Figures 13-20 from Nozaki et al. 2019*. Light microscopy of female strain of Volvox zeikusii Nozaki. Abbreviations: c, cytoplasmic bridges; d, daughter spheroid or developing embryo; e, egg; i, individual sheath; p, pyrenoid; s, stigma.
Figs 13–19. Asexual spheroids. Fig. 13. Optical section of spheroid. Scale bar = 50 μm. Fig. 14. Optical section of spheroid stained with methylene blue. Scale bar = 50 μm. Fig. 15. Front view of somatic cells showing cytoplasmic bridges. Scale bar = 20 μm. Fig. 16. Front view of somatic cells showing individual sheaths of the gelatinous matrix stained with methylene blue. Scale bar = 20 μm. Fig. 17. Lateral optical section of somatic cells positioned in anterior region of spheroid. Scale bar = 20 μm. Fig. 18. Surface view of somatic cells positioned in anterior region of spheroid. Scale bar = 20 μm. Fig. 19. Surface view of newly formed daughter spheroid. Scale bar = 50 μm. Fig. 20. Sexual female spheroid. Scale bar = 200 μm.
Hisayoshi Nozaki and colleagues have discovered a new species of Volvox, Volvox zeikusii. Or more accurately, they have discovered new strains of an old species and decided that some of the old strains with that name are something else.
How does that work, exactly? In 2015, Nozaki and colleagues collected a new Volvox strain in Thailand:
During our field survey in Thailand, we established a new strain in culture that produced spheroids greater than 2 mm diameter.
Two millimeters; that is a beast! I don’t think I’ve ever seen a Volvox spheroid more than half that size. The details of the new strain’s morphology and life cycle, though, didn’t match any of the known species, so they decided to investigate.
They compared the new strain to some similar strains from the Japanese culture collection (NIES) that were labeled Volvox dissipatrix. The new strain matched Walter Shaw’s original description of Volvox dissipatrix, and a segment of its DNA was a near-perfect match to a Volvox dissipatrix strain from Australia:
Figure 28 from Nozaki et al. 2019. Phylogenetic relationships of various strains of Volvox dissipatrix (W.R. Shaw) Printz and V. zeikusii, as inferred from internal transcribed spacer region 2 of nuclear ribosomal DNA. The tree was constructed by the maximum likelihood method. Branch lengths are proportional to the genetic distances. Numbers on the left or right side at the branches represent bootstrap values (≥50%, based on 1000 replicates) obtained with the maximum likelihood calculation and maximum parsimony analysis, respectively.
The new strain was also monoecious, that is, a single sexual spheroid produces both sperm and eggs. This is consistent with Volvox dissipatrix as Shaw described it. However, the other two strains (UTEX 2184 and 1869 above) were heterothallic, that is, they have genetic sex determination, so a given strain produces only male or only female spheroids (UTEX 2184 is female, 1869 male). As the tree above shows, the sequenced region of their DNA was also quite different from that of the other two strains (though nearly identical to each other).
A detailed study of their structure showed that the three available strains are all good matches to Shaw’s original description. The genetic differences and the difference in sexuality, though, suggest that the Thai and Australian strains are a different species from the other two. So a new species needs to be designated, but why not give the new name to the new strain?
The answer has to do with taxonomic rules. Taxonomy places a big emphasis on the original species description, in this case Shaw’s 1922 paper that established the species Volvox dissipatrix. The principle is that whatever species bears that name should be the same one that was originally described. In most taxonomic groups, this sort of continuity is ensured by the use of type specimens; that is, the person who describes a new species commits a preserved specimen to a museum. If questions of this sort come up in the future, new specimens can be compared to the type specimen. Whatever bears the species’ name should be the same species as the type specimen.
Unfortunately, type specimens are not available for volvocine algae. Spheroids do not preserve well, and much of the relevant diagnostic information has to do with the complete life cycle, which can’t be observed in dead specimens. Culture collections, such as the University of Texas Culture Collection (UTEX) and the Japanese National Institute for Environmental Studies (NIES), are a huge help in this regard, maintaining live strains that allow comparison with new strains, but they have limitations as well. Even in this study, one of the strains (UTEX 2184 above) is no longer available, though Nozaki and colleagues obviously have a DNA sample.
The strain that Shaw originally described is no longer available in any form: preserved, live, or DNA, so Nozaki and colleagues had to compare the available, living material with Shaw’s written description. Since the species Shaw described was homothallic and monoecious, like the Thai and Australian strains, those were designated Volvox dissipatrix, requiring that the other two strains be designated as a new species.
Fittingly, since the culture collections made much of this study possible, Nozaki and colleagues named the new species after the late Dr. Jeff Zeikus, who played an important role in establishing the University of Texas Culture collection.
*Unfortunately, the new paper is behind a paywall, but you can request a copy from Professor Nozaki through his ResearchGate page. If a non-paywalled copy becomes available, I will update this post to include a link. I also don’t have Shaw’s 1922 paper, so if you do have a pdf, please let me know.
UPDATE (2019-02-18): Professor Alexey Desnitskiy alerted me that Shaw’s 1922 paper is available for free from the Biodiversity Heritage Library at biodiversitylibrary.org/item/1131#page/222/mode/1up. I have updated the Stable links below to reflect this.
Stable links:
Nozaki H, Takusagawa M, Matsuzaki R, Misumi O, Mahakham W, Kawachi M. 2019 Morphology, reproduction and taxonomy of Volvox dissipatrix (Chlorophyceae) from Thailand, with a description of Volvox zeikusii sp. nov . Phycologia (doi:10.1080/00318884.2018.1540238)
Shaw W.R. 1922. Copelandosphaera, a new genus of the Volvocaceae. Philippine Journal of Science 21: 87–129. Available at biodiversitylibrary.org/item/1131#page/222/mode/1up.
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