Letters

Selection for Asymmetry

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Science  29 Oct 2004:
Vol. 306, Issue 5697, pp. 812-813
DOI: 10.1126/science.306.5697.812

In his thoughtful memorial (“In memory of John Maynard Smith,” Perspectives, 14 May, p. 979), R. Lewontin draws deserved attention to some of John Maynard Smith's often overlooked early experimental work on Drosophila. In one important particular, however, Lewontin is in error. Rather than being able to “produce heritable asymmetry in a normally bilaterally symmetrical organism,” Maynard Smith and Sondhi (1) actually showed the opposite: “Despite an apparent early trend towards left-handedness, later experiments failed to demonstrate the presence of consistent genetic or maternal effects on handedness. The explanation of these results is not clear. It seems likely that the handedness of individuals is a purely chance phenomenon in the sense that it is not influenced either genetically or maternally” (pp. 1045–46). What Maynard Smith and Sondhi achieved by way of selection was change in the variance of the difference between sides—a measure of stability of development—not change in the mean toward right- or left-handedness.

The complete absence of a heritable response for direction of asymmetry is in many ways even more extraordinary than the supposed revelation of “considerable hidden genetic variation” noted by Lewontin. How many other traits exhibit zero response to selection in Drosophila, particularly when the phenotypic variance has been greatly amplified in a mutant line, as in Maynard Smith's experiments? Although symmetry might be viewed as highly constrained and therefore unresponsive to selection, a simpler and more satisfying hypothesis is that bilateral symmetry is a default state. Additional information is required to cause one side of the body to develop differently from the other. Without such information, right and left sides end up as mirror images simply because no heritable effects make them otherwise. Subsequent studies extended Maynard Smith's result to other symmetrical traits in Drosophila (25) but, here as elsewhere, Maynard Smith was well ahead of things.

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Response

Palmer's characterization of the results of the work by Maynard Smith and Sondhi on ocelli in Drosophila is entirely correct. The discrepancy between his description and my characterization comes from the ambiguity that arises when one is forced to be too succinct. Maynard Smith and Sondhi distinguished between “asymmetry” and “handedness.” Normally, Drosophila has one anterior medial ocellus and two posterior ocelli, one on the left and one on the right side of the head. Using a developmentally destabilized line, Maynard Smith and Sondhi were able to increase the proportion of individuals that had the anterior medial and either the left or right posterior ocellus. These are characterized as “asymmetrical” and the increase was indeed heritable. What they were not able to do was to produce a heritable increase in “handedness,” a bias toward left posterior ocelli as opposed to right posterior ocelli by selecting asymmetrical parents with left ocelli. Palmer is correct that no one has ever suceeded in selecting for biased asymmetry as opposed to unbiased fluctuating asymmetry in a normally symmetrical organism.

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