Clonal and spatial genetic structure in natural populations of Luohanguo (Siraitia grosvenorii), an economic species endemic to South China,as revealed by RAPD markers

Clonal growth is generally expected to have significant effects on the spatial genetic structure within populations. In this study, random amplified polymorphic DNA (RAPD) markers were used to reveal clonal and spatial genetic structure of four natural populations of Luohanguo (Siraitia grosvenorii), an economic vine species endemic to South China. A total of 351 ramets were assigned to 76 distinct multi-locus genotypes (i.e. genets), with the G/N varying from 0.121 to 0.350. No widespread genet was found across different populations. The clonal diversity (D) and evenness (E) ranged from 0.333 to 0.828 and from 0 to 0.741, respectively. While most genets consisted of fewer than five ramets, we observed some dominant genets that had much more (up to 69) ramets and spread over large areas. Spatial autocorrelation analyses revealed a spatial genetic structure (i.e. significant positive autocorrelation within 20 m and negative autocorrelation beyond 40 m) in one population, but not in other three populations with smaller population size. This study highlights the importance of clonal growth in shaping the spatial genetic structure in Luohanguo, which may have complex effects on the dynamics and evolution of its declining populations.     

Size distribution and genetic structure in relation to clonal growth within a population of Magnolia tomentosa Thunb. (Magnoliaceae)

To establish a baseline for conservation of a threatened clonal tree, Magnolia tomentosa, we investigated size distribution and genetic structure within a population, using six microsatellite markers. Within the study site, 1044 living ramets (stems) were distinguished into 175 genets (individuals). The mean number of ramets per genet was 5.97, and 76% of all genets had multiple ramets. Genets, which apparently produced new ramets through sprouting and layering, were generally composed of several large ramets and many small ramets. Spatial autocorrelation analysis of microsatellite alleles revealed positive autocorrelation over short distances for both ramets and genets. The Moran's I-value of ramets in the shortest distance class was 3.8 times larger than that of genets, reflecting the effect of clonal growth. To analyse the size-class differences in genetic structure, the 175 genets were separated into two size classes, small and large. The correlogram for the small genets exhibited positive spatial autocorrelation in the shortest distance class, but this was not the case for the correlogram for the large genets, indicating that genetic structure is weakened or lost through self-thinning as the genets grow. The FIS value over all loci for the small genets was positive and deviated significantly from zero, while the corresponding value for the large genets was close to zero. The excess homozygotes in the small genets may be the result of genetic substructuring and/or inbreeding, and the reduction in homozygote frequency from the small to large genets may be because of loss of genetic structure and/or inbreeding depression.     

Local forest environment largely affects below-ground growth, clonal diversity and fine-scale spatial genetic structure in the temperate deciduous forest herb Paris quadrifolia

Paris quadrifolia (herb Paris) is a long-lived, clonal woodland herb that shows strong differences in local population size and shoot density along an environmental gradient of soil and light conditions. This environmentally based structuring may be mediated by differences in clonal growth and seedling recruitment through sexual reproduction. To study the interrelationship between environmental conditions and spatial patterns of clonal growth, the spatial genetic structure of four P. quadrifolia populations growing in strongly contrasting sites was determined. In the first place, plant excavations were performed in order to (i) determine differences in below-ground growth of genets, (ii) investigate connectedness of ramets and (iii) determine total genet size. Although no differences in internode length were found among sites, clones in moist sites were much smaller (genets usually consisted of 1-3 interconnected shoots, most of them flowering) than genets in dry sites, which consisted of up to 15 interconnected shoots, the majority of which were vegetative. Further, amplified fragment length polymorphism (AFLP) markers were used. Clonal diversity was higher in populations located in moist and productive ash-poplar forests compared to those found in drier and less productive mixed forest sites (G/N: 0.27 and 0.14 and Simpson's D: 0.84 and 0.75, respectively). Patterns of spatial population genetic structure under dry conditions revealed several large clones dominating the entire population, whereas in moist sites many small genets were observed. Nevertheless, strong spatial genetic structure of the genet population was observed. Our results clearly demonstrate that patterns of clonal diversity and growth form of P. quadrifolia differ among environments. Limited seedling recruitment and large clone sizes due to higher connectedness of ramets explain the low clonal diversity in dry sites. In moist sites, higher levels of clonal diversity and small clone sizes indicate repeated seedling recruitment, whereas strong spatial genetic structure suggests limited seed dispersal within populations.     

