Non-model model organisms

Jim Umen, the lead organizer of the upcoming Volvox meeting, has written a section for a new paper in BMC Biology, “Non-model model organisms.” Like all of the BMC journals, BMC Biology is open access, so you can check out the original.

The article surveys organisms that, while not among the traditional model systems, have been developed as model systems for studying particular biological questions. The paper has an unusual format, with a discrete section devoted to each species, each written by one or two of the authors. Aside from Volvox, there are sections on diatoms, the ciliates Stentor and Oxytricha, the amoeba Naeglaria, fission yeast, the filamentous fungus Ashbya, the moss Physcomitrella, the cnidarian Nematostella, tardigrades, axolotls, killifish, R bodies (a bacterial toxin delivery system), and cerebral organoids (a kind of lab-grown micro-brain).

Dr. Umen presents Volvox and its relatives as a model system for understand the evolution of traits related to the evolution of multicellularity:

Volvox and its close relatives (the volvocine green algae) are an alternative model for investigating multicellularity, including the early origins of traits such as cell adhesion and intercellular connections, cell-type differentiation with dedicated germ cells and terminally differentiated somatic cells, asymmetric cell divisions, morphogenetic patterning, and sexual dimorphism—all of which are found in more complex multicellular taxa. What differentiates volvocine algae from other taxa and makes them a unique model is their simplicity and their relatively recent transition to multicellularity, with several well-characterized genera that capture successive increases in morphological complexity. Conveniently, a close relative of all multicellular volvocine algae is the well-studied unicellular model organism Chlamydomonas reinhardtii (Chlamydomonas) that serves as an outgroup and a proxy for the ancestral state of the lineage.

Russell et al. Fig. 8

Figure 8 from Russell et al. 2017. Volvox and volvocine algae. a Cladogram of selected volvocine species shown in cartoon form with successive cellular and developmental innovations indicated by bulleted descriptions above or below the node in which they arose. Species with published sequenced genomes have names in blue-shaded boxes. b–e Light micrographs of vegetative phase Volvox carteri (Volvox) showing a mature pre-cleavage stage adult (b); a mother spheroid with juveniles (c); and an isolated gonidium (d) or somatic cell (e) from a mature pre-cleavage adult spheroid. f Light micrograph of a Chlamydomonas reinhardtii cell. g Schematic of the Volvox vegetative life cycle synchronized to a 48-h diurnal cycle. A boxed key showing cell types and extracellular matrix (ECM) is in the upper left. Development starts with mature pre-cleavage adults (~11:00 on diagram) and proceeds clock-wise through embryogenesis, cyto-differentiation of germ cells (gonidia) and somatic cells in juveniles, hatching of juveniles, and maturation to become the next generation of adults. After hatching the ECM and parental somatic cells of the previous generation are discarded. The cartooned stages corresponding to light micrographs in panels b and c are labeled.

I have to say, I’m happy about the focus on volvocine diversity as one of the primary advantages of the system:

The appeal of Volvox as a model for investigating the evolutionary and mechanistic bases of multicellularity derives not just from the potential to build on several decades of detailed developmental and genetic studies but also from increasing information on related genera whose genome sequences are enabling the history of developmental innovations and their genetic origins to be reconstructed.

My one quibble is that the phylogenetic tree in Fig. 8a is oversimplified, giving a misleadingly straightforward picture of the evolution of differentiated, multicellular Volvox from unicellular ancestors. To be fair, he does call it a ‘cartoon’. My guess is that this is due to space limitations, as each section seems to have been allowed only one (multipanel) figure. The tree’s not wrong, it just leaves out some of the important branches, for example the four-celled TetrabaenaceaeColemanosphaera, and Volvox section Volvox.

There is a short but useful review of the genetic, genomic, and other resources available for volvocine algae that includes a very brief summary of some of the major conclusions from comparative genomics of VolvoxChlamydomonas, and Gonium. The Volvox section concludes:

Most importantly, the opportunities for exciting discoveries far outnumber the researchers who are currently using Volvox and its relatives. A great way to learn more about Volvox and volvocine algae and to tap into this research community is to attend a meeting, or to visit a laboratory that uses these intriguing microcosms of multicellularity and experience first-hand their beauty and the scientific wonder they inspire.

I couldn’t agree more. The Volvox community is small, and there is still a lot to learn.


Stable links:

Russell JJ, Theriot JA, Sood P, Marshall WF, Landweber LF, Fritz-Laylin L, Polka JK, Oliferenko S, Gerbich T, Gladfelter A, Umen J, Bezanilla M, Lancaster MA, He S, Gibson MC, Goldstein B, Tanaka EM, Hu C-K, and Brunet A. 2017. Non-model model organisms. BMC Biology 15:55. doi: 10.1186/s12915-017-0391-5

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