA FENDLERI

Soil seed banks may accumulate and store seed genotypes produced over many seasons. If germination and establishment of these soil seeds are influenced by seed genotypes, then seed bank and seedling populations may differ genetically. I compared the genetic structure of dormant but viable soil seeds of the desert mustard Lesquerella fendleri with the genetic structure of Lesquerella seedlings at the Sevilleta Long-Term Ecological Research Site. In 1991 and 1992, soil seeds and seedlings were mapped and genetically analyzed using starch gel electrophoresis. When data from all loci were lumped, there were highly significant differences in allele frequencies between soil seeds and seedlings at the population level (all plots) in both years, in all subpopulation (adjacent plots) comparisons in 1991, and three of five subpopulations in 1992. Differences at some individual loci were also detected in one or both years. Analysis of data pooled across both years revealed highly significant differences in the distribution of multilocus soil seed and seedling heterozygosity, but no significant differences in mean heterozygosity. F_st values showed small but statistically significant genetic differentiation within soil seeds and seedlings in both years. F_st values also showed significant genetic differentiation between these two groups at three of seven loci in 1991, and at one locus in 1992. Soil seeds and seedlings showed a general pattern of decreasing genetic relationship with distance, as estimated by the coefficient of coancestry analyses. In 1991, seedlings were roughly twice as genetically related to each other than were soil seeds at fine spatial scales (0–0.25 and 0.25–0.50 m). This study suggests that Lesquerella seedlings in this system represent a nonrandom genetic subset of the underlying Lesquerella seed bank. Such temporal genetic change may be an important yet frequently overlooked mechanism for generating population genetic structure.     

Genetic interpretation of enzyme variation in sexual and agamospermous taxa of Taraxacum sections Vulgaria and Mongolica

The inheritance of 15 enzymes, comprising at least 22 genetic loci, was investigated in crosses between sexual diploid individuals of Taraxacum sections Vulgaria and Mongolica. Patterns were consistent with simple Mendelian segregation. From the inheritance information isozyme phenotypes in agamospermous plants from natural populations were inferred. In some crosses part or all of the progeny originated from self-fertilization, sofar a very rare phenomenon in the sections Vulgaria and Mongolica. It is possible that the probability of self-fertilization increases after pollination by triploid pollen, affecting the cohabitation dynamics of the various ploidy components in mixed natural stands.     

Molecular Mapping and Character Tagging in Mustard (Brassica juncea) II. Association of RFLP markers with Seed Coat Colour and Quantitative Traits

In Brassica juncea, segregation of 44 RFLP markers generated by homologous genomic DNA clones as probes was studied in a F₂ population obtained from an intervarietal cross. Linkage relationship among the markers was established using computer package ‘MAPMAKER’. Twenty five of the markers could be arranged in nine linkage groups, covering a total of 243.3 cM. Marker BJG357c showed tight linkage (3.9 cM) with yellow seed coat colour locus (r ₁). Based on single factor analysis of variance,17 significant marker-trait associations could be established in respect of six quantitative traits viz. days to flowering, plant height, number of primary branches, secondary branches per primary branch, siliquea per secondary branch, and seeds per siliqua. The proportion of phenotypic variation explained by individual marker-trait association ranged from 3.0% to 33.2%. The putative gene action at majority of the marked genomic regions was found to be partial dominance to dominance.     

Joint evolution of differential seed dispersal and self‐fertilization

Differential seed dispersal, in which selfed and outcrossed seeds possess different dispersal propensities, represents a potentially important individual‐level association. A variety of traits can mediate differential seed dispersal, including inflorescence and seed size variation. However, how natural selection shapes such associations is poorly known. Here, we developed theoretical models for the evolution of mating system and differential seed dispersal in metapopulations, incorporating heterogeneous pollination, dispersal cost, cost of outcrossing and environment‐dependent inbreeding depression. We considered three models. In the ‘fixed dispersal model’, only selfing rate is allowed to evolve. In the ‘fixed selfing model’, in which selfing is fixed but differential seed dispersal can evolve, we showed that natural selection favours a higher, equal or lower dispersal rate for selfed seeds to that for outcrossed seeds. However, in the ‘joint evolution model’, in which selfing and dispersal can evolve together, evolution necessarily leads to higher or equal dispersal rate for selfed seeds compared to that for outcrossed. Further comparison revealed that outcrossed seed dispersal is selected against by the evolution of mixed mating or selfing, whereas the evolution of selfed seed dispersal undergoes independent processes. We discuss the adaptive significance and constraints for mating system/dispersal association.     