Clonality in the Endangered <Emphasis Type="Italic">Ambrosia pumila</Emphasis> (Asteraceae) Inferred from RAPD Markers; Implications for Conservation and Management

Clonal plants have the ability to spread and survive over long periods of time by vegetative growth. For endangered species, the occurrence of clonality can have significant impacts on levels of genetic diversity, population structure, recruitment, and the implementation of appropriate conservation strategies. Here we␣examine clone structure in three populations of Ambrosia pumila (Nutt.) Gray (Asteraceae), a federally endangered clonal species from southern California. Ambrosia pumila is a perennial herbaceous species spreading from a rhizome, and is frequently found in dense patches of several hundred stems in a few square meters. The primary habitat for this species is upper terraces of rivers and drainages in areas that have been heavily impacted by anthropogenic disturbances and changing flood regimes. RAPD markers were employed to document the number and distribution of clones within multiple 0.25 m² plots from each of three populations. Thirty-one multi-locus genotypes were identified from the 201 stems sampled. The spatial distribution of clones was limited with no genotypes shared between plots or populations. Mean clone size was estimated at 9.10 ramets per genet. Genets in most plots were intermingled, conforming to a guerrilla growth form. The maximum genet spread was 0.59 m suggesting that genets can be larger than the sampled 0.25 m² plots. Spatial autocorrelation analysis found a lack of spatial genetic structure at short distances and significant structure at large distances within populations. Due to the occurrence of multiple genets within each population, the limited spread of genets, and a localized genetic structure, conservation activities should focus on the maintenance of multiple populations throughout the species range.     

Clonal and spatial genetic structure within populations of a coastal plant, Carex kobomugi (Cyperaceae)

Clarification of clonal growth pattern is critical for understanding the population dynamics and reproductive system evolution of clonal plant species. The contribution of clonality to the spatial genetic structure (SGS) within populations is also an important issue. I examined the spatial distribution of genetic variability within two populations of the coastal plant Carex kobomugi using seven microsatellite loci. Genotyping of 226 and 140 ramets within 14 × 40 m and 14 × 34 m plots on two populations revealed 36 and 33 multilocus genotypes, respectively. To quantify the extent of intermingling among clones, for each genet, I calculated the dominance of ramets belonging to a particular genet within a spatial range of the genet. Furthermore, I analyzed spatial distribution of genotypes within 2 × 2 m and 1 × 2 m quadrats using second-order spatial statistics. These analyses indicated that clones are highly intermingled, suggesting a low level of spatial interaction among clones. Spatial autocorrelation analysis of kinship coefficient including all pairs of ramets showed significantly stronger SGS than analysis considering only pairs between different genets. I conclude that clonal propagation largely contributes to SGS at a fine scale.     

Spatial distribution pattern of a clonal species: effects of differential production of clonal and sexual offspring

The spatial distribution patterns of genets and ramets within populations are expected to change as a function of the frequency with which clonal species recruit different types of offspring (sexual and clonal). We used an integrated approach to study the spatial arrangement of clonal plants by combining molecular and ecological data using Opuntia microdasys as a study system. The species is able to produce two types of clonal (plantlets and cladodes) and one type of sexual (seeds) offspring. Additionally it is found in three habitats that cause differences in the ability of each type of offspring to establish. In 2007, all individuals in the three habitats (162 in BH = bajada, 264 in IDH = hill-piedmont, and 136 at HPH = interdunes) were tagged and mapped. Amplified inter-simple sequence repeats (ISSR’s) were used to determine the multilocus genotype and relatedness of each individual ramet using 120 polymorphic bands (104 in BH, 128 in HPH and 180 in IDH). The spatial distribution pattern of genets and ramets was analyzed with the Hopkins test and spatial autocorrelation analysis. For all habitats we found that O. microdasys displayed a spatial distribution characterized by clumps of aggregated ramets, but habitats differed in the number of genets present. As for other clonal species a strong positive spatial autocorrelation exists within 20 m, although all analyses suggest that adjacent ramets are genetically less related to each other or belong to different genets, that is, ramets of different genets are intermingled. The spatial arrangement of genets and ramets in O. microdasys between habitats closely matches the frequency of establishment of each type of offspring (e.g. the more clonal areas are clumped groups of related individuals). These results confirm that in two habitats (BH and IDH) clonal recruitment had been more common than in the other habitat (HPH).     

