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.

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Another step toward understanding sex determination in Volvox

Volvox and its relatives are a great model system for understanding the evolution of multicellularity. Their simplicity (relative to most other multicellular groups) and the variety of ‘intermediate’ species (‘intermediate’ in terms of size and complexity) make them especially suitable for comparative studies of their morphology, development, genetics, genomics, and so on. David Kirk’s book on the topic thoroughly reviews the work done up through the late ’90s, and advances since then have only increased the pace of discovery.

But in the last ten years or so, I would argue that the volvocine algae have emerged as a leading model system for an entirely different set of questions related to the evolution of the sexes. Males and females are defined by the gametes they produce, and the sexes came into existence when their gametes diverged into two different types. The existence of different male and female gametes (sperm and eggs, in most cases) is called anisogamy, and the ancestral condition of similar gametes is isogamy.

In 2006, Hisayoshi Nozaki and colleagues reported that volvocine males evolved from the minus (isogamous) mating type. To the best of my knowledge, this is the only group for which we know this. Since then, more clues have been forthcoming, and these were competently reviewed last year by Takashi Hamaji and colleagues. A new paper in PLoS ONE, by Kayoko Yamamoto and colleagues, adds another piece to the puzzle.

Figure S2 from Yamamoto et al. 2017. Light microscopic images of Volvox africanus (homothallic, monoecious with males type) and V. reticuliferus (heterothallic, dioecious type). Scale bars = 50 μm. sp: sperm packet, e: egg. A-C. V. africanus strain 2013-0703-VO4. A. Asexual spheroid. B. Monoecious spheroid. C. Male spheroid. D, E. V. reticuliferus. D. Male spheroid in male strain VO123-F1-7. E. Female spheroid in female strain VO123-F1-6.

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J. S. Huxley part 2: Volvox

Last time, I wrote about Julian Huxley’s 1912 book, The Individual in the Animal Kingdom, and his use of the volvocine algae as an example. I liked most of what he had to say, though I took issue with his assertion that

…all the other members of the family except Volvox…are colonies and nothing more—their members have united together because of certain benefits resulting from mere aggregation, but are not in any way interdependent, so that the wholes are scarcely more than the sum of their parts.

This is, of course, a matter of how we define a multicellular organism, but I think any definition that excludes, for example, Eudorina, is not a very useful one.

This time, I’ll look at the rest of what Huxley had to say about the volvocine algae, most of which is about Volvox:

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Vandalism on the Volvox Wikipedia page

I’m set up to receive alerts when a few Wikipedia pages update, and I noticed an unusual amount of activity on the Volvox page. Turns out someone has been vandalizing the article. At one point, the first line read

”Volvoxiousmaximous”, discovered by Paul Hirn in 1869, is a polyphyletic blue jeans in the volvocine rainbow algae clade…

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Evolution of microRNAs in the volvocine algae

The following guest post was kindly provided by Dr. Kimberly Chen. I have edited only for formatting.

MicroRNAs (miRNAs) are a class of non-coding small RNAs that regulate numerous developmental processes in plants and animals and are generally associated with the evolution of multicellularity and cellular differentiation. They are processed from long hairpin precursors to mature forms and subsequently loaded into a multi-protein complex, of which the Argonaute (AGO) family protein is the core component. The small RNAs then guide the protein complex to recognize complementary mRNA transcripts and conduct post-transcriptional gene silencing.

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Message from David Kirk

I got an email out of the blue from David Kirk, and I thought some of it would be of interest. Dr. Kirk is one of the biggest names in Volvox research: he carried out much of the developmental genetics that forms the foundation of our field, he literally wrote the book on Volvox evo-devo, and my impression is that most of the PIs currently studying Volvox spent time in his lab as students and postdocs.

VolvoxBookCover

The email was prompted by the meeting review from the 2015 meeting in Cambridge (he liked it, whew! :-D), and he said that he’s looking forward to the 2017 meeting in St. Louis. The email also had a footnote with some interesting information, which I quote here with Dr. Kirk’s permission:

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