In­side a Choanofla­gel­late

Pic­tures Con­sid­ered #59

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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