The good, the bad and the flexible: plant interactions with pollinators and herbivores over space and time are moderated by plant compensatory responses.

BACKGROUND AND AIMS: Plants are sessile organisms that face selection by both herbivores and pollinators. Herbivores and pollinators may select on the same traits and/or mediate each others' effects. Erysimum capitatum (Brassicaceae) is a widespread and variable plant species with generalized pollination that is attacked by a number of herbivores. The following questions were addressed. (a) Are pollinators and herbivores attracted by similar plant traits? (b) Does herbivory affect pollinator preferences? (c) Do pollinators and/or herbivores affect fitness and select on plant traits? (d) Do plant compensatory responses affect the outcome of interactions among plants, pollinators and herbivores? (e) Do interactions among E. capitatum and its pollinators and herbivores differ among sites and years? METHODS: In 2005 and 2006, observational and experimental studies were combined in four populations at different elevations to examine selection by pollinators and herbivores on floral traits of E. capitatum. KEY RESULTS: Pollinator and herbivore assemblages varied spatially and temporally, as did their effects on plant fitness and selection. Both pollinators and herbivores preferred plants with more flowers, and herbivory sometimes reduced pollinator visitation. Pollinators did not select on plant traits in any year or population and E. capitatum was not pollen limited; however, supplemental pollen resulted in altered plant resource allocation. Herbivores reduced fitness and selected for plant traits in some populations, and these effects were mediated by plant compensatory responses. CONCLUSIONS: Individuals of Erysimum capitatum are visited by diverse groups of pollinators and herbivores that shift in abundance and importance in time and space. Compensatory reproductive mechanisms mediate interactions with both pollinators and herbivores and may allow E. capitatum to succeed in this complex selective environment.

Modular genetic architecture of floral morphology in Nicotiana: quantitative genetic and comparative phenotypic approaches to floral integration.

Animal-pollinated flowers are complex structures that may require a precise configuration of floral organs for proper function. As such, they represent an excellent system with which we can examine the role of phenotypic integration and modularity in morphological evolution. We use complementary quantitative genetic and comparative phenotypic approaches to examine correlations among floral characters in Nicotiana alata, N. forgetiana and their artificial fourth-generation hybrids. Flowers of both species share basic patterns of genetic and phenotypic correlations characterized by at least two integrated character suites that are relatively independent of each other and are not disrupted by four generations of recombination in hybrids. We conclude that these integrated character suites represent phenotypic modules that are the product of a modular genetic architecture. Intrafloral modularity may have been critical for rapid specialization of these species to different pollinators.

Dynamics of maternal and paternal effects on embryo and seed development in wild radish (Raphanus sativus).

BACKGROUND AND AIMS: Variability in embryo development can influence the rate of seed maturation and seed size, which may have an impact on offspring fitness. While it is expected that embryo development will be under maternal control, more controversial hypotheses suggest that the pollen donor and the embryo itself may influence development. These latter possibilities are, however, poorly studied. Characteristics of 10-d-old embryos and seeds of wild radish (Raphanus sativus) were examined to address: (a) the effects of maternal plant and pollen donor on development; (b) the effects of earlier reproductive events (pollen tube growth and fertilization) on embryos and seeds, and the influence of embryo size on mature seed mass; (c) the effect of water stress on embryos and seeds; (d) the effect of stress on correlations of embryo and seed characteristics with earlier and later reproductive events and stages; and (e) changes in maternal and paternal effects on embryo and seed characteristics during development. METHODS: Eight maternal plants (two each from four families) and four pollen donors were crossed and developing gynoecia were collected at 10 d post-pollination. Half of the maternal plants experienced water stress. Characteristics of embryos and seeds were summarized and also compared with earlier and later developmental stages. KEY RESULTS: In addition to the expected effects of the maternal plants, all embryo characters differed among pollen donors. Paternal effects varied over time, suggesting that there are windows of opportunity for pollen donors to influence embryo development. Water-stress treatment altered embryo characteristics; embryos were smaller and less developed. In addition, correlations of embryo characteristics with earlier and later stages changed dramatically with water stress. CONCLUSIONS: The expected maternal effects on embryo development were observed, but there was also evidence for an early paternal role. The relative effects of these controls may change over time. Thus, there may be times in development when selection on the maternal, paternal or embryo contributions to development are more and less likely.