Fine-scale genetic structure among genetic individuals of the clone-forming monotypic genus Echinosophora koreensis (Fabaceae)

• Background and Aims For rare endemics or endangered plantspecies that reproduce both sexually and vegetatively it iscritical to understand the extent of clonality because assessmentof clonal extent and distribution has important ecological andevolutionary consequences with conservation implications. Asurvey was undertaken to understand clonal effects on fine-scalegenetic structure (FSGS) in two populations (one from a disturbedand the other from an undisturbed locality) of Echinosophorakoreensis, an endangered small shrub belonging to a monotypicgenus in central Korea that reproduces both sexually and vegetativelyvia rhizomes. • Methods Using inter-simple sequence repeats (ISSRs) asgenetic markers, the spatial distribution of individuals wasevaluated using Ripley's L(d)-statistics and quantified thespatial scale of clonal spread and spatial distribution of ISSRgenotypes using spatial autocorrelation analysis techniques(join-count statistics and kinship coefficient, F_ij) for totalsamples and samples excluding clones. • Key Results A high degree of differentiation betweenpopulations was observed (_ST(g) = 0·184, P < 0·001).Ripley's L(d)-statistics revealed a near random distributionof individuals in a disturbed population, whereas significantaggregation of individuals was found in an undisturbed site.The join-count statistics revealed that most clones significantlyaggregate at 6-m interplant distance. The Sp statistic reflectingpatterns of correlograms revealed a strong pattern of FSGS forall four data sets (Sp = 0·072–0·154), butthese patterns were not significantly different from each other.At small interplant distances (2 m), however, jackknifed 95% CIs revealed that the total samples exhibited significantlyhigher F_ij values than the same samples excluding clones. • Conclusion The strong FSGS from genets is consistentwith two biological and ecological traits of E. koreensis: bee-pollinationand limited seed dispersal. Furthermore, potential clone matesover repeated generations would contribute to the observed highF_ij values among genets at short distance. To ensure long-termex situ genetic variability of the endangered E. koreensis,individuals located at distances of 10–12 m should becollected across entire populations of E. koreensis.     

Clonal and spatial genetic structures of aspen (Populus tremuloides Michx.)

To portray aspen clonal and spatial genetic structures, we mapped and genotyped trees in two 1-ha plots, each containing three aspen cohorts originating from fire or subsequent secondary disturbances. We used four microsatellite loci to identify aspen clones and increment core analysis to determine tree age. Clonal dimensions were measured by the maximum distance between two ramets and the number of ramets per genet. Standard normal deviate (SND) was used to assess the spatial distribution of aspen genets and cohorts, and multivariate spatial genetic autocorrelations to assess the spatial distribution of aspen genetic variation. Most aspen genets consisted of only one ramet (> 75%). Median clonal dimensions were 19 and 29 m (maxima: 104 and 72 m in the two plots). No segregation was observed between clones. Aspen cohorts were spatially segregated but trees were spatially aggregated within old and medium-aged cohorts. In contrast, trees were more randomly distributed within the youngest cohorts. This coincided with a spatial genetic autocorrelation at small scales (up to 30 m) in the older cohorts and a more random genetic distribution in the youngest ones. Our results suggest that aspen spatial genetic structuring reflects the spatial patterns produced by the regeneration of discrete cohorts at different stages of succession. Vegetative reproduction leads to aspen genetic spatial structuring at small scales (few metres) until midsuccession. However, as the stand gets older, the spatial distribution of aspen trees and genetic structure evolve from a structured pattern to a more random one under a gap disturbances regime.     

