Trait evolution and natural history of carnivorous pitcher plantsThe tropical pitcher plant genus Nepenthes consists of >160 species with striking morphological diversity; their pitchers (modified leaves) vary in shape and color. Like Darwin’s finches, the Nepenthes radiation may be driven by dietary adaptations. Several species possess specialized nutrient acquisition strategies, including detritivory and coprophagy. These specialized diets can be linked to interspecific morphological diversity. Intraspecific trait diversity is also pronounced in the genus, including pitcher dimorphism (each species produces two morphologically distinct pitcher traps, "lower" pitchers borne from terrestrial rosettes and "upper" pitchers borne from arboreal climbers) and color polymorphism. We have studied intraspecific pitcher variation in Nepenthes gracilis, investigating the potential influence of interactions with symbionts, herbivores, and prey in maintaining color polymorphism in nature, and found evidence suggesting herbivory—more than prey capture or light environment—has a role in the evolution of red pigmentation in lower pitchers (Gilbert et al. 2018). Future work will examine traits, including color polymorphism, in the convergently evolved North American pitcher plants (Sarracenia).
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Regulation of abiotic factors with consequences to symbiontsIn general, plants interface with and modify external environments across their surfaces, and this can significantly impact interactions with symbiotic animals and microbes. This is readily observable in the aquatic communities inhabiting the digestive fluid in the pitchers of Nepenthes (this is an example of miniature ecosystems known as phytotelmata). The pitcher is physiologically active and can thus regulate the properties of that fluid. In a common garden experiment, we found that different Nepenthes species differ in how they regulate various fluid properties (including pH, viscosity, and coloration), and of these plant-regulated factors, fluid pH has an especially strong influence over the community composition of microbial eukaryotes and bacteria. As species vary in the characteristic range of pH levels they produce, this results in species differences in microbiome assembly; thus host species identity effectively acts as an ecological filter (Gilbert et al. 2020, Sci. Rep.). More broadly, plants in general have the capacity to modify the pH levels of their leaf surfaces, and in current USDA-funded work, we are investigating this trait in two orders (Caryophyllales and Malvales), investigating both the molecular underpinnings of species differences in pH regulation as well as the consequences that it has on leaf surface microbes.
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Community ecology in a geographic contextAs Nepenthes pitchers are host to diverse communities of arthropods, microbes, and vertebrates, they can be used to compare community assembly patterns for multiple taxa simultaneously. In one project, we studied Nepenthes mindanaoensis phytotelm communities along an elevational gradient on Mt. Hamiguitan, Mindanao, the Philippines, simultaneously examining both the bacterial and eukaryotic (microbes as well as macroscopic arthropods) communities within the pitchers. We found that fluid pH was the factor most strongly affecting bacterial community composition, whereas elevation had the strongest effect on eukaryotic community composition, with or without metazoans (Gilbert et al. 2020, Microb. Ecol.). Thus, patterns of community structure are concordant between eukaryotic microbes and "macrobes", though different for bacteria in this case.
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