Genet structure and determinants of clonal structure in a temperate deciduous woodland herb, Uvularia perfoliata

1 We used isozyme variation to examine the genet structure of Uvularia perfoliata patches in gap and closed canopy habitats in a temperate deciduous forest in Maryland, USA.
2 A large patch in a gap habitat was composed of a small number of widely spread genets with many ramets, and a large number of genets with more restricted distribution and few ramets. Genets with many ramets were patchily distributed at a metre scale. Analysis of genet structure on a scale of square centimetres, however, revealed that the genets were highly intermingled with no clear boundaries between them. The presence at both scales of sampling of many genets with unique multilocus genotypes indicated continuing genet recruitment within the population.
3 In the closed canopy habitat, the patches examined were each composed of a single unique multilocus genotype, suggesting that each had developed by asexual propagation following the establishment of a single genet.
4 The clonal structure of U. perfoliata patches in both gap and closed canopy habitats therefore appears to depend on recruitment patterns of genets. Populations in closed canopy habitats are characterized by a 'waiting' strategy, in which asexual ramet production maintains populations until genet recruitment by seed production can occur under the more optimal conditions associated with canopy gaps. Extended sampling suggests that the genetic diversity of U. perfoliata populations is primarily controlled by the disturbance regime of the forest canopy.     

Large clones on cliff faces: expanding by rhizomes through crevices

BACKGROUND AND AIMS: Large clones of rhizomatous plants are found in many habitats, but little is known about whether such clones also occur on cliff faces where environmental conditions are extremely harsh and heterogeneous. METHODS: Using molecular (intersimple sequence repeat, ISSR) markers, the genotypic composition of a cliff-face population of Oxyria sinensis in Sichuan, China, was investigated. KEY RESULTS: The 98 O. sinensis ramets sampled belonged to 12 different genotypes (clones). The three most frequent clones were represented with 45, 22 and 12 ramets, respectively; the remaining nine were represented with only one to five ramets. The three largest clones spanned at least 2.7 m in the vertical direction and 4.6-6.9 m in the horizontal direction on the cliff face. CONCLUSIONS: On the cliff face, large clones of O. sinensis are formed by rhizomes growing along the crevices. Expansion by rhizomes may help O. sinensis to exploit the patchy resources and support establishment and growth of new ramets. Moreover, rooted ramets connected by rhizomes may effectively reduce the susceptibility of O. sinensis to rock fall and erosion and thus greatly improve the chances for long-term survival. The multi-clone structure indicates that sexual reproduction is also important for the long-term persistence of O. sinensis populations on cliffs.     

Patch establishment and development of a clonal plant,Polygonum cuspidatum,on Mount Fuji

Microsatellite analysis was used to investigate the patch establishment and development of Polygonum cuspidatum Sieb. et Zucc, a clonal herbaceous plant that dominates the primary succession on the southeast slope of Mount Fuji. Genotypes of P. cuspidatum in 155 patches at the study site differed from each other. This indicates that P. cuspidatum patches are initially established by seed dispersed on the bare scoria field, and not by clonal rhizome extension. Genetic differentiation was estimated using the FST values between subpopulations at the study site. There was almost no genetic differentiation between subpopulations, indicating the presence of massive gene flow. The pollen fathers of seeds and maternal genets of current-year seedlings were inferred from the microsatellite allele composition by a simple exclusion method. The wide, random distribution of pollen fathers suggests that pollen dispersal occurs over a broad area. Maternal analysis showed a tendency for seed dispersal to be biased to the area nearby and down slope from the mother plants. Patch establishment under massive gene flow may result from such pollen and seed dispersal. To understand the process of patch development, aerial photographs taken from 1962 to 1999 were compared, and then genets in each of 36 patches were identified from the microsatellite genotypes of P. cuspidatum shoots. The comparison of aerial photographs showed that most of the patches enlarged each year and that some neighbouring patches combined during growth. Genet analysis demonstrated a high correlation between patch area and the area of the largest genet within it, and that new genets were recruited at the patch periphery. These findings indicate that both vegetative and sexual reproduction, i.e. rhizome extension and the establishment of new seedlings, contribute to the development of P. cuspidatum patches.     