Population genetic structure of the medicinal plant Vitex rotundifolia in China: implications for its use and conservation

Vitexrotundifolia L.is an important plant species used in traditional Chinese medicine.For its efficient use and conservation,genetic diversity and clonal variation of V.rotundifolia populations in China were investigated using inter-simple sequence repeat markers.Fourteen natural populations were included to estimate genetic diversity,and a large population with 135 individuals was used to analyze clonal variation and fine-scale spatial genetic structure.The overall genetic diversity (GD) of V.rotundifolia populations in China was moderate (GD=0.190),with about 40% within-population variation.Across all populations surveyed,the average within-population diversity was moderate (P = 22.6%; GD = 0.086).A relatively high genetic differentiation (Gst=0.587)among populations was detected based on the analysis of molecular variance data.Such characteristics of V.rotundifolia are likely attributed to its sexual/asexual reproduction and limited gene flow.The genotypic diversity (D=0.992) was greater than the average values of a clonal plant,indicating its significant reproduction through seedlings.Spatial autocorrelation analysis showed a clear within-population structure with gene clusters of approximately 20 m.Genetic diversity patterns of V.rotundifolia in China provide a useful guide for its efficient use and conservation by selecting particular populations displaying greater variation that may contain required medicinal compounds,and by sampling individuals in a population at >20 m spatial intervals to avoid collecting individuals with identical or similar genotypes.     

High variation in clonal vs. sexual reproduction in populations of the wild strawberry, Fragaria virginiana (Rosaceae)

Background and Aims

Many plants reproduce both clonally and sexually, and the balance between the two modes of reproduction will vary among populations. Clonal reproduction was characterized in three populations of the wild strawberry, Fragaria virginiana, to determine the extent that reproductive mode varied locally between sites. The study sites were fragmented woodlands in Cook County, Illinois, USA.

Methods

A total of 95 strawberry ramets were sampled from the three sites via transects. Ramets were mapped and genotyped at five variable microsatellite loci. The variability at these five loci was sufficient to assign plants to clones with high confidence, and the spatial pattern of genets was mapped at each site.

Key Results

A total of 27 distinct multilocus genotypes were identified. Of these, 18 genotypes were detected only once, with the remaining nine detected in multiple ramets. The largest clone was identified in 16 ramets. No genets were shared between sites, and each site exhibited markedly different clonal and sexual recruitment patterns, ranging from two non-overlapping and widespread genets to 19 distinct genets. Only one flowering genet was female; the remainder were hermaphrodites.

Conclusions

Local population history or fine-scale ecological differences can result in dramatically different reproductive patterns at small spatial scales. This finding may be fairly widespread among clonal plant species, and studies that aim to characterize reproductive modes in species capable of asexual reproduction need to evaluate reproductive modes in multiple populations and sites.Key words: Clonal structure, gynodioecy, Fragaria virginiana, microsatellites, population genetic structure     

The impact of extensive clonal growth on fine-scale mating patterns: a full paternity analysis of a lily-of-the-valley population (Convallaria majalis)

Background and Aims

The combination of clonality and a mating system promoting outcrossing is considered advantageous because outcrossing avoids the fitness costs of selfing within clones (geitonogamy) while clonality assures local persistence and increases floral display. The spatial spread of genetically identical plants (ramets) may, however, also decrease paternal diversity (the number of sires fertilizing a given dam) and fertility, particularly towards the centre of large clumped clones. This study aimed to quantify the impact of extensive clonal growth on fine-scale paternity patterns in a population of the allogamous Convallaria majalis.

Methods

A full analysis of paternity was performed by genotyping all flowering individuals and all viable seeds produced during a single season using AFLP. Mating patterns were examined and the spatial position of ramets was related to the extent of multiple paternity, fruiting success and seed production.

Key Results

The overall outcrossing rate was high (91 %) and pollen flow into the population was considerable (27 %). Despite extensive clonal growth, multiple paternity was relatively common (the fraction of siblings sharing the same father was 0·53 within ramets). The diversity of offspring collected from reproductive ramets surrounded by genetically identical inflorescences was as high as among offspring collected from ramets surrounded by distinct genets. There was no significant relationship between the similarity of the pollen load received by two ramets and the distance between them. Neither the distance of ramets with respect to distinct genets nor the distance to the genet centre significantly affected fruiting success or seed production.

Conclusions

Random mating and considerable pollen inflow most probably implied that pollen dispersal distances were sufficiently high to mitigate local mate scarcity despite extensive clonal spread. The data provide no evidence for the intrusion of clonal growth on fine-scale plant mating patterns.     

Clonal Diversity in Differently-Aged Populations of the Pseudo-Annual Clonal Plant Circaea lutetiana L.