Preformation, architectural complexity, and developmental flexibility in Acomastylis rossii (Rosaceae).

The duration of preformation and the seasonal pattern of development were studied in the architecturally complex alpine perennial Acomastylis rossii. Each leaf and inflorescence requires 3 yr to progress from initiation through structural and functional maturity to senescence. As a consequence, three cohorts of preformed organs, initiated in successive years, are borne simultaneously by each individual plant. The oldest cohort matures immediately following snowmelt, after which no additional leaves are matured until the following spring. A second cohort remains below ground in the apical bud and continues development, while a third cohort is initiated. Initiation and development of primordia proceed below ground throughout the summer and continue for at least 2.5 mo after aboveground structures have senesced. Acomastylis rossii maintains numerous dormant vegetative buds containing preformed leaf primordia in the axils of senesced leaves. Developmental preformation has been widely reported in arctic and alpine tundra environments and has been theorized to severely constrain rapid responses to environmental variation. The presence of many such preformed structures may mitigate some of the constraint on plant response to environmental variation imposed by the long developmental trajectories of leaves and inflorescences in apical buds.

Mechanisms of differential pollen donor performance in wild radish, Raphanus sativus (Brassicaceae).

In order to understand the characters on which sexual selection might operate in plants, it is critical to assess the mechanisms by which pollen competition and mate choice occur. To address this issue we measured a number of postpollination characters, ranging from pollen germination and pollen tube growth to final seed paternity, in wild radish. Crosses were performed using four pollen donors on a total of 16 maternal plants (four each from four families). Maternal plants were grown under two watering treatments to evaluate the effects of maternal tissue on the process of mating. The four pollen donors differed significantly in number of seeds sired and differed overall in the mating characters measured. However, it was difficult to associate particular mechanistic characters with ability to sire seeds, perhaps because of interactions among pollen donors within styles or among pollen donors and maternal plants. The process of pollen tube growth and fertilization differed substantially among maternal watering treatments, with many early events occurring more quickly in stressed plants. Seed paternity, however, was somewhat more even among pollen donors used on stressed maternal plants, suggesting that when maternal tissue is more competent, mating is slowed and is more selective.

Developmental analysis of the evolutionary origin of vegetative propagules in Mimulus gemmiparus (Scrophulariaceae).

Mimulus gemmiparus (Scrophulariaceae), a rare endemic of Colorado, has a novel life history that depends on an unusual method of vegetative reproduction. The plants are functionally annuals; however, reproduction is asexual via propagules that have been termed gemmae. The morphological identity and the evolutionary antecedent of these propagules are unclear. We approached this problem through comparative developmental analyses of M. gemmiparus and the presumed progenitor species, Mimulus guttatus. In M. gemmiparus there are two meristems initiated in the axil of each leaf primordium. The distal meristem has the potential to produce either a lateral branch or a flower, and the proximal meristem becomes a vegetative propagule (the gemma) that is ultimately surrounded by an expanded, ensheathing petiole. The first leaves of the propagules are thickened and are the site of nutrient storage. Consequently, these propagules can be characterized morphologically as brood bulbils. Mimulus guttatus also has two meristems in each leaf axil; however, the proximal meristem typically remains dormant and serves no function in the life history of this species. Based on architectural and developmental correspondence, we hypothesize that the propagule of M. gemmiparus is homologous to the proximal meristem of M. guttatus. Comparative analysis shows that evolution of the bulbil has involved both the incorporation of features present in shoots of M. guttatus and the acquisition of novel features.