What is a (Volvox) species?


Hisayoshi Nozaki and colleagues have just described some Volvox samples from two lakes and a pond in Japan.

Figure 1A from Nozaki et al. 2016. Volvox sp. Sagami asexual spheroid with daughter colonies (d).

Figure 1A from Nozaki et al. 2016. Volvox sp. Sagami asexual spheroid with daughter colonies (d).

The newly collected strains have a lot in common with another recently described species, Volvox ferrisii, but there are some important differences as well:

…it could be clearly distinguished from all previously described monoecious species of Volvox sect. Volvox by its small number of eggs or zygotes (5–25) in sexual spheroids, with short acute spines (up to 3 μm long) on the zygote walls and elongated anterior somatic cells in asexual spheroids.

In spite of these differences, Nozaki and colleagues stop short of calling the newly collected strains a new species. Why?

Describing new volvocine species is practically Dr. Nozaki’s hobby. Yet in this case he has sat the fence, concluding neither that the new strains are a new species nor that they belong to an established species:

Volvox sp. Sagami represents a lineage that is morphologically and genetically distinct from V. ferrisii. However, genetic differences between Volvox sp. Sagami and V. ferrisii were small (Figs 2 and 3), falling within a single biological species in other volvocalean algae. Thus, these two lineages may represent a single species that has extensive morphological and ecological diversity, or they may be very closely related different species.

So is Volvox sp. Sagami a new species, or is it just a variant of Volvox ferrisii? How do we know?

Like fitness and individual, what exactly we mean by the word species is one of those topics that generates endless debate among biologists and philosophers of biology (in sharp contrast to the claim from Uncommon Descent that this discussion “never happens”). If I were so inclined, I could turn this blog into nothing but discussions of the relative merits of various species concepts. Instead, I’ll refer interested readers to this review paper by Kevin de Queiroz.

For the purposes of this post, it’s sufficient to say that the predominant view of what constitutes a species, at least sexually reproducing species, is Ernst Mayr’s Biological Species Concept: roughly, organisms that can mate to produce viable, fertile offspring are members of the same species. Those that either can’t cross or produce sterile offspring (e.g. horse x donkey –> mule) are separate species. I’m glossing over a book’s worth of complications, but this is the basic principle.

One of the problems with the Biological Species Concept is that it can be hard to test, and this is certainly the case with volvocine algae. It’s especially the case for monoecious species, such as Volvox sp. Sagami and Volvox ferrisii. Monoecious, in this context, means that a given sexual spheroid (or colony, or coenobium) produces both sperm and eggs. So if we were to mix some Volvox sp. Sagami with some Volvox ferrisii and induce sexual reproduction, we would have no way to know whether the resulting offspring were Volvox sp. Sagami Volvox ferrisiiVolvox sp. Sagami Volvox sp. Sagami, or Volvox ferrisii x Volvox ferrisii. In other words, we have no practical way to judge whether or not Volvox sp. Sagami and Volvox ferrisii are interfertile:

Intercrossing experiments would determine one of these two possibilities. However, such experiments are not practically possible because the two lineages produced monoecious sexual spheroids with sperm packets and eggs.

Instead, we have to rely on proxies, differences that suggest that the strains in question are sufficiently diverged to qualify as separate species. Nozaki and colleagues used both phenotypic and genetic proxies to evaluate the distinctness of Volvox sp. Sagami from Volvox ferrisii:

…[Volvox sp. Sagami] could be clearly distinguished from all previously described monoecious species of Volvox sect. Volvox by its small number of eggs or zygotes (5–25) in sexual spheroids, with short acute spines (up to 3 μm long) on the zygote walls and elongated anterior somatic cells in asexual spheroids. Based on sequences of internal transcribed spacer (ITS) regions of nuclear ribosomal DNA (rDNA; ITS-1, 5.8S rDNA and ITS-2) and plastid genes, however, the Volvox sp. Sagami lineage and its sister lineage (the monoecious species V. ferrisii) showed very small genetic differences, which correspond to the variation within a single biological species in other volvocalean algae.

In the age before molecular systematics, the phenotypic differences would have been all we had to go on, and Volvox sp. Sagami would probably have been described as a new species. But the DNA sequence data seem to indicate otherwise. Among the three sequenced genes, there were only two nucleotide differences between Volvox sp. Sagami and Volvox ferrisii. That’s a much smaller amount of variation than has previously been observed within some volvocine species. Furthermore, there’s a particular kind of change in the rDNA sequences (compensatory base changes) that has previously been treated as diagnostic of distinct volvocine species, and that difference is not observed between Volvox sp. Sagami and Volvox ferrisii. There is an intron in one of the plastid genes in Volvox sp. Sagami that is missing from Volvox ferrisii, but this too varies within some volvocine species.

So there are some indications that Volvox sp. Sagami and Volvox ferrisii are different species: the morphological differences certainly support this, and the presence of the intron provides some support as well. But the genetic variation between the two is well within the range that’s typically observed within a singe volvocine species.

Are the differences between Volvox sp. Sagami and Volvox ferrisii just intraspecific variation, or are they truly different species? In the end, we may have to accept that, for the time being at least, we just don’t know.

 

Stable links:

Coleman, A. W. 2009. Is there a molecular key to the level of “biological species” in eukaryotes? A DNA guide. Molecular Phylogenetics and Evolution 50:197–203.

De Queiroz, K. 2007. Species concepts and species delimitation. Systematic Biology 56:879–886.

Isaka, N., H. Kawai-Toyooka, R. Matsuzaki, T. Nakada, and H. Nozaki. 2012. Description of two new monoecious species of Volvox sect. Volvox (Volvocaceae, Chlorophyceae), based on comparative morphology and molecular phylogeny of cultured material. Journal of Phycology 48:759–767.

Mayr, E. 1942. Systematics and the origin of species from the viewpoint of a zoologist. New York: Columbia University Press.

Nozaki, H., N. Ueki, N. Isaka, T. Saigo, K. Yamamoto, R. Matsuzaki, F. Takahashi, K. Wakabayashi, and M. Kawachi. 2016. A new morphological type of Volvox from Japanese large lakes and recent divergence of this type and V. ferrisii in two different freshwater habitats. PLoS One 11:e0167148.

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