Thursday, May 21, 2009
(After yesterday's tour de force, this post's going to be a lot lighter, especially on the references.)
Let's start by defining the individual. Are two human identical twins separate individuals? Of course. We humans have an ability to develop independent personalities, so our definition of "individual" is based on that. Even if you started with a hundred identical human clones, and raised them in a hundred similar environments/families, they'd develop separate personalities. Very similar, perhaps, they might all e.g. have a liking for dipping their buttered toast in their coffee, but still separate.
But that's not true in an evolutionary sense. Natural selection works, ultimately, on the genome.A1 And, for all intents and purposes, the genome of identical twins, and other types of clones, are identical. For purposes of natural selection, it makes no difference whether a person sacrifices his/her life for his/her own children or that of his/her identical twins. Similar applies to other sacrifices or benefits.
What we often don't realize is how the whole idea of reproduction as we usually think of it is at odds with this fact. We talk about single-celled creatures "reproducing" when they undergo mitosis, even though the result is two cells with identical genomes.
Most Eukaryotes have a time in their life cycle when they undergo sexual reproduction, with re-shuffling of the genome and creation of new ones. This is also seen as reproduction, and the distinction is often lost.
I'd like to propose a new paradigm here: let's not call mitosis "reproduction", but consider all the cells with identical genomes created through mitosis as parts of a single multi-celled creature, despite the fact that the cells are independently free-living. Thus mitosis creates "growth" rather than "reproduction" just as it does in multi-celled creatures.
Why? Because how we think about these things affects what questions we ask, and that affects what we find out through research. Consider a population of amoebae growing in a well-fed environment. These free-living cells may well consist of a small number of competing genomes, each present in many copies. The could well have the ability to recognise other cells with an identical genome, perhaps by means of a number of "identity" proteins on their outer surfaces, with enough different proteins that there would be many on each chromosome. Of course, there might be a little mis-recognition in cases of recombination (crossover), but in general they could distinguish self from non-self.
Not only that, but they could well leave very complex messages for members of their own genome. It's been demonstrated that mammalian cells are capable of sending packages of cytochrome full of protein and RNA messengers to one another,1, 2 and it's hard to believe that this capability isn't present in "single-celled" creatures as well. These packages are called exosomes, or microvesicles, and are probably large enough to contain a full set of recognition proteins on their outer surface. This means that an individual consisting of a large number of free-living amoeboid cells with identical genomes could keep its cells in touch, and coordinate its activities, at least to some extent.
The same logic applies to multicellular clones, for instance in the case of Cnidaria, where clonal populations "can form distinctive anemone-free zones, several centimetres across", due to hostile interactions between different individuals of the same species.3 Indeed, clones of the same genome can differentiate into various types depending on need, in a way similar to how cells differentiate during development of multi-celled creatures. We should not assume that individual cells are less capable of this sort of behavior, the cell is actually pretty smart.
All in all, we should probably assume that many "single celled" creatures are likely to be multi-celled, just not with their cells hooked together.
Hunter, M., Ismail, N., Zhang, X., Aguda, B., Lee, E., Yu, L., Xiao, T., Schafer, J., Lee, M., Schmittgen, T., Nana-Sinkam, S., Jarjoura, D., & Marsh, C. (2008). Detection of microRNA Expression in Human Peripheral Blood Microvesicles PLoS ONE, 3 (11) DOI: 10.1371/journal.pone.0003694
Appendix 1. "Evolution works, ultimately, on the genome."
Only ultimately. It works directly on the expressed phenotype, as modified by various epigenetic and other forms of information transmission. See Evolution in Four Dimensions for an extended discussion.
1. Membrane-derived microvesicles: important and underappreciated mediators of cell-to-cell communication by Hadi Valadi, Karin Ekström, Apostolos Bossios, Margareta Sjöstrand, James J. Lee, and Jan O. Lötvall
2. Detection of microRNA Expression in Human Peripheral Blood Microvesicles by Melissa Piper Hunter, Noura Ismail1, Xiaoli Zhang, Baltazar D. Aguda, Eun Joo Lee, Lianbo Yu, Tao Xiao, Jeffrey Schafer, Mei-Ling Ting Lee, Thomas D. Schmittgen, S. Patrick Nana-Sinkam, David Jarjoura, and Clay B. Marsh
3. Behind anemone lines: factors affecting division of labour in the social cnidarian Anthopleura elegantissima by Ayre, DJ and Grosberg, RK
Posted by AK at 2:30 PM