Abstract: In many clonal plant species seedling recruitment is restricted to short colonization episodes early in the development of the population, and clonal diversity (i.e., genet diversity) in the population is expected to decrease with increasing population age. In established populations of the pseudo-annual Circaea lutetiana seedling recruitment has previously not been observed. Therefore, we expected established populations to have low clonal diversities. We analysed number and frequency of genets and spatial distribution of genets in six differently-aged C. lutetiana populations with the use of four informative RAPD primers. We found relatively low clonal diversities in young populations but very high clonal diversities in established populations. Therefore, the hypothesis was rejected that seedling recruitment does not occur in established populations. Moreover, we did not find large genet size asymmetries in established populations. Genet size differences can be caused by stochastic processes or by fitness related traits, such as differences in vegetative reproduction. Because vegetative propagation of ramets is dependent on ramet size, and the number of ramets and the size of each ramet determine genet size, we expected that large genets produced, on average, large ramets. However, this was not the case, suggesting that stochastic processes caused genet size differences. Genet size may also be bounded if spatial distribution of genets is affected by micro-habitat differences. For this we expected to find a clumped spatial distribution of ramets of the same genet. However, ramets of large genets were always found intermingled with ramets belonging to other genets.     

Clonal dynamics in western North American aspen (Populus tremuloides)

Clonality is a common phenomenon in plants, allowing genets to persist asexually for much longer periods of time than ramets. The relative frequency of sexual vs. asexual reproduction determines long‐term dominance and persistence of clonal plants at the landscape scale. One of the most familiar and valued clonal plants in North America is aspen (Populus tremuloides). Previous researchers have suggested that aspen in xeric landscapes of the intermountain west represent genets of great chronological age, maintained via clonal expansion in the near absence of sexual reproduction. We synthesized microsatellite data from 1371 ramets in two large sampling grids in Utah. We found a surprisingly large number of distinct genets, some covering large spatial areas, but most represented by only one to a few individual ramets at a sampling scale of 50 m. In general, multi‐ramet genets were spatially cohesive, although some genets appear to be fragmented remnants of much larger clones. We conclude that recent sexual reproduction in these landscapes is a stronger contributor to standing genetic variation at the population level than the accumulation of somatic mutations, and that even some of the spatially large clones may not be as ancient as previously supposed. Further, a striking majority of the largest genets in both study areas had three alleles at one or more loci, suggesting triploidy or aneuploidy. These genets tended to be spatially clustered but not closely related. Together, these findings substantially advance our understanding of clonal dynamics in western North American aspen, and set the stage for a broad range of future studies.     

Genetic neighbourhood of clone structures in eelgrass meadows quantified by spatial autocorrelation of microsatellite markers

Limited dispersal distances in plant populations frequently cause local genetic structure, which can be quantified by spatial autocorrelation. In clonal plants, three levels of spatial organization can contribute to positive autocorrelation; namely, the neighbourhood of (a) ramets, (b) clone fragments and (c) entire clones. Here we use data from an exhaustive sampling scheme on a clonal plant to measure the contribution of the neighbourhoods of each distinct clonal structure to total spatial autocorrelation. Four plots (256 grid points each) within dense meadows of the marine clonal plant Zostera marina (eelgrass) were sampled for clone structure with nine microsatellite markers ( approximately 80 alleles). We found significant coancestry (f(ij)), at all three levels of spatial organization, even when not allowing for joins between samples of identical genets. In addition, absolute values of f(ij) and the maximum distance with significant positive f(ij) decreased with the progressive exclusion of joins between alike genotypes. The neighbourhood of this clonal plant thus consists of three levels of organization, which are reflected in different kinship structures. Each of these kinship structures may affect the level of biparental inbreeding and the physical distance between flowering shoots and their outcrossing neighbourhood. These results also emphasize the notion that spatial autocorrelation crucially depends on the scale and intensity of sampling.     

