Volvox 2015: taxonomy, phylogeny & ecology

Volvox africanus

Volvox africanus (from Herron et al. 2010)

The worst-kept secret among Volvox researchers is that the current volvocine taxonomy is a train wreck. Within the largest family, the Volvocaceae, five nominal genera are polyphyletic (Pandorina, Volvulina, Eudorina, Pleodorina, and Volvox). Of the remaining three, two are monotypic (Platydorina and Yamagishiella). Only the newly described Colemanosphaera is monophyletic with more than one species. The extent of the problem was suspected long before it was confirmed by molecular phylogenetics, and ad hoc attempts to deal with it have led to the existence of such taxonomic abominations as ‘sections,’ ‘formas,’ and ‘syngens.’ An overhaul is called for, but it is complicated by the aforementioned loss of type cultures.

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Pathways to pluralism: Beckett Sterner on biological individuality, part 2

Aphids on dandelion

Aphids on dandelion. Public domain image from Wikimedia Commons.

Previously, I introduced Beckett Sterner’s new paper comparing and critically evaluating the views of Ellen Clarke and Peter Godfrey-Smith on biological individuality. For Clarke, individuality is recognized by the presence of ‘individuating mechanisms’: traits that increase the capacity for among-unit selection or decrease the capacity for within-unit selection. Godfrey-Smith recognizes different kinds of individuals, but at a minimum, populations of individuals must have Lewontin’s criteria of phenotypic variation, differential fitness, and heritability of fitness, i.e. be capable of adaptive change.

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Why don’t we revise volvocine taxonomy?

Volvocine taxonomy is in a sorry state. Most nominal genera, and some nominal species, are almost certainly polyphyletic. More than once, I’ve been asked during a talk, “Why is Volvox scattered all over the tree?”

JPhycol2010Fig2a

Fig. 2A from Herron et al. 2010. The traits characteristic of the genus Volvox—asexual forms with >500 cells, only a few of which are reproductive, and oogamy in sexual reproduction—have arisen at least three times independently: once in the section Volvox (represented by V. globator, V. barberi, and V. rousseletii), once in V. gigas, and once or possibly twice in the remaining Volvox species. Branch shading indicates maximum-parsimony reconstruction (white = absent, black = present, dashed = ambiguous). Pie charts indicate Bayesian posterior probabilities at selected nodes. Numbers to the left of cladograms indicate log-Bayes factors at selected nodes: positive = support for trait presence, negative = support for trait absence. Interpretation of log-Bayes factors is based on Kass and Raftery’s (1995) modification of Jeffreys (1961, Theory of probability. 3rd edn. Oxford Univ. Press, Oxford, UK.): 0 to 2, barely worth mentioning; 2 to 6, positive; 6 to 10, strong; >10, very strong. Boldface numbers following species names indicate Volvox developmental programs following Desnitski (1995).

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Volvox 2015: biophysics

In a session chaired by Ray Goldstein, we heard about recent advances in the biophysics of Volvox and Chlamydomonas. Over the last decade or so, Volvox has proven to be an experimentally tractable model system for several questions in hydrodynamics and flagellar motility. Volvox colonies can be grown in large numbers (even by physicists!), clonal cultures have relatively little among-colony variation, and they are large enough to be manipulated in ways that most single-celled organisms can’t. Furthermore, their simple structure accommodates the kind of simplifying assumptions physicists are fond of, leading Kirsty Wan (among others at the meeting) to refer to them as “spherical cows.”

In a series of papers, Douglas Brumley and colleagues have explored flagellar dynamics in Volvox carteri. Amazingly, these studies have shown that the synchronized beating of V. carteri‘s ~1000 pairs of flagella is entirely due to hydrodynamic coupling. In other words, in spite of the apparent high degree of coordination among the flagella of separate cells within a colony, no actual coordination among cells takes place. Synchronization emerges from indirect interactions mediated by the liquid medium. An elegant demonstration of this is shown in Brumley et al.’s 2014 eLife paper, in which somatic cells were physically separated from a colony and held at various distances from each other. Despite there being no direct physical connection between the cells, they beat synchronously when close together, with a phase shift that increased with increasing cell to cell distance:

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Volvox 2015: evolution

This is taking much longer than I ever expected; hopefully I can get through blogging about Volvox 2015 before registration opens for Volvox 2017!

