| MadSci Network: Botany |
Let me begin by saying that I am no expert on chloroplasts. Further, I have no ready access to research on the significance and evolution of chloroplast shape, if such exists. I can give you some well-reasoned ideas from the standpoint of as good general biologist, however.
To attack your first question first, I would say that variation exists in the shape of chloroplasts because one shape does not suit all situations. Your answer, that it was developed by these various organisms in due course through evolution is as good as any as far as I know. I know of no other factor other than the limitations in plasticity of the entity itself (the endosymbiont prokaryote that constitutes the chloroplast).
This automatically addresses your concern about the most effective shape. If there were one, most effective, shape then all (or most) organisms would have it. As you suggest, I would think that the ultimate shape of the chloroplast was an evolutionary product of the dynamic relationship between surface area and volume in the context of the structure and function of the cell host to which it must conform.
I sincerely hope this has been helpful.
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