Volvox 2015: biophysics


In a session chaired by Ray Goldstein, we heard about recent advances in the biophysics of Volvox and Chlamydomonas. Over the last decade or so, Volvox has proven to be an experimentally tractable model system for several questions in hydrodynamics and flagellar motility. Volvox colonies can be grown in large numbers (even by physicists!), clonal cultures have relatively little among-colony variation, and they are large enough to be manipulated in ways that most single-celled organisms can’t. Furthermore, their simple structure accommodates the kind of simplifying assumptions physicists are fond of, leading Kirsty Wan (among others at the meeting) to refer to them as “spherical cows.”

In a series of papers, Douglas Brumley and colleagues have explored flagellar dynamics in Volvox carteri. Amazingly, these studies have shown that the synchronized beating of V. carteri‘s ~1000 pairs of flagella is entirely due to hydrodynamic coupling. In other words, in spite of the apparent high degree of coordination among the flagella of separate cells within a colony, no actual coordination among cells takes place. Synchronization emerges from indirect interactions mediated by the liquid medium. An elegant demonstration of this is shown in Brumley et al.’s 2014 eLife paper, in which somatic cells were physically separated from a colony and held at various distances from each other. Despite there being no direct physical connection between the cells, they beat synchronously when close together, with a phase shift that increased with increasing cell to cell distance:

One cool thing about physicists, they make great videos. It reminds me a bit of this demonstration of synchronization emerging in a set of metronomes, except that in this case the synchronization is mediated by wood instead of water:

Along with Douglas Brumley, Timothy Pedley has explored the physics of Volvox rotation. Swimming Volvox colonies rotate around their anterior-posterior axis, similar to the way a rifle bullet rotates (although likely for a different reason). This is achieved by having the flagella beat not directly toward the posterior of the colony but at a ~20 degree angle (“azimuthal offset”).

This rotation turns out to be crucial for motility. The eyespots found in the somatic cells of Volvox sense not only the presence of light but also its direction*, and a full rotation gives each cell a full circuit of facing toward and away from the light. The details of how this works are complex and species-specific, but the basic idea is that the flagella of cells facing away from the light beat their flagella faster (or forward when the others are beating backwards; see figure below), and this steers the colony toward the light, just as paddling on one side of a canoe will cause it to turn toward the other side. Noriko Ueki has explored the mechanics of phototaxis in both Volvox and Chlamydomonas. Chlamydomonas also rotates around its anterior-posterior axis (though for different reasons), and as in Volvox this rotation combined with directional light sensing facilitates phototaxis. Dr. Ueki’s new results on Chlamydomonas motility are (I think) unpublished, so I won’t say much about them here other than to say that I found them quite surprising. Dr. Ueki’s work is moving us ever closer to having a complete mechanistic explanation of volvocine phototaxis.

Fig. 8 from Ueki et al. 2010. Schematic representation of the phototactic movements in V. rousseletii. (a) Straight-ahead swimming in the dark. (b) A sudden dark-light switch causes the flagellar beating to reverse in the anterior hemisphere and the deceleration of the spheroid's forward movement (photophobic response). (c) After approximately 2 seconds, only cells on the illuminated side of the anterior hemisphere of the rotating spheroid show the reversed flagellar beating direction, resulting in an acceleration of the spheroid's forward movement and turning toward the light source. Gravity assists the phototactic movements because it pulls more on the posterior hemisphere due to an anisotropic mass distribution caused by the denser daughter spheroids within the posterior hemisphere and probably also by the closer spacing of the somatic cells in the posterior hemisphere. (d) Time scale.

Fig. 8 from Ueki et al. 2010. Schematic representation of the phototactic movements in V. rousseletii. (a) Straight-ahead swimming in the dark. (b) A sudden dark-light switch causes the flagellar beating to reverse in the anterior hemisphere and the deceleration of the spheroid’s forward movement (photophobic response). (c) After approximately 2 seconds, only cells on the illuminated side of the anterior hemisphere of the rotating spheroid show the reversed flagellar beating direction, resulting in an acceleration of the spheroid’s forward movement and turning toward the light source. (d) Time scale.

Kirsty Wan addressed the evolutionary origins of flagellar motility in colonial algae. Dr. Wan’s previous work has focused on flagellar dynamics of individual cells of Chlamydomonas, especially the synchronization between the two flagella. Her more recent work zooms out to a comparative view of flagellar coordination in colonial species.

This is a direction that I think holds a lot of promise. We know an awful lot about Chlamydomonas reinhardtii and Volvox carteri forma nagariensis, but there is a limit to how much we can learn from comparisons involving just these two species. To me, the greatest advantage of the volvocines as a model system is the existence of so many species that are intermediate in terms of size and complexity. I have already written about the work of Hiroko Kawai-Toyooka (“all about sex“), Zach Grochau-Wright (“cell differentiation“), Erik Hanschen (“evolution“), Shota Yamashita and Alexey Desnitskiy (“development“), all of whom presented either explicitly comparative studies or studies of species other than the canonical two. I am always happy to see studies of this sort, because I think the more we know about more species, the more we learn about how the traits in which we’re interested evolved.

*It would be more accurate to say that they can tell whether or not they are facing the light.

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