by Roberto
In last Monday's post I presented Mike Vellend's "Theory of Community Ecology," which posits that selection, drift, speciation, and dispersal are the processes that interact to determine community dynamics. Today I'm going to expand on the process of "ecological drift." What is that? I confess, when I was first exposed to the term "drift" while learning population genetics, I just didn't get it. "Drift" to me meant to be carried along by currents, as in snowdrift or driftwood. Genetic drift? Genes being carried along by currents? It made no sense, until I realized genetic drift was really jargon, a short way of saying random genetic drift. The word random is the key. It's the concept that the frequency of an allele in a population can sometimes be due to random chance. Borrowing from population genetics, Vellend proposes that in ecosystems there are some changes in species abundance that are due to random chance.
In terms of microbial communities, consider that a mixture of similar strains of a particular species are migrating through a landscape. They may encounter "nooks," where just a few cells can migrate into. Such nooks cause a physical bottleneck where cells of one strain will, by chance, arrive first and colonize the entire nook. Does this ring a bell from STC? It easily could, if you've read Christoph's "Pore Hopping" post from last October. In it he describes the beautiful paper from Tami Lieberman's lab on how different lineages of Cutibacterium acnes co-exist on human skin, but individual skin pores are usually dominated by a single lineage. Christoph called it "neutral bottlenecking," I see that as another way of saying "ecological drift." In that work, the populations inside the skin pores were already established. Is there a way one could directly visualize ecological drift in action? For that, one would need to study pristine (uncolonized) nooks and watch as they colonize. A recent paper does just that.
Fig. 1. Top: Second instar squash bug dissected to reveal the midgut, showing the crypts colonized with Caballeronia expressing a GFP. Bottom: The posterior end the midgut from a second instar squash bug that was fed two Caballeronia strains, one expressing a GFP and the other expressing an RFP. This composite image was adapted from images from source.
Chen et al. (2024) use the colonization of the squash bug (Anasa tristis) midgut by its bacterial symbiont Caballeronia as an experimental model system to study ecological drift. The insect normally acquires the bacteria from the environment during development (when they molt into the second instar). This means the midgut does not contain Caballeronia before that. Importantly, the bacteria are eventually housed in hundreds of "crypts" along the midgut; these are the nooks. Using bacterial strains that express either green (GFP) or red (RFP) fluorescent proteins they can follow colonization. The top panel of Fig. 1 gives a dramatic view of the midgut colonization by Caballeronia. In the paper, they show both between hosts and within-host heterogeneity in colonization due to ecological drift. I was most struck by the results that show the stochastic nature of within-host gut colonization. The beauty of the image in the bottom panel of Fig. 1 should explain my excitement. A second instar squash bug was fed a 50/50 mix of GFP- and RFP-expressing strains. By the time these labelled Caballeronia reached the posterior end of the midgut, colonization of individual crypts was random; most crypts contain either red or green fluorescing bacteria. Wow! Clever, simple, clean. A beautiful demonstration that random ecological drift can play an important role in microbial community dynamics.
Do you want to comment on this post? We would be happy about it! Please comment on Mastodon, Bluesky, or on 𝕏 (formerly Twitter).
Comments