Fine-scale spatial structure of genets and sexes in the dioecious plant <Emphasis Type="Italic">Dioscorea japonica</Emphasis>, which disperses by both bulbils and seeds

To understand the evolution of clonal reproduction and the diversity of clonal plants, it is necessary to clarify the characteristics of each clonal habit. There has been little research on whether bulbils alter spatial genetic structure (SGS) because of the lack of connection to maternal ramets. We used simple-sequence-repeat (SSR) markers to determine the fine-scale SGS of the dioecious plant Dioscorea japonica, which disperses both as bulbils and as seeds. We also evaluated the contributions of sexual and clonal reproduction and tested for spatial sex segregation (SSS). We discovered 111 genets from 394 ramets in a 2.8-ha plot. Genotypic richness (R = 0.28) and clonal diversity (Simpson’s D = 0.94, Fager’s E = 0.90) were high. We did not find SSS, suggesting that the population does not suffer from a shortage of mating pairs due to clonal reproduction. The Sp values revealed moderate SGS at the genet level (Sp = 0.013–0.014), and the genets intermingled at a local scale. Significant SGS at the ramet level showed that ramets within the same genet tended to aggregate. We also found a skewed clonal spatial distribution. The spatial extent of genets was positively correlated with the number of ramets within a genet. The contribution of bulbil production to the variance of parent–offspring gene dispersal was about one–fifth the contribution from sexual reproduction. These results suggest that the dispersal via bulbils affects the SGS in D. japonica, although its contribution to gene dispersal is small compared to the contribution of sexual reproduction.     

Clonal diversity and spatial genetic structure of Potamogeton pectinatus in managed pond and river populations

Higher levels of genetic diversity of river macrophytes are expected in downstream parts because of potential accumulation of various genotypes from upstream sites. We assessed the clonal diversity and spatial genetic structure of fennel pondweed (Potamogeton pectinatus or Stuckenia pectinata) populations with emphasis on the estimation of dispersal via clonal propagules along a river in connection to upstream ponds. We analysed genetic diversity of 354 plant shoots sampled in 2005 and 2006 at three pond and five river sites in the Woluwe river catchment (Belgium). Nine microsatellite DNA markers revealed 88 genets of which 89% occurred in only one site. Clonal propagule dispersal was detected up to 10 km along the river. Few multilocus genotypes were repeatedly present along a major part of the river indicating vegetative spread. Populations of ponds contained a higher amount of clonal diversity, indicating the importance of local seed recruitment. A fine-scaled spatial genetic structure indicated that most seedling recruitment occurred at a distance <5 m in pond populations whereas clones in river sites were unrelated and showed no spatial autocorrelation. The clonal diversity decreased along the river from upstream to downstream due to establishment of few large clones.     

Clonal diversity and structure within a population of the pondweed Potamogeton pectinatus foraged by Bewick's swans

Clonal diversity within plant populations is affected by factors that influence genet (clone) survival and seed recruitment, such as resource availability, disturbance, seed dispersal mechanism, propagule predation and the age of the population. Here we studied a population of Potamogeton pectinatus, a pseudo-annual aquatic macrophyte. Within populations reproduction appears to be mainly asexually through subterranean propagules (tubers), while recruitment via seeds is believed to be relatively unimportant. RAPD markers were used to analyse clonal diversity and genetic variation within the population. Ninety-seven genets were identified among 128 samples taken from eight plots. The proportion of distinguishable genets (0.76) and Simpson's diversity index (0.99) exhibited high levels of clonal diversity compared to other clonal plants. According to an analysis of molecular variance (amova) most genetic variation occurred between individuals within plots (93-97%) rather than between plots (8-3%). These results imply that sexual reproduction plays an unexpectedly important role within the population. Nevertheless, autocorrelation statistics revealed a spatial genetic structure resulting from clonal growth. In contrast to genetic variation, clonal diversity was affected by several ecological factors. Water depth and silt content had direct negative effects on clonal diversity. Tuber predation by Bewick's swans had an unexpected indirect negative effect on clonal diversity through reducing the tuber-bank biomass in spring, which on its turn was positively correlated to clonal diversity. The disturbance by swans, therefore, did not enhance seed recruitment and thus clonal diversity; on the contrary, heavily foraged areas are probably more prone to stochastic loss of genets leading to reduced clonal diversity.     

