It is common knowledge that different plant individuals have dissimilar pathogen resistance. Based on the random convergence of their parent’s genes, some plants are more fit than others, as some of the passed down genetic information encodes for proteins and enzymes that are involved in an immune response. This facet of disease and parasite ecology usually ends here, with scientists locating regions of the genome contributing to enhanced resistance. Though, Heather Slinn and her team in December 2016 took this a few steps further, and studied how disease resistance against a fungal pathogen translated up the food chain to predatory spiders.
The fungus this study focused on is from the pathogenic genus Taphrina. Taphrina sp. cause leaf blistering that ultimately render leaf tissue functionless. The ascomycete fungus has two morphological phases that differ ecologically. The yeast state is saprophytic, consuming dead organic material, while the filamentous sexual state is strictly phytoparasitic, infecting and breaking down living plant tissue. The genus is comprised of nearly 100 species, but the species of focus in this study infect only leaves, and no other plant tissue.
The plant central to this study is the black cottonwood, (Populus trichocarpa). The riparian tree inhabits areas around moving bodies of water like rivers or streams. Like many other Populus species, the black cottonwood times the release of its buoyant seeds with the seasonal post winter flooding, ensuring that its potential offspring germinate on the shade-less banks of the river or stream. These trees are vital to these riparian ecosystems, because they strengthen its banks, which greatly reduce bank erosion and silt making its way into the flowing water.
Several studies have shown the importance of spiders on riparian ecosystems. The large majority of insects occupy an aquatic habitat in their early life. These aquatic instars soon molt into their adult form, and leave the water. Spiders take full advantage of this biology, and build webs on the stream, creek, and river banks in hope of collecting newly emerging adults. A tremendous number of insects succumb to riparian web-building spiders. This interaction radiates throughout the surrounding ecosystem, as fewer insect herbivores impact plant communities, and less blood sucking mosquitoes vector diseases-just to name a few..
These researchers initially made the observation that many spiders built webs on the leaves being attacked by Taphrina fungus. So, they built hypotheses around this, and asked a series of questions. First, they wanted to know if spiders selected these infected leaves more than healthy ones. Then, they wanted to see if black cottonwoods had genetic variation in their susceptibility to the fungus. Finally, they tied the two previous questions together and asked if the difference in spider density was related the plants fungal resistance.
By growing five different plant genotypes in a common garden experiment, these researchers acquired data and skillfully applied statistical analyses to answer their initial questions. They found quite clearly that spiders prefer leaves infected with Taphrina. Spiders were 35 times more likely to set up shop on Taphrina blistered leaves as opposed to healthy, photosynthetic ones. In just the five genotypes of black cottonwood, these researchers uncovered a remarkable amount of variation to fungal resistance. Both the density and size of Taphrina blisters differed more than 2-fold amongst the trees used. The five genotypes represent just a miniscule fraction of the genetic diversity within black cottonwood communities found in nature, and still, pathogen resistance still varied greatly in the experiment. Ultimately, this comprehensive study showed that cottonwood tree genotype affects Taphrina blister size and density, which is closely correlated with spider density.
All and all, this research shows that Taphrina susceptible trees promote spider colonization. Who would’ve thought that a fungal pathogen and the plant genes associated with resistance drive predatory spider abundance? Cottonwoods are notorious for their tiny, wide dispersing seeds, so it is unlikely that closely related Taphrina susceptible trees would be grouped together like other tree genera (ect. Quercus sp.). Therefore, spider infested trees should occur in a patchy distribution, and not completely dominate entire sections of their riparian habitat. Still, these susceptible trees in nature should be dominated by web building predatory spiders, and their surrounding communities on this small scale will function with fewer water emerging insects. The results from this standout paper published in Ecology provide a ton of ecological insight, and inform us yet again how interconnected species living from the forest floor are.