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.

Even species-level taxonomy is often questionable, since a lot of the older descriptions were based entirely on morphological traits, which we know are often convergent. The opposite problem occurs, too. Hisayoshi Nozaki described fixed differences between Volvox strains collected from Lake Biwa in Japan. Although they are nearly identical in terms of their asexual morphology, substantial differences exist in the sexual cycle. As a result Professor Nozaki split the strains into two species. I won’t name them here, since this work is unpublished, but I’ll certainly devote a post to the paper when it comes out.

Thomas Pröschold presented phylogenetic reconstructions, based on morphological and DNA sequence data, that differ substantially from most other recent reconstructions. Among the novel results are a close relationship between the Tetrabaenaceae and certain species of Vitreochlamys, as well as several unicellular taxa (Chlamydomonas and Vitreochlamys) nested within the (otherwise) multicellular clade. I’m skeptical of these results (and I’ve expressed this to Dr. Pröschold), but I don’t want to make a judgement until I’ve seen the resulting paper (and read the full methods). If he is right, it would imply a radical revision of our current understanding of volvocine evolution.

Ecology was not well represented at the meeting, and I think this is a shame, because there is a lot we don’t know. Alexey Desnitskiy’s biogeographic studies (see Volvox 2015: Development) made some ecological inferences, but the most explicitly ecological talk was that of Cristian Solari. In a series of chemostat experiments, Dr. Solari tested how population size at equilibrium responds to temperature. In phosphorus-limited cultures, the response to temperature varied considerably among volvocine species of different sizes. The relationship between equilibrium population size and intrinsic rate of increase also differed among species, suggesting that intrinsic rate of increase may not be a good predictor of population size.


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