Relatively rare tropical trees all agree: avoiding the 'rain of death' seems like a good call

When you think of a tropical jungle, what’s the first thing that comes to mind? Probably a lush green landscape with trees, vines, flowers, and let’s be real, at least one toucan. Tropical forests are made up of diverse groups of tree species, in contrast to other more temperate forests, that might only have one dominant type of tree, something you might see in Yosemite, perhaps.

But, por qué why?? Understanding the underlying cause of this observation is an essential question in tropical forest ecology. Recent research has pointed to the role of negative density dependence (NDD) in tree distribution, a fancy way of saying: the further one tree is from another of the same species, the higher its chances of survival.
In the most recent News and Views section of Molecular Ecology, Marco Todesco and Quentin Cronk discuss what is now called the “Janzen-Connell hypothesis”, based on a theory originally proposed by Jan Gillet. The theory states that “Any seedling germinating near their parent is subjected to a ‘rain of death’ of insect herbivores and fungal spores”. I don’t know about you, but describing this process as a ‘rain of death’ made me interested in what might be happening in this risky ‘apple not falling far from the tree’ situation.

“We hypothesized that the strength of pathogen-mediated NDD is greatest in locally less abundant plant species because founder effects (a form of genetic drift) reduce allelic diversity in a small local population compared with the regional metapopulation …..”

So it turns out, trees are people too…well okay, not really, but just like humans and other organisms, they are susceptible to pathogens (yikes! beware the rain of death!). Specifically, the key players responsible for NDD are thought to be host specific soil pathogens. Previous observations noted that locally rare species are more impacted by NDD than others, however, the mechanism behind this was unknown.

“This study reveals possible connections between genetics and ecological dynamics of tropical trees.”

In a recent article in Molecular Ecology, Marden et al., collected seeds from under 5 fruiting trees of 6 species in Panama. The seeds were surface sterilized and a total of 75 seedlings from each of the tropical tree species (15 per parent tree) were planted in sterile soil. Of li’l sets of 15 seedlings: 5 were the controls (no treatment), 5 were given live soil inoculum (simulating the rain of death), and 5 were exposed to a salicylic acid (SA) treatment. The exposure to SA simulated exposure to a plant defense hormone, known to activate R genes, including those related to pathogenesis.
The authors hypothesized that either reduced genetic diversity (possibly from inbreeding), or a change in essential resistance genes could be responsible for an individual tree’s increased pathogen susceptibility. Specifically, the authors examined the transcriptomes of these six species to check out levels of gene expression and polymorphism among the seedlings.
The authors were able to examine between 7 and 10 thousand genes in each species they analyzed, which in most cases, included plenty of resistance (R) genes. Across the board, these R genes were weird, they had highly elevated polymorphism in all of the six species compared to the rest of the transcriptome, pointing to “long-term diversifying selection and accumulation of old alleles with amino acid diversity important for immune recognition and response to microbial pathogens….”.
Marden and colleagues also found that species with fewer individuals had less diverse protein coding genes related to pathogen resistance (R genes), but when the rest (non-R gene related) of the genes were examined no differences were observed. These data point to an interesting explanation as to why NDD might be more rampant in locally rare species.

“Further examination of these processes may bridge the gap between molecular evolution studies that indicate ongoing selection for multiple resistance alleles at many loci, and how this diversity functions in nature.”

It was interesting that the authors also exposed seedlings to a defense signaling hormone (SA) to see how its genes might actually react in nature. They found that there was no evidence that rare species were less able to activate R genes, further showing that it isn’t something broken in the signaling pathway, but instead the lower R gene diversity affects these seedling’s ability to fight off pathogens. Generally, these results are cool since they provide information on not only plant-pathogen interactions but also tropical forest community ecology. It’s also an interesting example of how the little guys, once again, play a big role in the environment.
Marden, J.H., Mangan, S.A., Peterson, M., Wafula, E., Fescemyer, H.W., Der, J., dePamphilis, C.W. and Comita, L.S., 2017. Ecological genomics of tropical trees: how local population size and allelic diversity of resistance genes relate to immune responses, co‐susceptibility to pathogens, and negative density dependence. Molecular EcologyDOI: 10.1111/mec.13999
Todesco, M. and Cronk, Q. (2017), The genetic dimension of pest pressure in the tropical rainforest. Mol Ecol, 26: 2407–2409. doi:10.1111/mec.14078

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