I got an email from a science teacher in India. This is the internet being what we thought it would be in the 1990s. I did my best to answer, but feel free to weigh in in the comments.
This is Subhashini, I am a science teacher and a content writer for higher secondary school in India. I have gone through your research papers about Volvox. I still have few questions about Volvox. As I do not want children to get confused need some clarification. I would appreciate if you can help me in answering few questions regarding the same.
Q.1 Is Volvox unicellular, multicellular or colonial organism? Why? (I understand the evolutionary process and the relation of the same but need the explanation about specific cellularity.)
Q.2 If I remove few cells from volvox or cut Volvox in half will it still survive and function like naturally occuring Volvox? or it functions differently?
[Q. 3 Why can not Volvox be called as multicellular organism? (I personally feel, Volvox should be called as multicellular. I read about it but have never worked with it, hence the above question ]
Is it okay to mail you for further more queries regarding the same? Looking forward to hearing from you.
Subhashini kindly gave me permission to post this here. I replied,
Thank you for the great questions!
Regarding Q.1., in my opinion, Volvox is a multicellular organism. However, different people use different criteria to decide this. For many, the dividing line between a colonial organism and a multicellular organism is cellular differentiation (or a germ-soma division of labor), which Volvox has. Some authors require cell-cell communication, and there is no evidence that this occurs in Volvox. I take a third view, namely that the complex developmental process of inversion, which occurs in all members of the Volvocaceae, implies a degree of integration that suffices to call them multicellular. Please note that I do sometimes call individual Volvocaceans colonies, but this is just a lazy informality; ‘spheroids’ or ‘coenobia‘ would be preferable.
Regarding Q.2., a colony that is cut in half will suffer several detriments. Unless the cut is made at the equator, it is unlikely that the halves will be capable of phototaxis. Some experiments by Cristian Solari showed that embryos from disrupted spheroids develop abnormally. Finally, I suspect (without experimental evidence) that disrupted colonies would be more vulnerable to pathogens, since the cells and extracellular matrix would no longer be protected by the physical barrier of the colonial boundary.
Regarding Q.3., I agree with you, but as I have noted in Q.1., not everyone does. It depends on what criteria are applied, and some people set a higher bar than others.
Please do feel free to email me with additional questions; I am always happy to hear that students are learning about Volvox!
Do you mind if I post your questions to the Fierce Roller blog? I would redact your name if you prefer.