Who are you calling lower?


Weismann Fig. 62

Fig. 62 from Weismann, A. 1904. The Evolution Theory. London: Edward Arnold. Pandorina morum; after Pringsheim. I, A young colony, consisting of 16 cells. II, Another colony, whose cells have reproduced daughter-colonies; all the cells uniformly alike. III, A young Volvox-colony; sz, somatic cells; kz, germ-cells.

I needed to cite some information from August Weismann’s 1904 book The Evolution Theory1 yesterday, so I did something I rarely do anymore: walked over to the library and checked out a physical copy. The University of Montana library has a first edition, two-volume set of the translation by Arthur Thomson. I’m always interested to see how biologists thought about Volvox before people like Richard Starr, David Kirk, and Rüdiger Schmitt came on the scene. All of the quoted text is from pages 257-261 in Volume I.
Among the lower Algae there is a family, the Volvocinæ, in which the differentiation of the many-celled body on the principle of division of labour has just set in; in some genera it has been actually effected, though in the simplest way imaginable, and in others it has not yet begun.

Lower! Hmmph. Nearly 70 years after Darwin pointed out the absurdity of “…one animal being higher than another,” this unfortunate language was still in use (for that matter, nearly 180 years later, it is still in use). As for so many later authors, Volvox interested Weismann largely because of its simplicity:
Thus in the genus Pandorina the individual consists of sixteen green cells; united into a ball (Fig. 62, I), each one exactly like the other, and all functioning alike. They are all united into a spherical body, a whole, by the gelatinous matrix which they all secrete; and thus they form a cell-colony, a cell-stock, a many-celled individual…Each one of these cells thus performs all the somatic functions, that is, all that are necessary to the maintenance of the individual life. But each also possesses the power of reproducing the whole colony from itself, that is, it also performs the function of reproduction necessary to the maintenance of the species.
[Emphasis mine] Clearly Weismann shares, to some extent, our modern ambiguity over what exactly constitutes an individual. This is still a hot topic in biology and philosophy of biology; for example, the latest meeting of the Philosophy of Science Association had two sessions on the nature of biological individuality.
This Pandorina shows no trace of a differentiation of its component cells to particular and different functions, but a nearly allied genus of the same family, the genus Volvox (Fig. 62, III), consists of two kinds of cells — on the one hand of small cells (sz) which occur in large numbers and compose the wall of the hollow gelatinous mass, forming, so to speak, the skeleton of the Volvox; and, on the other hand, of a much smaller number of cells which are very much larger (kz). The former, the ‘body’ or ‘somatic’ cells, are green, and have a red ‘eye-spot’ and two flagella; they are connected with each other by processes from their cell-bodies*, and are able, by means of the co-ordinated action of their flagella, to propel the whole colony with a slow rotary movement through the water.
[Emphasis mine] *Cytoplasmic bridges, in other words. This description is not very different from what might be found in the introduction to a modern paper, although Noriko Ueki and colleagues have shown that no coordination among cells is necessary.
In Volvox we have, for the first time, a cell-colony in which a distinction has been established between body or somatic cells and reproductive or germ-cells. In contrast to Pandorina, a large number, indeed the majority of the cells of the colony, have lost the power of reproducing the whole by division, and only the few reproductive cells possess this, while they, in turn, have lost other functions, notably that of locomotion.
This was a central point for Weismann, a concrete example of his germ-plasm theory, according to which only the germ cells pass on genetic information (“ids,” in his terminology) to the next generation.
It is the Volvocinæ which show us, so to speak, the exact point at which natural death set in, at which it was introduced into the world of life. In Pandorina the state of things is still the same as in single-celled organisms, for each cell is still all in all, each can bring forth the whole, none dies from physiological causes involved in the course of development, and they are therefore ‘immortal’ in the sense stated. But in Volvox the ‘individual’ dies when it has given off its reproductive cells, because here the contrast between germ-cells and body has developed. Only the body is mortal in the sense of being subject to natural death; the germ-cells possess the potential immortality of the single-celled animals, and it is necessary that they should possess it if the species is to continue to exist.
Interestingly, this view of unicellular immortality has changed since Weismann’s time. Careful experiments have shown that fission yeast, budding yeast, and bacteria including Caulobacter and E. coli all senesce. Nevertheless, Volvox and related algae have clearly been informing our views of death for a long time, as they still do today.
Edit: Weismann’s Fig. 63 added.
Weismann2
1. Weismann, A. 1904. The Evolution Theory. London: Edward Arnold. The quoted text is from pages 257-261.

Comments

  1. says

    Yes, this picture (fig. 62, page 257) of Volvox (without species identification) from Weismann’s book is really fine!
    However, the next picture (fig. 63, page 270) is of great interest. This is Volvox aureus. But note a detail which is very unusual: the colony is monoecious!!! As a rule, the colonies of Volvox aureus are dioecious.

    Best regards,
    Alexey Desnitskiy

    • says

      Alexey, do you think it is possible that Weismann’s Volvox aureus is not the same species that we call Volvox aureus today, or is intraspecific variation a more likely explanation? I’ll add Fig. 63 to the post.

      • says

        Matt, actually it is Klein’s Volvox aureus (not Weismann’s). I think it might be helpful to check Klein’s papers with respect to his descriptions of Volvox aureus. But I admit that the intraspecific variation in sexual differentiation can be a likely explanation (particularly in the light of the recent paper by Sa Geng et al., 2014 in PLOS Biology – Evolution of sexes from an ancestral mating-type specification pathway).
        According to old classical descriptions (e.g. Smith, 1944), Volvox aureus is dioecious (but sometimes monoecious). Later, Darden and Starr regarded Volvox aureus as dioecious. It is appropriate to remind that in 19-th century Volvox aureus and Volvox globator had additional names – V. dioicus and V. monoicus respectively.
        In any case we must not forget that Volvox aureus (the only cosmopolitan representative of the genus Volvox ) might have a complicated species structure.

  2. moarscienceplz says

    Interestingly, this view of unicellular immortality has changed since Weismann’s time. Careful experiments have shown that fission yeast, budding yeast, and bacteria including Caulobacter and E. coli all senesce.

    That is very interesting. That means that many sections of the Lazarus Long stories by Robert Heinlein are scientifically invalid. I guess Heinlein will have to update them. Oh wait! He can’t – he has fully senesced.

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