贺兰山丁香自然居群克隆生长格局及遗传多样性的ISSR分析

运用ISSR分子标记技术,通过制定挖掘采样、"+"形采样及"垂直"采样3种采样方案,对贺兰山丁香(Syringa pinnatifolia var.alanshanica)不同居群的克隆多样性、克隆生长格局及其遗传多样性进行了分析.克隆多样性分析表明:挖掘采样方式采到的3个克隆系内,各自所包含的单株间具有完全相同的基因型;"垂直"采样及"+"形采样的7个居群、 239个样品表现出136个不同的基因型或克隆,显示贺兰山丁香具有较高的克隆多样性(D=0.994)及基因型分布均匀性(E=0.985).克隆生长空间格局分析表明,贺兰山丁香为密集型克隆植物.每个居群都由多克隆组成,克隆生长只发生在同一丛内,多数基株只含有1个分株,最多可达8个.物种水平上的平均克隆大小(N_C)和平均基因型比例(PD)分别为1.757和0.569.对贺兰山丁香遗传多样性分析的结果显示,在居群水平和物种水平上都保持着较高的遗传多样性,其遗传变异主要存在于居群内;但居群间分化程度较低(G_ST=0.320),表明自然居群间基因交流有限.     

Clonal spread of invasive Ludwigia hexapetala and L. grandiflora in freshwater wetlands of California

Ludwigia hexapetala and L. grandiflora are recent, aggressive invaders of freshwater wetlands in California. To assess the relative role of sexual versus clonal reproduction in invasive spread, we used AFLP markers to genotype 794 ramets of L. hexapetala sampled from 27 populations in three watersheds of California, and 150 ramets of L. grandiflora from five populations in a fourth watershed. We then used two analytical methods, similarity thresholds and character incompatibilities, to distinguish genotypic variation within genets (clones) from variation between genets. Our analyses revealed extremely limited genotypic and genet variation in invasive L. hexapetala and L. grandiflora within California. Within L. hexapetala, 95% of the ramets analyzed represented a single genet. The genet was the only one detected in 20 populations. The remaining seven populations contained two to nine genets. Within L. grandiflora, all ramets were of only one genotype. Thus, invasive spread within and between populations, and across watersheds, appears to be almost exclusively clonal and brought about by the dispersal of vegetative propagules. The extremely low seedling recruitment indicates that management should target vegetative dispersal and growth.     

不同年龄巴山木竹种群克隆结构

很多竹类植物是典型的克隆植物,也是大熊猫的食物。研究典型竹子种群克隆结构的形成和发展对竹林的生产和抚育具有理论和实践意义,可为预测该竹林群落的演替趋势和大熊猫保护提供科学依据。利用SSR标记研究不同年龄A(7龄)、B(30龄)和C(60龄)巴山木竹种群的克隆结构和多样性,探讨小尺度范围内不同年龄巴山木竹种群的克隆结构及斑块的建立和发展。8对SSR引物共扩增出了118个位点,3个种群样地的256个样本共检测到了49个克隆(基因型),A、B和C种群分别检测出31、10个和8个克隆。随着种群年龄的增长,巴山木竹克隆面积增加,克隆数量减少;A和B样地各克隆分布格局为团块状,而C样地克隆既有团块状又有离散状。这一结果显示出在幼苗定居的初期,基株可能以短距离的克隆延伸为主从而呈现出团块状;而随着年龄的增长,克隆面积不断扩大,当复轴混生型的巴山木竹克隆受到强大的压迫时,基株可能会进行较多的单轴和长距离克隆延伸,呈现出离散状。Mantel检测和空间自相关分析都支持3个样地在小尺度范围内存在明显的克隆空间遗传结构。3个样地在10 m等级下显著的正相关空间遗传结构距离为3.1、28、48 m,X-轴截距为9.051、30.698和50.536,空间自相关系数的范围分别为0.1—0.167、0.008—0.703和0.006—0.735。由此可推断,随着年龄的增长,巴山木竹克隆斑块的规模在不断地扩大,同一克隆的分株数量增加,在均匀取样情况下,正相关空间遗传结构距离范围内取到具有相同基因型的可能性越大。A、B和C 3样地的基因型比率(G/N)为1、0.14和0.055,Simpson多样性指数(D)分别为1、0.876和0.744。这说明巴山木竹幼苗期基因型比例远远高于成年的竹林,随着年龄的增长巴山木竹克隆多样性虽有所降低,但由于有性繁殖的作用仍然保持了较高的多样性。聚类和主坐标分析均表明总体上各样地的克隆被聚为一类,但不同样地少数克隆的基因型有重叠和聚集,可推断出不同巴山木竹种群之间可能存在着基因流动和近似的克隆起源。