Mechanics of Volvox inversion


Variation is everywhere in biology. Structural variation is present at molecular, cellular, organismal, and population levels, and functional variation occurs in processes from metabolism to development to behavior. In spite of this, we often describe biology in typological terms, and this is often a source of confusion.

Some variation is crucial; for example, evolution is dependent on genetic variation, and behavioral variation within ant and bee colonies ensures that all the necessary jobs get done. Much variation, though, is simply biological noise, an unavoidable consequence of the mostly analogue nature of living systems. In extreme cases, variation of this sort can complicate and even derail development, but in general development is remarkably robust. A variety of regulatory mechanisms prevent small amounts of variation early in development from being amplified into large variations in adults.

Pierre Haas and colleagues have posted a preprint to arXiv describing variation in the developmental process of inversion in Volvox globator. Facultatively sexual organisms such as Volvox are great for studying non-genetic sources of variation, because it’s pretty simple to produce millions of genetically identical individuals. When they are raised in identical conditions, variation due to environmental differences is minimized, and most of the observed variation is stochastic.

Inversion is a crucial process in Volvox and related algae, which I’ve written about previously (“The most important time in your life,” “Volvox 2015: development,” “Pleodorina inversion,” “Spheroids without inversion: Astrephomene development“). Briefly, members of the family Volvocaceae (Eudorina, Pandorina, Platydorina, Pleodorina, Volvox, Volvulina, and Yamagishiella) start off wrong-side out early in development, and they have to turn themselves inside out to get their flagella on the outside. If they didn’t do this, they wouldn’t be able to swim.

Inversion involves a combination of cell movements and changes in cell shape, but the specifics vary quite a bit across species. In this case, Haas and colleagues were looking at Volvox globator, which uses so-called ‘type B’ inversion. They used selective plane illumination microscopy to record 3-dimensional time-lapse movies of developing V. globator embryos (see the video above, kindly provided by Stephanie Höhn). As the figure below shows, the process varies substantially, even though they are looking at genetically identical embryos:

Haas et al. 2017 Figure 5

Figure 5 from Hass et al 2017. Average Stages of Inversion. 𝑁 = 22 overlaid and scaled Volvox globator embryo halves from experimental data (lines in shades of blue), and averages thereof (red lines), for ten stages of inversion.

They used a mathematical model to simulate the process of inversion. I’d love to do a deep dive into this, but the math is frankly over my head:

Haas et al. 2017 Equation 28

Scary math! Equation 28 from Haas et al. 2017.

Their models show that the variation in V. globator inversion results from a combination of geometry, mechanics, and active regulation:

The simplest scenario with which the observed shape variations are consistent is that type-B inversion in Volvox globator results from two separate processes, with most of the variability at the invagination stage attributed to the relative timing of these processes in individual embryos. The difference between these processes is mirrored, at a mechanical level, by the different types of deformations driving them: the first process, to invert the posterior hemisphere, mainly relies on active bending, whereas the second process, to invert the anterior hemisphere, is mainly driven by active expansion and contraction.

Through these processes, inversion reaches the same end product–a right-side-out embryo–despite considerable spatial and temporal variation in the particular trajectories. As the authors point out, their methods and results may inform similar studies in other species:

We anticipate that these ideas and methods can be applied to other morphogenetic events in other model organisms to add to our understanding of the regulation of morphogenesis: what amount of regulation, be it spatial or temporal, of the cell-level processes is there, and how does it relate to the amount required mechanically for the processes to be able to complete?

 

Stable links:

Haas, P. A., S. S. M. H. Höhn, A. R. Honerkamp-Smith, J. B. Kirkegaard, and R. E. Goldstein. 2017. Mechanics and variability of cell sheet folding in the embryonic inversion of Volvox. arXiv 1708.07765.

Comments

  1. VolcanoMan says

    Hi. I have an algae question, and you are an algae expert. I saw these (mostly oblate) spheroids of blobby green translucent-to-opaque (they transmit just a bit of light through them) stuff that I assume are algal colonies, in a damp depression on a granite substrate on basically dry land (a hundred meters from a freshwater lake) in Central Canada. They were in the 0.4 mm to almost 2 cm diameter range, and tended to cluster together, up to ~20 blobs in each cluster. I’ve done a bit of looking around and think they may be genus Nostoc, but I am not certain. Do you have any ideas what this could be? I thought they were really cool looking anyway; I have never seen anything like this at that location (and I’ve been going there for decades). Are we experiencing an alien invasion of weird protoplasmic blobs that will eat and/or enslave all of humanity?

    Photo attached here: https://imgur.com/a/JTPmb

    Thanks for your time, and keep writing about algae!

    • Matthew Herron says

      Nostoc would be my guess, but I’m by no means certain. Yes, algae will eat and enslave humanity; I’m just not sure if it will be these algae.

  2. VolcanoMan says

    Thanks for the quick reply. Weird stuff, algae, but very interesting. Now that I have poked around the internet learning about it, I can see why one would devote a career to its study. On the other hand, I actually found a couple websites that treat it as a pest, like a weed in the lawn, something to be destroyed, so I’m now thinking maybe it would be better if algae DID eat and enslave us – it would do a better job of running the planet.

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