by Roberto
Fig. 1. Cut-away view through the center of a 70-cell B. monosierra choanoflagellate rosette. 3D reconstruction generated from transmission electron micrographs of serial ultrathin sections. Color code: cell bodies (cyan), microvilli (orange), flagella (green), bacteria (red); ECM (white); intercellular bridges (yellow), filopodia (purple). Source
Choanoflagellates (endearingly referred to as "choanos") are the closest living relative of animals and as such they can provide insights into the evolution of animal multicellularity. Interestingly, at least two choano developmental features are induced by bacterial products. For one, choano mating is induced by an enzyme released by the marine bacterium Vibrio fischeri. This result clearly fascinated us here at STC, dedicating not one but two posts to it, in 2017 and 2019! My own fascination with choanos goes back over a decade, when I first learned about them from my colleague Jon Clardy who was working in collaboration with Nicole King. They had discovered that a bacterial sulfonolipid with femtomolar potency induced the choanos to become multicellular, joining of individual flagellated cells to form spherical "rosettes." So, it was not serendipity that I discovered a third fascinating feature of a novel species of choano, they appear to have an associated microbiome, as described in a preprint by Kayley Hake, Nicole King, and collaborators. The new species, for now named Barroeca monosierra (honoring the choano investigator Barry Leadbeater and the site of its isolation, Mono Lake in the California Sierra), is the largest choano thus far described. As such, the rosette forms a sphere big enough to have a lumen and this lumen is filled with bacteria. The beautiful 3D reconstruction of serial sections of a rosette was imaged using transmission electron microscopy. Present in the lumen of the choano and colored in red are the bacterial cells. The authors did some initial molecular characterization of these bacteria. The most abundant "phylotype" – present in all rosettes investigated and accounting for some two thirds of the bacterial load – is a Gammaprotebacterium of the family Oceanospirillaceae. There are at least nine other abundant phylotypes though none of them were found in all rosettes investigated. While much work remains to be done to define the "mini microbiome" functionality these early results hold the promise that this might become a good model system to study the early evolution of stable bacterial-animal associations.
There's another motive, beyond choanos, as to why I wanted to highlight this beautiful image. At a very basic level, a key reason we pursue science is our curiosity to understand the world around us and our role therein. One marvelous way to awaken that curiosity in others is through the visual arts. If we present a beautiful image, accompanied with a bit of our enthusiasm for it, questions will arise in the viewers that will lead to productive communication. It is easy to imagine presenting this image to diverse audiences, from school children to members of a lab in a group meeting setting, and seeing the wonderful learning that will ensue. So, many thanks Kayley for producing this beautiful image and many others (I encourage STC readers to peruse Kayley's scientific artwork)! I think your work not only gives material for starting scientific conversations, it should also prove inspiring to aspiring scientists.
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