A simulation study of the effects of architectural constraints and resource translocation on population structure and competition in clonal plants

(1) Spatially explicit simulation of clonal plant growth is used to determine how ramet-level traits affect ramet density, spatial pattern of ramets and competitive ability of a clonal plant. The simulation model used combines elements of (i) an individual-based model of plant interactions, (ii) an architectural model of clonal plant growth, and (iii) a model of resource translocation within a set of physiologically integrated plant individuals. (2) The effects of two groups of parameters were studied: growth and resource acquisition parameters (resource accumulation, density-dependence of resource accumulation, resource translocation between ramets) and architectural rules (branching angle and probability of branching, internode length). The model was parameterised by values approximating those of clonally growing grasses as closely as possible. The basic parameter values were chosen from a short-turf grassland. Sensitivity analysis was carried out on relevant parameters around three basic points in the parameter space. Both single-species and two-species systems were studied. (3) It is shown that increasing resource acquisition and growth parameters increase ramet density, genet number and competitive ability. Translocation parameters and architectural parameters modify the effects of resource acquisition and growth, but their effect in single-species stands was smaller. (4) The simulations of species with fixed ramet sizes showed that ramet density in single-species stands cannot be used for predicting competitive ability. Increase in resource acquisition and growth parameters was correlated with an increase in equilibrium ramet density and competitive ability. Increasing branching angle, branching probability or internode length lead to an increased competitive ability, but did not affect equilibrium ramet density. Change of architectural parameters could therefore affect competitive ability independently of their effect on the final ramet density. (5) Spatial pattern both in single-species and two-species stands was also highly parameter-dependent. Changes in architectural parameters and in translocation usually lead to pronounced change in the spatial pattern; change in growth and resource acquisition parameters generally had little effect on spatial pattern.     

无性系植物种群生态学研究进展及有关概念

无性系植物种群生态学研究进展及有关概念刘庆钟章成（中国科学院成都生物研究所,６１００４１）（西南师范大学生物系,重庆６３０７１５）ＡｄｖａｎｃｅｓｉｎＥｃｏｌｏｇｉｃａｌＲｅｓｅａｒｃｈｏｆＣｌｏｎａｌＰｌａｎｔＰｏｐｕｌａｔｉｏｎａｎｄＳｏｍｅＲｅ...     

clonality V.0.4: a randomization-based program to test for heterozygosity-genet size relationships in clonal organisms

clonality V.0.4 is a program for testing heterozygosity-genet size relationships in clonal organisms using a randomization procedure. The software has been developed under the Borland Delphi developing environment and a Windows-executable version is freely downloadable from http://gemi.mpl.ird.fr/SiteSGASS/Prugnolle/ClonalityPage.html. The program compares the observed F(IS) of the population with the F(IS) expected if genets (multilocus genotypes present in multiple copies within the population) were chosen randomly from the set of different multilocus genotypes. The randomization procedure is performed with the same number of genets and the same number of repetitions per genet as what is observed in the original data set.     

Investigating the effects of topography and clonality on genetic structuring within a large Norwegian population of Arabidopsis lyrata

Background and Aims

The gene flow through pollen or seeds governs the extent of spatial genetic structure in plant populations. Another factor that can contribute to this pattern is clonal growth. The perennial species Arabidopsis lyrata ssp. petraea (Brassicaceae) is a self-incompatible, clonal species found in disjunctive populations in central and northern Europe.

Methods

Fourteen microsatellite markers were employed to study the level of kinship and clonality in a high-altitude mountain valley at Spiterstulen, Norway. The population has a continuous distribution along the banks of the River Visa for about 1·5 km. A total of 17 (10 m × 10 m) squares were laid out in a north–south transect following the river on both sides.

