Ecological convergence in phytochemistry and flower-insect visitor interactions along an Andean elevation gradient.

The diversity of specialized molecules produced by plants radiating along ecological gradients is thought to arise from plants' adaptations to local conditions. Therefore, closely related species growing in similar habitats should phylogenetically converge, or diverge, in response to similar climates, or similar interacting animal communities. We here asked whether closely related species in the genus Haplopappus (Asteraceae) growing within the same elevation bands in the Andes, converged to produce similar floral odors. To do so, we combine untargeted analysis of floral volatile organic compounds with insect olfactory bioassay in congeneric Haplopappus (Asteraceae) species growing within the same elevation bands along the Andean elevational gradient. We then asked whether the outcome of biotic interactions (i.e., pollination vs. seed predation) would also converge across species within the same elevation. We found that flower odors grouped according to their elevational band and that the main floral visitor preferred floral heads from low-elevation band species. Furthermore, the cost-benefit ratio of predated versus fertilized seeds was consistent within elevation bands, but increased with elevation, from 6:1 at low to 8:1 at high elevations. In the light of our findings, we propose that climate and insect community changes along elevation molded a common floral odor blend, best adapted for the local conditions. Moreover, we suggest that at low elevation where floral resources are abundant, the per capita cost of attracting seed predators is diluted, while at high elevation, sparse plants incur a higher herbivory cost per capita. Together, our results suggest that phytochemical convergence may be an important factor driving plant-insect interactions and their ecological outcomes along ecological gradients.

La gran diversidad de moléculas especializadas producidas por plantas a lo largo de gradientes ecológicos se atribuye a la adaptación de plantas a sus condiciones locales. Por tanto, especies de plantas estrechamente relacionadas que crecen en hábitats similares deberían de converger en la producción de fitoquímicos similares, en respuesta a climas similares o comunidades de animales con las que interactúan. En este estudio exploramos esta hipótesis caracterizando la metabolómica no dirigida de compuestos orgánicos volátiles florales y conduciendo bioensayos olfativos de insectos en especies cogenéticas del género Haplopappus (Asteraceae) que crecen dentro de las mismas bandas altitudinales a lo largo de un gradiente altitudinal andino. Conjuntamente investigamos si el resultado de las interacciones bióticas (ej. polinización versus depredación) también convergen entre especies que crecen dentro de la misma banda altitudinal. Encontramos que los olores de las flores se agrupan de acuerdo con su banda altitudinal, y que el visitante floral más común prefiere los capítulos florales de las especies de bandas de baja elevación. Además, la relación entre el costo (depredación) y beneficio (polinización) es consistente dentro de las bandas de elevación pero incrementa con elevación, de 6:1 en elevaciones bajas a 8:1 en elevaciones altas. Por lo tanto, proponemos que los cambios climáticos y la comunidad de insectos a lo largo de la elevación resultaron en una mezcla común de olores de flores, mejor adaptada a las condiciones locales. Asimismo, sugerimos que a baja altura donde los recursos florales son abundantes, el costo per cápita de atraer a los depredadores de semillas se diluye, mientras que en sitios altos, las plantas escasas incurren en un costo per cápita de herbivoría más alto. Nuestros resultados sugieren que la convergencia fitoquímica puede ser un factor importante que impulsa las interacciones planta­insecto y sus resultados ecológicos a lo largo de gradientes ecológicos.

Macroevolutionary decline in mycorrhizal colonization and chemical defense responsiveness to mycorrhization.

Arbuscular mycorrhizal fungi (AMF) have evolved associations with roots of 60% plant species, but the net benefit for plants vary broadly from mutualism to parasitism. Yet, we lack a general understanding of the evolutionary and ecological forces driving such variation. To this end, we conducted a comparative phylogenetic experiment with 24 species of Plantago, encompassing worldwide distribution, to address the effect of evolutionary history and environment on plant growth and chemical defenses in response to AMF colonization. We demonstrate that different species within one plant genus vary greatly in their ability to associate with AMF, and that AMF arbuscule colonization intensity decreases monotonically with increasing phylogenetic branch length, but not with concomitant changes in pedological and climatic conditions across species. Moreover, we demonstrate that species with the highest colonization levels are also those that change their defensive chemistry the least. We propose that the costs imposed by high AMF colonization in terms of reduced changes in secondary chemistry might drive the observed macroevolutionary decline in mycorrhization.

Variable effects on growth and defense traits for plant ecotypic differentiation and phenotypic plasticity along elevation gradients.

Along ecological gradients, phenotypic differentiation can arise through natural selection on trait diversity and magnitude, and environment-driven plastic changes. The magnitude of ecotypic differentiation versus phenotypic plasticity can vary depending on the traits under study. Using reciprocal transplant-common gardens along steep elevation gradients, we evaluated patterns of ecotypic differentiation and phenotypic plasticity of several growth and defense-related traits for two coexisting but unrelated plant species, Cardamine pratensis and Plantago major. For both species, we observed ecotypic differentiation accompanied by plasticity in growth-related traits. Plants grew faster and produced more biomass when placed at low elevation. In contrast, we observed fixed ecotypic differentiation for defense and resistance traits. Generally, low-elevation ecotypes produced higher chemical defenses regardless of the growing elevation. Yet, some plasticity was observed for specific compounds, such as indole glucosinolates. The results of this study may suggest that ecotypic differentiation in defense traits is maintained by costs of chemical defense production, while plasticity in growth traits is regulated by temperature-driven growth response maximization.

Biological Control beneath the Feet: A Review of Crop Protection against Insect Root Herbivores.

Sustainable agriculture is certainly one of the most important challenges at present, considering both human population demography and evidence showing that crop productivity based on chemical control is plateauing. While the environmental and health threats of conventional agriculture are increasing, ecological research is offering promising solutions for crop protection against herbivore pests. While most research has focused on aboveground systems, several major crop pests are uniquely feeding on roots. We here aim at documenting the current and potential use of several biological control agents, including micro-organisms (viruses, bacteria, fungi, and nematodes) and invertebrates included among the macrofauna of soils (arthropods and annelids) that are used against root herbivores. In addition, we discuss the synergistic action of different bio-control agents when co-inoculated in soil and how the induction and priming of plant chemical defense could be synergized with the use of the bio-control agents described above to optimize root pest control. Finally, we highlight the gaps in the research for optimizing a more sustainable management of root pests.