Embryogenesis in Gonium and Tetrabaena

Back when I was a cocky grad student, I wrote a paper that was, in some ways, critical of the work of one of the biggest names in my field. David Kirk, who passed away last year, was among the most important figures in establishing Volvox as a model system for development, genetics, and evolution, among other things. He had published a paper that I thought was unnecessarily progressivist, and I said so in terms that, in retrospect, could have been more diplomatic. In response, Dr. Kirk, whom I had never met, sent me a very thoughtful email thanking me for pointing out some of the problems and politely disagreeing on some other points. Its tone was kind and respectful when annoyed and argumentative would have probably been justified.

In that email, he offered a bet, the stakes of which were to be a beer, that one of the things I had suggested would turn out to be wrong. The issue had to do with inversion, a process that the (mostly) spheroidal algae in the family Volvocaceae undergo during development. I have written about inversion many times on Fierce Roller; in a nutshell, these algae start their lives inside-out, with their flagella on the inside, and invert to get the flagella on the outside, where they can be used for swimming. Their relatives in the genus Gonium also undergo a process of partial inversion, changing from cup-shaped (with the flagella on the concave side) to flat or slightly cup-shaped in the other direction. Dr. Kirk had interpreted Gonium‘s partial inversion as a probable intermediate step that led to the complete inversion characteristic of the Volvocaceae. My reconstructions suggested that incomplete inversion in Gonium had evolved separately from complete inversion in the Volvocaceae, and Dr. Kirk bet me that this would turn out to be wrong.

[Read more…]

Convergence part 6: the deepest of deep homologies

You are more closely related to a mushroom than a kelp is to a plant. It’s strange to think about, but it’s true. Kelps seem very plant-like, with their root-like holdfasts, stalk-like stipes, and leaf-like blades. But kelps are brown algae, part of the stramenopile (or heterokont) lineage of eukaryotes, which are very distant from the land plants and their green algal relatives, all of which are within the archaeplastida (the direct descendants of the primary origin of chloroplasts). Mushrooms (fungi) and humans (animals), on the other hand, are both opisthokonts, practically cousins at the scale we’re talking about.

Pawlowski 2013 Fig. 1

Figure 1 from Pawlowski 2013. Deep phylogeny of eukaryotes showing the position of small eukaryotic lineages that branch outside the seven supergroups (modified after Burki et al. 2009; drawings S Chraiti). You are represented by a fish, which at this scale you might as well be.

[Read more…]

MicroRNAs in Chlamydomonas

One of the biggest changes in evolutionary theory in the late 20th century was the growing appreciation for the central role of changes in gene expression in macroevolution. Developmental genes, especially Hox genes, turned out to be remarkably conserved across lineages that diverged over half a billion years ago. The subsequent huge changes in morphology were more often due to changes in when and where those genes were expressed than to changes in the coding sequences of the genes themselves.

Even more recently, an entire new class of regulatory mechanisms was discovered and found to be important in developmental processes. MicroRNAs (miRNAs) are short (21-24 nucleotides) sequences of RNA that reduce gene expression by promoting the breakdown of messenger RNAs (mRNAs) and by repressing translation of mRNAs into proteins. We have only known that microRNAs even existed since the early 1990’s, and their importance in gene regulation and development wasn’t appreciated until the 2000’s.

Although they are structurally similar, plant and animal microRNAs repress gene expression through very different mechanisms. A new paper by Betty Y-W. Chung and colleagues in Nature Plants shows that the regulatory mechanisms of Chlamydomonas microRNAs have both striking similarities and important differences with animal miRNAs:

[Read more…]

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.

[Read more…]

Spheroids without inversion: Astrephomene development

Algae in the family Volvocaceae are (with one exception) little spheroids that swim around in freshwater lakes, ponds, and puddles. Volvox is by far the most famous of these algae, but there are a number of smaller genera, including Eudorina, Pleodorina, and Pandorina:

Fig. 1 from Herron 2016. Examples of volvocine species. (A) Chlamydomonas reinhardtii, (B) Gonium pectorale, (C) Astrephomene gubernaculiferum, (D) Pan- dorina morum, (E) Volvulina compacta, (F) Platydorina caudata, (G) Yamagishiella unicocca, (H) Colemanosphaera charkowiensis, (I) Eudorina elegans, (J) Pleodorina starrii, (K) Volvox barberi, (L) Volvox ovalis, (M) Volvox gigas, (N) Volvox aureus, (O) Volvox carteri. Figure Credit for A and B: Deborah Shelton.

Fig. 1 from Herron 2016. Examples of volvocine species; D-O are in the family Volvocaceae. (A) Chlamydomonas reinhardtii, (B) Gonium pectorale, (C) Astrephomene gubernaculiferum, (D) Pandorina morum, (E) Volvulina compacta, (F) Platydorina caudata, (G) Yamagishiella unicocca, (H) Colemanosphaera charkowiensis, (I) Eudorina elegans, (J) Pleodorina starrii, (K) Volvox barberi, (L) Volvox ovalis, (M) Volvox gigas, (N) Volvox aureus, (O) Volvox carteri. Figure Credit for A and B: Deborah Shelton.

All of the members of this family have a problem: at the end of cell division, they find themselves in an awkward configuration, with their flagella on the inside. Each cell has two flagella, and the algae need them on the outside to be able to swim. They achieve this through a developmental process called inversion, essentially turning themselves completely inside-out during embryogenesis. Even the one member of the family that is not spheroidal, Platydorina (F in the figure above), undergoes inversion before flattening into a horseshoe shape. The ways in which they do this are complex and diverse (see for example “Pleodorina inversion” and “The most important time of your life“), but not the topic of this post.

The sister group to the Volvocaceae, the Goniaceae, also includes a spheroidal genus, Astrephomene (C in the figure above). Although Astrephomene looks a lot like some of the Volvocaceae, say Eudorina (I) or Pleodorina (J), it doesn’t undergo inversion!

[Read more…]

Volvox 2015: development

Replica of Antonie van Leeuwenhoek's microscope.

Ray Goldstein‘s working (!) replica of Antonie van Leeuwenhoek’s microscope.

At the start of the Development session, I asked for a show of hands of people who self-identify as developmental biologists. About four went up. That’s not quite fair, since there’s some ambiguity in the question (primarily? exclusively?), but my point was that what all of us who are interested in the evolution of multicellularity study is the evolution of development. In fact, it might fairly be said that the origin of multicellularity is the origin of development.

[Read more…]