Pattern formation in a ts-cell-lethal mutant ofDrosophila: The range of phenotypes induced by larval heat treatments.

We describe the range of phenotypes caused by cell death when larvae of the heat-sensitive cell-lethal mutant,l(1)ts726 ofDrosophila, are subjected to heat treatment at different stages of development. When the treatment extends into the pupal period, certain bristles fail to develop but the disc derivatives are otherwise normal. Earlier treatments cause the replacement of sets of leg and eye-antennal markers by mirror image duplications of neighbouring sets. The results are compared in detail with those expected under a gradient model proposed earlier to account for the phenotype. It is found that although the results for the second leg are in excellent agreement with the predictions of the model, a more elaborate hypothesis is necessary to account for the eye-antennal disc data. Abnormal head patterns fall into several distinct categories, any one of which could be explained by postulating the existence of a gradient, if other categories did not also occur. The markers affected in each case belong to overlapping sets, and each category of pattern can be induced by heat treatments administered throughout the temperature-sensitive period. The statistical distribution of the data is such that only one category of pattern would be detected in a small scale experiment. The possible implications relative to pattern formation in normal development are discussed.