Summary 1In 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.

Many flowering plants exhibit dual reproductive modes, producing both sexual and asexual offspring. The commonest form of asexual reproduction is clonal growth, in which vegetative modules (ramets) are produced by the parental genotype (genet). In plants, sexual and asexual reproduction usually occur simultaneously, and this can lead to allocation trade-offs and antagonism between reproductive modes. Our review considers the ecological and evolutionary consequences of functional interactions between clonal reproduction and pollination and mating. Clonal reproduction is commonly associated with mass flowering, restricted pollen dispersal, and geitonogamous self-pollination, processes that can result in inbreeding depression and pollen discounting. We review evidence for the correlated evolution of clonality and sexual systems, particularly self-incompatibility, and identify several floral mechanisms that function to reduce mating costs by limiting selfing and pollen discounting. We conclude by discussing the loss of sexuality in clonal plants and consider the genetic and environmental basis of sexual dysfunction. 193 Review in Advance first posted online on August 10, 2010. (Changes may still occur before final publication online and in print.) Changes may still occur before final publication online and in print.

Summary 1The phenotypic plasticity of vegetative traits is a characteristic feature of aquatic plants, promoting survival and growth in the heterogeneous environments typical of wetlands. Less is known about plastic responses of life-history and reproductive traits, particularly patterns of sex allocation. 2 We investigated the plasticity of vegetative and reproductive traits in Sagittaria latifolia , a clonal aquatic plant whose populations are either monoecious or dioecious. Plants of the two sexual systems exhibit divergent life-history characters associated with the disturbed vs. competitive habitats in which monoecious and dioecious populations occur, respectively. We evaluated the prediction that populations of the two sexual systems would have different patterns of phenotypic plasticity because of the contrasting habitats in which they occur. 3 We grew four clonal replicates of 10 genotypes from seven monoecious and five dioecious populations (total = 480 plants) in two fertilizer treatments under glasshouse conditions and measured components of life history, leaf and flower morphology, and sex allocation. 4 The two sexual systems displayed divergent patterns of plasticity for four life-history traits but only flowering time and ramet production showed the expected pattern of greater plasticity in monoecious populations, and the reverse was true for flower production. Fertilization had opposite effects in the two sexual systems for corm production (increased in monoecious populations) and time to flowering (delayed in dioecious populations). 5 Leaf size generally increased due to the addition of fertilizer; however, this increase was substantially greater in dioecious populations. Larger leaf size in dioecious populations was associated with more convex leaves and greater surface area, potentially increasing light capture in the shaded and more competitive habitats in which these populations occur. 6 We found significant plasticity for female sex allocation in monoecious populations, with more female flowers at higher nutrient levels. However, the majority of populations had a significant genetic component to variation in sex allocation and/or significant genotype × environment interactions. These patterns are consistent with monoecy representing a flexible reproductive strategy for regulating mating opportunities in heterogeneous habitats.

Plant species rarely exhibit both monoecious and dioecious sexual systems. This limits opportunities to investigate the consequences of combined versus separate sex function on mating patterns and genetic variation and the analysis of factors responsible for the evolution and maintenance of the two sexual systems. Populations of the North American clonal aquatic Sagittaria latifolia are usually either monoecious or dioecious and often grow in close geographic proximity. We investigated mating patterns, genetic structure, and relationships between the two sexual systems using allozyme variation in populations from southern Ontario, Canada. As predicted, selfing rates in monoecious populations (n = 6, mean = 0.41) were significantly higher than in dioecious populations (n = 6, mean = 0.11). Moreover, marker-based estimates of inbreeding depression (delta) indicated strong selection against inbred offspring in both monoecious (mean delta = 0.83) and dioecious (mean delta = 0.84) populations. However, the difference in selfing rate between the sexual systems was not reflected in contrasting levels of genetic variation. Our surveys of 12 loci in 15 monoecious and 11 dioecious populations revealed no significant differences in the proportion of polymorphic loci (P), number of alleles per locus (A), or observed and expected heterozygosity (H(o) and H(e), respectively). Strong inbreeding depression favoring survival of outcrossed offspring may act to maintain similar levels of diversity between monoecious and dioecious populations. Despite geographical overlap between the two sexual systems in southern Ontario, a dendrogram of genetic relationships indicated two distinct clusters of populations largely corresponding to monoecious and dioecious populations. Reproductive isolation between monoecious and dioecious populations appears to be governed, in part, by observed differences in habitat and life history. We suggest that selfing and inbreeding depression in monoecious populations are important in the transition from monoecy to dioecy and that the maintenance of distinct sexual systems in S. latifolia is governed by interactions between ecology, life history, and mating.

Summary• Many plants combine sexual reproduction with vegetative propagation, but how trade-offs between these reproductive modes affect fitness is poorly understood. Although such trade-offs have been demonstrated at the level of individual shoots (ramets), there is little evidence that they scale up to affect genet fitness. For hermaphrodites, reproductive investment is further divided between female and male sexual functions. Female function should generally incur greater carbon costs than male function, which might involve greater nitrogen (N) costs.• Using a common garden experiment with diclinous, clonal Sagittaria latifolia we manipulated investment in reproduction through female and male sex functions of 412 plants from monoecious and dioecious populations.• We detected a 1 : 1 trade-off between biomass investment in female function and clonal reproduction. For male function, there was no apparent trade-off between clonal and sexual reproduction in terms of biomass investment. Instead, male function incurred a substantially higher N cost.• Our results indicate that: trade-offs between investment in clonal propagation and sexual reproduction occur at the genet level in S. latifolia; and sexual reproduction interferes with clonal expansion, with investment in female function limiting the quantity of clonal propagules produced, and investment in male function limiting the nutrient content of clonal propagules.