RE-EVALUATING THE SIGNIFICANCE OF CORRELATIONS BETWEEN SEED NUMBER AND SIZE: EVIDENCE FROM A NATURAL POPULATION OF THE LILY,CLINTONIA BOREALIS

The basis for the negative correlation between seed number and seed size was experimentally investigated in a natural population of Clintonia borealis. Clones of this species vary significantly in estimated self-compatibility (ratio of seed set with selfing to that with outcrossing) and this appears to affect the number and size of seeds set in individual flowers of each. Clones estimated to be largely self-compatible set more seeds per flower than incompatible ones under natural pollination. However, naturally pollinated flowers of self-compatible clones set smaller seeds than those of incompatible clones, and the significance of the negative relationship between seed number and size in individual flowers was removed by holding variation due to compatibility constant. Supplementing resources per flower (by reducing the number of fruits competing for resources per stem) significantly increased total seed mass but had no effect on the negative relationship between seed number per flower and seed size. In contrast, supplementing cross pollination did not significantly influence total seed mass per flower but changed the relationship between seed number and size to positive, regardless of resource level. In other words, with plentiful cross pollination maternal genets capable of setting more seeds per flower also produced heavier ones. Thus, evidence is provided that the balance between seed number and seed size in this population is regulated by the interaction of maternal self-compatibility with natural pollination.     

Bayesian Inference of the Evolution of a Phenotype Distribution on a Phylogenetic Tree

The distribution of a phenotype on a phylogenetic tree is often a quantity of interest. Many phenotypes have imperfect heritability, so that a measurement of the phenotype for an individual can be thought of as a single realization from the phenotype distribution of that individual. If all individuals in a phylogeny had the same phenotype distribution, measured phenotypes would be randomly distributed on the tree leaves. This is, however, often not the case, implying that the phenotype distribution evolves over time. Here we propose a new model based on this principle of evolving phenotype distribution on the branches of a phylogeny, which is different from ancestral state reconstruction where the phenotype itself is assumed to evolve. We develop an efficient Bayesian inference method to estimate the parameters of our model and to test the evidence for changes in the phenotype distribution. We use multiple simulated data sets to show that our algorithm has good sensitivity and specificity properties. Since our method identifies branches on the tree on which the phenotype distribution has changed, it is able to break down a tree into components for which this distribution is unique and constant. We present two applications of our method, one investigating the association between HIV genetic variation and human leukocyte antigen and the other studying host range distribution in a lineage of Salmonella enterica, and we discuss many other potential applications.     

Post‐dispersal seed predation and its relations with seed traits: a thirty‐species‐comparative study

Post‐dispersal seed predation is a key process determining the variability in seed survival in forests, where most seeds are handled by rodents. Seed predation is thought to affect seedling regeneration, colonization ability and spatial distribution of plants. Basic seed traits are the essential factors affecting rodent foraging preferences and thus seed survival and seedling recruitment. Many studies have discussed several seed traits and their effects upon seed predation by rodents. However, the results of those previous studies are usually equivocal, likely because few seed traits and/or plant species tend to be incorporated into these studies. In order to elucidate the relationships between seed predation and seed traits, we surveyed the predation of 48 600 seeds in a natural pine forest, belonging to 30 species, for three consecutive years. The results demonstrated that: (i) seed size and seed coat hardness did not significantly affect seed predation; (ii) total phenolics had a negative effect upon seed predation; (iii) positive effects of nitrogen content upon seed predation were found. From our study, it seems that the better strategy to prevent heavy predation is for plants to produce seeds with higher total phenolics content rather than physical defenses (i.e. hard seed coat) or larger seeds. Additionally, rodent foraging preference may depend more on Nitrogen content than other nutrient content of seeds.