I hope Q doesn't mind be butting in here, but this is something I find fasinating.
I also don't recall off hand a species of Volvox with a free living individual stage of the life cycle. I know that Chlamydomonas is often given as the 'unicell' ancestor of the colonial Volvocaceae and that some are classed as colonial and some as multicellular. What particular species of volvox were you thinking of? The seasonality in your example makes me think of the seasonal sexual reproduction of V. carteri which produces a spore like zygote which can survive a ponds drying up
I'm not too sure (not being too versed in these matters), but I think he may indeed be talking about
V. carteri. I can't give any definitive references (I'm at home now), but there's a few pages about it in the first chapter of
Developmental Biology by Scott F. Glbert if you've got one lying around. The sexual stage of their lifecycle is not the only interesting aspect of them apparently; the asexual stage has a definite division of labour.
From what I can gather from the pictures and the text, they resemble a ball of somatic cells which have flagella surrounding a collection of larger cells that never have functional flagella and are purely reproductive. Once these inner (germ?)-cells mature they divide rapidly (including some asymmetrical divisions) to form an embryo which eventually gets released from the 'parental' somatic cells (which subsequently commit suicide). You and Q are bound to know a lot more about this kind of thing than I do, so does this ring any bells?
The book mentions a couple of interesting things
1. A simple mutation can cause the somatic cells to
not commit suicide, and go on to become gonidia cells in their own right. This IMO shows that the step between a colony made up of identical multipurpose cells and a differentiated one is relatively small.
2. The embryos that are inside the colony are originally inside out (ie with all the larger cells dotted around on the outside). They turn themselves the right way around by folding movements which resemble (superficially at least) those involved in gastrulation, (and the production of the different types of cells in more complex organisms).
Any thoughts?
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