Key Results

It is shown that clonal growth is far more common than previously shown in this species, although the overall size of the genets is small (mean diameter = 6·4 cm). Across the whole population there is no indication of isolation by distance, and spatial genetic structure is only visible on fine spatial scales. In addition, no effect of the river on the spatial distribution of genotypes was found.

Conclusions

Unexpectedly, the data show that populations of small perennials like A. lyrata can behave like panmictic units across relatively large areas at local sites, as opposed to earlier findings in central Europe.     

Longevity of clonal plants: why it matters and how to measure it

Background

Species'' life-history and population dynamics are strongly shaped by the longevity of individuals, but life span is one of the least accessible demographic traits, particularly in clonal plants. Continuous vegetative reproduction of genets enables persistence despite low or no sexual reproduction, affecting genet turnover rates and population stability. Therefore, the longevity of clonal plants is of considerable biological interest, but remains relatively poorly known.

Scope

Here, we critically review the present knowledge on the longevity of clonal plants and discuss its importance for population persistence. Direct life-span measurements such as growth-ring analysis in woody plants are relatively easy to take, although, for many clonal plants, these methods are not adequate due to the variable growth pattern of ramets and difficult genet identification. Recently, indirect methods have been introduced in which genet size and annual shoot increments are used to estimate genet age. These methods, often based on molecular techniques, allow the investigation of genet size and age structure of whole populations, a crucial issue for understanding their viability and persistence. However, indirect estimates of clonal longevity are impeded because the process of ageing in clonal plants is still poorly understood and because their size and age are not always well correlated. Alternative estimators for genet life span such as somatic mutations have recently been suggested.

Conclusions

Empirical knowledge on the longevity of clonal species has increased considerably in the last few years. Maximum age estimates are an indicator of population persistence, but are not sufficient to evaluate turnover rates and the ability of long-lived clonal plants to enhance community stability and ecosystem resilience. In order to understand the dynamics of populations it will be necessary to measure genet size and age structure, not only life spans of single individuals, and to use such data for modelling of genet dynamics.     

Patch size and distance: modelling habitat structure from the perspective of clonal growth

Background and Aims

This study considers the spatial structure of patchy habitats from the perspective of plants that forage for resources by clonal growth. Modelling is used in order to compare two basic strategies, which differ in the response of the plant to a patch boundary. The ‘avoiding plant’ (A) never grows out of a good (resource-rich) patch into a bad (resource-poor) region, because the parent ramet withdraws its subsidy from the offspring. The ‘entering plant’ (E) always crosses the boundary, as the offspring is subsidized at the expense of the parent. In addition to these two extreme scenarios, an intermediate mixed strategy (M) will also be tested. The model is used to compare the efficiency of foraging in various habitats in which the proportion of resource-rich areas (p) is varied.

Methods

A stochastic cellular automata (CA) model is developed in which habitat space is represented by a honeycomb lattice. Each cell within the lattice can accommodate a single ramet, and colonization can occur from a parent ramet''s cell into six neighbouring cells. The CA consists of two layers: the population layer and the habitat. In the population layer, a cell can be empty or occupied by a ramet; in the habitat layer, a cell can be good (resource-rich) or bad (resource-poor). The habitat layer is constant; the population layer changes over time, according to the birth and death of ramets.

Key Results

Strategies M and E are primarily limited by patch distance, whereas A is more sensitive to patch size. At a critical threshold of the proportion of resource-rich areas, p = 0·5, the mean patch size increases abruptly. Below the threshold, E is more efficient than A, whilst above the threshold the opposite is true. The mixed strategy (M) is more efficient than either of the pure strategies across a broad range of p values.

Conclusions

The model predicts more species/genotypes with the ‘entering’ strategy, E, in habitats where resource-rich patches are scattered, and more plants with the ‘avoiding’ strategy, A, in habitats where the connectivity of resource-rich patches is high. The results suggest that the degree of physiological integration between a parent and an offspring ramet is important even across a very short distance because it can strongly influence the efficiency of foraging.