Kirsty Wan and Ray Goldstein have posted a new paper to arXiv*: “Coordinated Beating of Algal Flagella is Mediated by Basal Coupling.” The paper examines in unprecedented detail the mechanics of intracellular flagellar coordination. That’s cool and all, but first: hexadecaflagellates!
Wan & Goldstein compared algal cells with 2, 4, 8, and, yes, 16 flagella. I never knew there was such a thing. Pyramimonas cyrtoptera has 16, and its relative P. octopus has…well, you can probably guess.
Members of the Goldstein lab have previously shown that the apparent coordination of flagella among the cells in multicellular volvocine algae does not require any direct connection (see Volvox 2015: biophysics). Rather, the synchrony of flagella results from hydrodynamic coupling, indirect interactions mediated by the liquid medium. However, Dr. Wan’s new research shows that coordination of the flagella of a single cell does require direct interactions.
Chlamydomonas swims in a breaststroke, with the flagella at the anterior beating in synchrony. The flagella of Chlamydomonas (and of all volvocine algae) are anchored to the basal bodies, and these are connected by a “distal fiber.” This distal fiber is thus an obvious suspect in the coordination of flagella, and indeed Wan & Goldstein show that Chlamy mutants without the distal fiber fail to coordinate their flagella. Experiments with isolated flagella show that hydrodynamic coupling does have an effect on coordination, but in this case the effect appears to be minor and insufficient to explain the observed synchrony.
Patterns of coordination among the flagella of quadriflagellate species turn out to be amazingly variable:
Analogues to the gaits of a quadruped, quadriflagellate swimming involves diverse gaits of actuation of four flagella that is species specific: including the A) trot (Pyramimonas parkeae), B) pronk (Pyramimonas tetrarhynchus), C) rotary gallop (Carteria crucifera), and D) transverse gallop (Tetraselmis suecica).
A simple experiment shows that these gaits are coordinated by something other than hydrodynamic coupling: using micromanipulation, the authors immobilized one flagellum and tested for changes in the beating of the other three.
I guess we have to forgive them for this algae torture, because what they found is pretty cool: when one flagellum is immobilized, the other three beat in the same pattern as normal. Eight-celled Pyramimonas octopus and sixteen-celled P. cyrtoptera use a gait similar to the “pronk” of P. tetrarhynchus.
The authors suggest that coordination among the flagella of these various species is likely to involve contractile connections among basal bodies along with some sort of controller acting as a pacemaker. The observed variety of gaits suggests lineage-specific changes to the controller or the connections or both. The details of these mechanisms remain speculative for now.
*Under the category “Soft Condensed Matter.” Apparently, this is how physicists describe biology!