Tropical forests are known for their high biological diversity, and this is partly due to the large numbers of rare species they contain. One reason for rarity in tropical trees seems to be high rates of death from diseases that spread between individuals of the same species. When a seedling of a rare species is surrounded by neighbors of the same species, its risk of dying from pathogens is much higher than that of a seedling of a common species surrounded by neighbors of that species. This project will test the new hypothesis that rarity goes along with low resistance to disease because having just a small number of individuals of a species leads to also having a low diversity of the genes that are responsible for disease resistance. The hypothesis will be tested through collaboration between scientists at the Smithsonian Institution and Pennsylvania State University. Plant tissue will be collected from related pairs of rare and common species of tropical trees at the Smithsonian's Tropical Research Institute in Panama. The tissue will be shipped to Pennsylvania, where a new technique in molecular genetics known as next generation transcriptome sequencing will be used to quantify the overall and resistance gene diversity of each species. Back in Panama, experiments in the forest will test whether enhancing cross-pollination between trees can increase the genetic diversity of their offspring and give them greater resistance to disease and chances of survival.
This study will provide useful insights into how the abundance of a species can shape its ability to resist disease. Knowing this will help us understand why some species are rare while others are common, and help forest managers make more informed decisions. This project will be the first to obtain a large collection of resistance gene sequences from trees, information that can used in future studies of genetic diversity in agricultural and horticultural plants. The project will also provide training for students and early career biologists in both the United States and Panama.
Understanding what determines why some species within a community are common while others are rare is a longstanding goal in the field of ecology and has implications for conservation and management efforts. The goal of this project was to address a novel hypothesis about the causes and consequences of rarity. Specifically, we proposed that rare species have reduced diversity of genes that confer resistance to pathogens, such that rare species are kept rare because they are unable to escape their host-specific pathogens. We sought to test this hypothesis in a highly diverse tropical tree community in Panama, where our previous studies have revealed that locally rare tree species suffer more when they occur at high densities, primarily due to attack by host-specific soil pathogens. We received initial funding to demonstrate the feasibility of our proposed approach and methods, specifically that we would be able to grow up seedlings of wild tropical tree species in Panama and induce the expression of resistance genes in those seedlings in order to obtain high-quality RNA, which would then allow us to quantify the diversity of resistance genes and test whether it was related to species abundance in the community. Intellectual merit: We were able to successfully induce R gene expression in tropical tree seedlings using our protocol. We also showed that de novo assembly of root transcriptomes works very well with wild tropical trees, and that putative resistance genes can be identified and related to the ecological variables of interest. Furthermore, the project has produced a large collection of resistance gene sequences for wild tropical tree species and will provide gene sequence data and alleles that could be linked to existing, publicly available, long-term forest dynamics data and that will be informative for future studies of genetic diversity in tropical forests. Broader impacts: The project provided valuable training and research experience for a postdoctoral researcher, a PhD student, an undergraduate student, and a field technician. In addition, the resistance gene sequences produced by this study could prove useful for agriculture and horticulture, specifically for defending economically important plants against soil pathogens, which would have significant economic and societal benefits.