Maternal determinants of seed dispersal in Cakile edentula: Fruit, plant, and site traits
Seed dispersion patterns are largely determined by the maternal plant. Characters of the progeny, such as dispersal, that are determined by the maternal parent and that vary with the maternal environment are said to exhibit maternal environmental effects. Because dispersal is maternally determined, the response to selection on dispersal depends on the correlations between generations for maternal traits that influence dispersal. These correlations depend on the direction and degree to which the maternal plant responds to its environment. Although it is widely appreciated that seed dispersal is maternally determined, the evolution of dispersal has not yet been addressed within the context of maternal character evolution. In this study, I identify maternal traits that influence seed dispersion patterns and characterize their variation as a function of maternal environmental conditions in order to determine whether maternal environmental effects on dispersal are possible and to predict the direction of correlations between generations for these traits. The traits included morphological traits of the fruits and architectural traits of the maternal plant. Laboratory and field studies were combined to investigate dispersal of Cakile edentula var. lacustris, an annual beach plant with heteromorphic fruit segments. Fruit traits influenced dispersal under laboratory conditions and in the field. Taller, more sparsely branched plants with a greater percentage of fruits on their primary stem had impeded dispersal, and plants with more fruit segments had denser seed shadows. Path analysis revealed that the total effect of a trait on dispersal often differed from its direct effect because of intercorrelations among variables. Environmental factors influenced dispersal directly, but they also strongly influenced the expression of plant traits. By considering both the effects of plant traits on dispersal and the influence of environmental variables, such as density, on the expression of these plant traits, this study predicts positive phenotypic correlations between maternal and progeny generations for some traits and negative correlations for others. The extent to which each maternal trait influences dispersal, the extent to which it varies with the environment, and the extent to which the environment varies as a function of dispersal will all influence phenotypic correlations between maternal parent and offspring. Consequently, these factors are expected to influence the response of dispersal to selection.
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