That trade‐offs result from the allocation of limited resources is a central concept of life history evolution. We quantified trade‐offs between sexual and clonal reproduction in the aquatic plant, Butomus umbellatus, by experimentally manipulating sexual investment in two distinct nutrient environments. Increasing seed production caused a significant but nonlinear trade‐off. Pollinating half of all flowers strongly reduced clonal bulbil production, but pollinating the remaining flowers did not cause any further trade‐off. Trade‐offs were not stronger under low nutrient conditions that clearly limited plant growth. Experimentally induced trade‐offs were not reflected in negative phenotypic correlations between sexual and clonal allocation among plants within eight populations grown in a uniform greenhouse environment. Diminishing effects of increased sexual allocation plus a lack of accord between experimental manipulations and phenotypic correlations suggest that trade‐offs between sexual and clonal reproduction are unlikely to constrain the evolution of reproductive strategy in this species.

The mode of reproduction may influence the spread of invasive species by affecting evolutionary potential and dispersal ability. We sampled 51 introduced North American populations of the clonal aquatic plant Butomus umbellatus L. (flowering rush) and found extreme variation in sexual fertility caused by polyploidy. Populations consisted of diploids that produced thousands of viable seeds or of sexually sterile triploids. Although a trade-off between sexual and clonal reproduction predicts that the sexual sterility of triploids would be compensated for by greater clonal reproduction, a greenhouse experiment involving eight diploid and 10 triploid populations showed that diploid plants not only invest substantially in sexual structures but also make hundreds of tiny clonal bulbils on both rhizomes and inflorescences. In contrast, triploids do not make bulbils and have very limited scope for clonal multiplication and dispersal. Diploid populations were more frequent than triploid populations, especially in the Great Lakes region. This is probably because of the difference between cytotypes in clonal rather than sexual reproduction, as genetic analyses indicate a general lack of sexual recruitment in North America. Although triploids were less common, they have a wider geographical distribution. This could be due to a greater ecological tolerance resulting from polyploidy. However, genetic evidence suggests that triploids have become widespread via their use in and escape from horticulture. North America is being colonized by two distinct forms of B. umbellatus that differ strongly in reproductive strategy as well as the vectors and pathways of invasion.

Menyanthes trifoliata L. is a distylous, clonal aquatic macrophyte found in shallow bogs and river margins throughout the boreal ecosystem, including the island of Newfoundland. A combination of long-distance dispersal and colonization after deglaciation, and pollen limitation resulting from reduced pollinator diversity and abundance documented on islands, predicts the breakdown of heterostyly to favour the establishment of self-compatible homostyles on islands. To test if self-fertilizing homostyles have been selected, variation in floral characters and compatibility relationships were examined in M. trifoliata populations from the Avalon Peninsula of Newfoundland. Morph ratio and its effect on fruit and seed set were examined in nine populations. Of the seven dimorphic populations, morphs occurred in a 1:1 ratio in four populations and deviated significantly from a 1:1 ratio in three populations. The two populations monomorphic for either pin or thrum morphs set few fruits or seeds (<15%). A strictly reciprocal arrangement of stigma height and anther length was not observed between pin and thrum morphs in the majority of populations studied. Stigma-anther separation showed a bimodal distribution with few intermediate "homostylous" flowers, rather than the discreet bimodal distribution typical of distylous species. Fruit and seed set were high (>60%) in equal morph ratio populations and were not significantly correlated to stigma-anther separation, indicating that there was no selective advantage of being homostylous. All three populations tested were highly self-incompatible, confirming that there has not been a breakdown of heterostyly on the island of Newfoundland. A reduced pollinator fauna typical of island environments may have relaxed stabilizing selection for strict herkogamy between floral morphs, resulting in the observed lack of reciprocity.Résumé : Le Menyanthes trifoliata L. est une macrophyte aquatique clonale distyle qu'on retrouve dans les marais peu profonds et au bord des rivières partout dans l'écosystème boréal, incluant l'île de Terre-Neuve. Une combinaison de dispersion à distance et de colonisation après le départ du glacier accompagnées d'une limitation de la dispersion du pollen liée à une faible diversité des pollinisateurs ainsi que l'abondance observée sur des îles, permettent de prédire la disparition de l'hétérostylie en faveur de l'établissement d'homostyles compatibles, sur les îles. Afin de vérifier si des plantes homostyles auto-fertiles ont été sélectionnées, les auteurs ont examiné la variation des caractères floraux et les relations de compatibilité dans des populations de M. trifoliata situées sur la péninsule d'Avaléon, dans l'île de Terre-Neuve. Ils ont examiné le rapport des morphes et son effet sur la mise à fruits et à graines dans neuf populations localisées sur la péninsule d'Avaléon. Sur les sept populations dimorphes, le rapport des morphes est de 1 : 1 dans quatre cas et il dévie significativement dans deux populations. Les deux autres populations monomorphes pour ...

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