The final session on day 1 (August 20) was chaired by Aurora Nedelcu from the University of New Brunswick. Dr. Nedelcu’s introduction emphasized some of the basic questions in evolutionary biology, aside from the origins of multicellularity and sex, on which volvocine research has provided insights: the evolution of morphological innovations, the relative importance of cis-regulatory changes vs. protein-coding changes, kin vs. group selection as competing explanations for the evolution of altruism, the evolution of soma and of indivisibility, the genetic basis of cellular differentiation, and the role of antagonistic pleiotropy (my hastily scribbled notes seem to say “antagonistic pleiotropy of olsl.” Is that supposed to be rls1? This is the cost of waiting too long to write. Maybe Aurora can clarify.).

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Volvox 2015: cell differentiation

One of the most studied aspects of Volvox development is the differentiation of its 2000 or so cells into two types: a few (usually 12-16) large reproductive cells (germ) and the rest small, biflagellate cells that provide motility (soma). The main genes controlling this differentiation have long been known, but the details of how they work are still being worked out.

Erik Hanschen (left) with Cristian Solari, David Smith, and Jillian Walker

Erik Hanschen (left) with Cristian Solari, David Smith, and Jillian Walker

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(Probably not) Precambrian Volvox

A new(ish) paper in National Science Review evaluates the evidence for various interpretations of Ediacaran microfossils from the Weng’an biota in South China (Xiao et al. 2014. The Weng’an biota and the Ediacaran radiation of multicellular eukaryotes. Natl. Sci. Rev., 1:498–520.). I recommend checking it out; it’s open access, and there’s a lot of interesting stuff in there that I’m not going to address.

These fossils are undoubtedly multicellular, probably eukaryotic, and extremely enigmatic. Their age (582-600 million years) means they could have important implications for the evolution of multicellularity, and their exceptional preservation in great numbers creates the potential for reconstructing their life cycles in great detail. Some of the Weng’an fossils have been interpreted as volvocine algae, an interpretation that I find highly unlikely.

Some of the Weng’an fossils are thought to represent red algae, and this would not be terribly surprising, since red algae have been around for at least 1.2 billion years. Others, for example the tubular fossils, are more problematic, with interpretations as diverse as cyanobacteria, eukaryotic algae, crinoids, and cnidarians.

Fig. 8 from Xiao et al. 2014

Figure 8 from Xiao et al. 2014: Schematic diagram showing diagnostic features of the five recognized species of tubular microfossils in the Weng’an biota.

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Evolution of eusociality

Last month, two papers on the evolution of eusociality were published in high-profile journals: one by Karen M. Kapheim and colleagues in Science, the other by Sandra M. Rehan and Amy L. Toth in Trends in Ecology & Evolution (TREE). Social and eusocial insects are an attractive system for studying major transitions, sharing some of the key features that make the volvocine algae so good for this purpose: multiple, independent origins of traits thought to be important to the transition and extant species with intermediate levels of sociality. These features make the social insects, like the volvocine algae, well-suited for comparative studies.
Figure 1 from Rehan & Toth: (A) Overview of phylogeny of aculeate Hymenoptera (with the nonhymenopteran but eusocial termites as an outgroup), highlighting independent origins of sociality (colored branches), groups with species ranging from solitary to primitively social (green), primitively social to advanced eusocial (orange), solitary to advanced eusocial (blue), and all species advanced eusocial (grey). (B) The full range of the solitary to eusocial spectrum (blue) and predictions of which genomic mechanisms are hypothesized to operate at different transitional stages of social evolution (broken arrows).

Figure 1 from Rehan & Toth: (A) Overview of phylogeny of aculeate Hymenoptera (with the nonhymenopteran but eusocial termites as an outgroup), highlighting independent origins of sociality (colored branches), groups with species ranging from solitary to primitively social (green), primitively social to advanced eusocial (orange), solitary to advanced eusocial (blue), and all species advanced eusocial (grey). (B) The full range of the solitary to eusocial spectrum (blue) and predictions of which genomic mechanisms are hypothesized to operate at different transitional stages of social evolution (broken arrows).

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