Flowering plants possess an unrivaled diversity of mechanisms for achieving sexual and asexual reproduction, often simultaneously. The commonest type of asexual reproduction is clonal growth (vegetative propagation) in which parental genotypes (genets) produce vegetative modules (ramets) that are capable of independent growth, reproduction, and often dispersal. Clonal growth leads to an expansion in the size of genets and increased fitness because large floral displays increase fertility and opportunities for outcrossing. Moreover, the clonal dispersal of vegetative propagules can assist "mate finding," particularly in aquatic plants. However, there are ecological circumstances in which functional antagonism between sexual and asexual reproductive modes can negatively affect the fitness of clonal plants. Populations of heterostylous and dioecious species have a small number of mating groups (two or three), which should occur at equal frequency in equilibrium populations. Extensive clonal growth and vegetative dispersal can disrupt the functioning of these sexual polymorphisms, resulting in biased morph ratios and populations with a single mating group, with consequences for fertility and mating. In populations in which clonal propagation predominates, mutations reducing fertility may lead to sexual dysfunction and even the loss of sex. Recent evidence suggests that somatic mutations can play a significant role in influencing fitness in clonal plants and may also help explain the occurrence of genetic diversity in sterile clonal populations. Highly polymorphic genetic markers offer outstanding opportunities for gaining novel insights into functional interactions between sexual and clonal reproduction in flowering plants.

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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.

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Comparisons of the causes and consequences of cross- and self-fertilization have dominated research on plant mating since Darwin's seminal work on plant reproduction. Here, I provide examples of these accomplishments, but also illustrate new approaches that emphasize the role of floral design and display in pollen dispersal and fitness gain through male function. Wide variation in outcrossing rate characterizes animal-pollinated plants. In species with large floral displays, part of the selfing component of mixed mating can arise from geitonogamy and be maladaptive because of strong inbreeding depression and pollen discounting. Floral strategies that separate the benefits of floral display from the mating costs associated with geitonogamy can resolve these conflicts by reducing lost mating opportunities through male function. The results from experiments with marker genes and floral manipulations provide evidence for the function of herkogamy and dichogamy in reducing self-pollination and promoting pollen dispersal. Evidence is also presented indicating that increased selfing resulting from changes to floral design, or geitonogamy in large clones, can act as a stimulus for the evolution of dioecy. The scope of future research on mating strategies needs to be broadened to include investigations of functional links among flowers, inflorescences and plant architecture within the framework of life-history evolution.

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1. In flowering plants the balance between sexual and clonal, asexual reproduction can vary widely. We quantified variation in sexual reproduction in a tristylous, clonal, aquatic plant, Decodon verticillatus, and investigated the role of ecological and genetic factors in causing this variation. 2. We surveyed components of sexual fertility and vegetative growth in 28 populations distributed along a 500-km latitudinal transect in New England, USA. Northerly populations tend to be monomorphic (M) for style length, and probably therefore have reduced sexual reproduction compared with southerly, trimorphic (T) populations. 3. Compared with T populations (n = 10), M populations (n = 18) exhibited large reductions for all components of sexual reproduction, including flower production, pollen deposition, pollen tube growth, fertilization, fruit set and seeds per fruit. Seven M populations produced no seed at all, and the other 11 very little (mean = 24 vs. 1139 seeds per plant in trimorphic populations). Clonal propagation was also greatly reduced in M populations. 4. A survey of three polymorphic allozyme loci detected only single, usually heterozygous, genotypes in 15 M populations, whereas all T populations were genotypically diverse. The other three M populations contained three or fewer genotypes and one always predominated. Sexual recruitment is therefore extremely rare. 5. Comparison of the sexual fertility of M and T populations in a concurrent common glasshouse experiment with our field data revealed that reduced sexual performance in northern M populations is principally due to genetic factors, but is also caused by ecological factors that covary with latitude. 6. This abrupt shift away from sexual reproduction in populations at the northern periphery of the geographical range in D. verticillatus may greatly limit their evolutionary potential and restrict further northward expansion.

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The fitness consequences of self‐fertilization are largely determined by how self‐pollination occurs. Within‐flower self‐pollination (autogamy) may be advantageous, since it can provide reproductive assurance without much seed or pollen discounting. In contrast, between‐flower self‐pollination (geitonogamy) provides no reproductive assurance and can cause severe seed and pollen discounting. I used floral emasculations with marker‐gene analysis to estimate the components of self‐fertilization in a tristylous, self‐compatible, clonal, mass‐flowering plant, Decodon verticillatus. This species produces 30% of progeny through selfing. I assessed the contribution of autogamy to selfing by comparing pollen deposition, seed production, and the selfing rate of flowers emasculated before anther dehiscence with intact flowers. Emasculation had no effect on pollen deposition, caused a small increase in seed production, and only reduced self‐fertilization by 14%, suggesting that most selfing occurs through geitonogamy. Geitonogamy can be due to self‐pollination between flowers on the same branch, different flowering branches of the same plant, or different ramets of the same clonal genet. I assessed the contribution of within‐branch geitonogamy by emasculating all flowers on a branch, which reduced selfing from 0.29 ± 0.03 (mean ± 1 se) to 0.19 ± 0.04. Between‐branch geitonogamy was estimated by comparing the selfing rate of plants with only a single flowering branch (0.16 ± 0.07) to plants with multiple flowering branches (0.27 ± 0.04). Selfing rates of individual branches also correlated positively with the daily number of flowers open on nonfocal branches of the same plant. Between‐ramet geitonogamy was suggested by significant self‐fertilization by single‐branch plants, even when all flowers were emasculated (0.10 ± 0.07). Selfing rates of individual branches also correlated negatively with measures of local clonal diversity. Based on these results, autogamy accounts for only 18 ± 14% of self‐fertilization, with the remainder (82 ± 17%) due to geitonogamy, which occurs about equally through pollination within branches (31 ± 22%), between branches (38 ± 32%), and between ramets (31 ± 28%). Because selfing occurs mostly through geitonogamy and is associated with strong inbreeding depression, it seems disadvantageous. Selfing in D. verticillatus has probably not been selected directly, but is a by‐product of self‐compatibility, large plant size, mass‐flowering, and clonal propagation.

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