Local differentiation and plasticity in size and sex expression in jack-in-the-pulpit, Arisaema triphyllum (Araceae)

Deer Indirectly Alter the Reproductive Strategy and Operational Sex Ratio of an Unpalatable Forest Perennial

Environmental conditions impose restrictions and costs on reproduction. Multiple reproductive options exist when increased reproductive costs drive plant populations toward alternative reproductive strategies. Using 4 years of demographic data across a deer impact gradient, where deer alter the abiotic environment, we parameterize a size-dependent integral projection model for a sexually labile and unpalatable forest perennial to investigate the demographic processes driving differentiation in the operational sex ratio (OSR) of local populations. In addition to a relative increase in asexual reproduction, our results illustrate that nontrophic indirect effects by overabundant deer on this perennial result in delayed female sex expression to unsustainably large plant sizes and lead to more pronounced plant shrinkage following female sex expression, effectively increasing the cost of reproduction. Among plants of reproductive age, increased deer impact decreases the size-dependent probability of flowering and reduces reproductive consistency over time. This pattern in sex expression skews populations toward female-biased OSRs at low deer impact sites and male-biased OSRs at intermediate and high deer impact sites. While this shift toward a male-biased OSR may ameliorate pollen limitation, it also decreases the effective population size when coupled with increased asexual reproduction. The divergence of reproductive strategies and reduced lifetime fitness in response to indirect deer impacts illustrate the persistent long-term effects of overabundant herbivores on unpalatable understory perennials.

Sex-related differences in stress tolerance in dioecious plants: a critical appraisal in a physiological context

Sex-related differences in reproductive effort can lead to differences in vegetative growth and stress tolerance. However, do all dioecious plants show sex-related differences in stress tolerance? To what extent can the environmental context and modularity mask sex-related differences in stress tolerance? Finally, to what extent can physiological measurements help us understand secondary sexual dimorphism? This opinion paper aims to answer these three basic questions with special emphasis on developments in research in this area over the last decade. Compelling evidence indicates that dimorphic species do not always show differences in stress tolerance between sexes; and when sex-related differences do occur, they seem to be highly species-specific, with greater stress tolerance in females than males in some species, and the opposite in others. The causes of such sex-related species-specific differences are still poorly understood, and more physiological studies and diversity of plant species that allow comparative analyses are needed. Furthermore, studies performed thus far demonstrate that the expression of dioecy can lead to sex-related differences in physiological traits-from leaf gas exchange to gene expression-but the biological significance of modularity and sectoriality governing such differences has been poorly investigated. Future studies that consider the importance of modularity and sectoriality are essential for unravelling the mechanisms underlying stress adaptation in male and female plants growing in their natural habitat.

Phenotypic differentiation is associated with gender plasticity and its responsive delay to environmental changes in Alternanthera philoxeroides--phenotypic differentiation in alligator weed

Phenotypic plasticity is common in many taxa, and it may increase an organism's fitness in heterogeneous environments. However, in some cases, the frequency of environmental changes can be faster than the ability of the individual to produce new adaptive phenotypes. The importance of such a time delay in terms of individual fitness and species adaptability has not been well studied. Here, we studied gender plasticity of Alternanthera philoxeroides to address this issue through a reciprocal transplant experiment. We observed that the genders of A. philoxeroides were plastic and reversible between monoclinous and pistillody depending on habitats, the offspring maintained the maternal genders in the first year but changed from year 2 to 5, and there was a cubic relationship between the rate of population gender changes and environmental variations. This relationship indicates that the species must overcome a threshold of environmental variations to switch its developmental path ways between the two genders. This threshold and the maternal gender stability cause a significant delay of gender changes in new environments. At the same time, they result in and maintain the two distinct habitat dependent gender phenotypes. We also observed that there was a significant and adaptive life-history differentiation between monoclinous and pistillody individuals and the gender phenotypes were developmentally linked with the life-history traits. Therefore, the gender phenotypes are adaptive. Low seed production, seed germination failure and matching phenotypes to habitats by gender plasticity indicate that the adaptive phenotypic diversity in A. philoxeroides may not be the result of ecological selection, but of gender plasticity. The delay of the adaptive gender phenotype realization in changing environments can maintain the differentiation between gender systems and their associated life-history traits, which may be an important component in evolution of novel traits and taxonomic diversity.