Update: kin selection in plants and bacteria?
Two papers I've discussed that seem to show the importance of kin selection in plants and bacteria merit some additional commentary.
Susan Dudley, an author of the plant paper, was just here for a seminar, at my invitation. Someone in the audience (Jeannine Cavender-Bares, but it's not her fault if I'm misinterpreting her) raised an interesting possibility. Could the results Dudley reported (differences in root growth depending on whether neighboring plants are more or less related) be the result of roots sensing the genetic diversity of roots around them , rather than relatedness per se? This could actually be a possible mechanism of predicting relatedness without a plant needing to "know" what genotype it itself is. If three neighboring plants are identical to each other, (as indicated, perhaps, by similar chemicals exuded from their roots), maybe a bunch of seeds from the same mother plant fell in the same place, and the plant detecting the chemicals is also from the same mother plant. It should be possible to test this hypothesis by seeing how a plant of genotype A responds to 3 plants of genotypes A vs. B, (same diversity but high vs. low relatedness) and also three different genotypes A,B,C (high diversity but intermediate relatedness).
Meanwhile, John Dennehy calls our attention to a post by Rosie Redfield, who comments on the "quorum-sensing" paper I reviewed recently. She notes that the Diggle paper ignores another possible reason bacteria might release and measure "quorum-sensing signals", namely, measuring how fast an excreted enzyme disappears due to diffusion or fluid flow. She published this idea a few years ago. If you're a bacterium in a fast-moving stream, there's no point in releasing enzymes, even if you have a lot of clone-mates nearby, because the enzymes will be washed away before they can do anything useful. On the other hand, maybe even a single cell could benefit from releasing enzymes if it's in a little crevice somewhere with no fluid flow. (Has anyone manipulated fluid flow around immobilized quorum-sensing bacteria?)
Redfield also points out that the "high relatedness" treatment may be unrealistic -- essentially pure cultures, so Hamilton's r = 1. Manipulating relatedness in a way that is more representative of the real world would be nice. For example, earlier I discussed an experiment that used a thicker or thinner culture medium to control ease of movement. A thicker medium would lead to greater relatedness, as dividing cells would stay near each other.
Redfield is also the author of this gem, providing an alternative view of horizontal gene transfer in bacteria: Genes for breakfast: The have-your-cake-and-eat-it-too of bacterial transformation. J Hered 84: 400-404