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:
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:
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?
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.