Sturtevant's Guess

The genetic approach to embryology: The case of Limnaea

There's a certain irony that one of the first early mutations discovered was a maternal effect mutation. Not only that, it was Morgan's former student and eventual successor, Alfred Sturtevant, who did the analysis that showed the maternal inheritance! The mutation, of course, is the sinistral shell coiling variant in the snail Limnaea. This variant was widely discussed in embryology texts, and Boycott and Diver (1923) did a study of its inheritance (Proc. Roy. Soc. B 95: 207) They claimed to get 1:1 and 3:1 right-left ratios from their matings of left-and right-coiling snails, but they cannot explain how this would be accomplished. Sturtevant's 1923 paper is only two columns long. But in these two columns long, he tells Boycott and Diver that their data are wrong (merely fortuitous ratios) and that they should have gotten different results if they had done the experiments properly. He then tells them what the results should have been and explains why.

If one does interpret these ratios as merely chance ones, it becomes possible to formulate a much simpler interpretation than the one proposed by these authors. An analysis of the data presented suggests that the case is a simple Mendelian one, with the dextral character dominant, but with the nature of the individual determined, not by its own constitution, but by that of the unreduced egg from which it arose.

He concludes, "it seems likely that we shall have a model case of the Mendelian inheritance of an extremely 'fundamental' character, and a character that is impressed on the egg by the mother."

Eventually, Boycott worked with the senior embryologist, S. L. Garstang, and they get (after some 6000 crosses) exactly what Sturtevant predicted. Boycott et al. (1930) called Sturtevant's analysis "his inspired guess."

However, this is not so much an inspired guess as a beautiful example of predictive theory. Sturtevant did not have to do the laborious crosses to "know" the result. He had freed his mind from the actual material of the experiments (see Latour, 1988). In the battle going on in the 1920-1930s between genetics and embryology, Sturtevant's "inspired guess" had a great deal of power. It showed that while embryologists considered embryology to be a "lawless science", geneticists might be able to discover underlying principles. The geneticists were claiming to have superseded the embryologists, and this was a case in their favor.

Figure 1
Figure 1   Alfred Sturtevant, circa 1925. (From Kohler, 1994.)

Literature Cited

Boycott, A. E., Diver, C., Garstang, S. L., and Turner, F. M. 1930. The inheritance of sinestrality in Limnaea peregra (Mollusca: Pulmonata). Phil. Trans. Roy. Soc. London B 219 [1930]: 51-131.

Kohler, R. E. 1994. Lords of the Fly: Drosophila Genetics and the Experimental Life. University of Chicago Press, Chicago.

Latour, B. 1988. The politics of explanation. In Knowledge and Reflexivity. (S. Woolgar, ed.) Sage Press, London. Pp. 155-177.

Sturtevant, A. 1923. Inheritance of direction of coiling in Limnaea. Science 58: 269-270.

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