DEVELOPMENTAL BIOLOGY 42, 211-221(1975) Minutes: Mutants of Drosophila Autonomously Affecting Cell Division Rate GIN& MORATA’ AND PEDRO RIPOLL Znstituto de Gendtica y Antropologia, Centro de Znvestigaciones Biolbgicas, C.S.Z.C., Velhzquez, 144, Madrid 6, Spain Accepted October 1, 1974 “Minute” (iV) mutants of Drosophila show a characteristic prolongation of developmental time. We chose three different Minute mutants to examine the proposition that this slower rate of development is also reflected in a decreased mitotic rate in imaginal disc cells. The division rate of normal cells (M+) dividing in Minute individuals and of Minute cells dividing in normal individuals has been analyzed by inducing mitotic recombination in the growing wing disc and the abdominal histoblasts. In both kinds of experiments, clones of non-Minute cells are always larger than simultaneously arising Minute cell clones implying a higher mitotic rate for M+, as compared to M, cells. In addition, Minute cells seem to be unable to compete with normal cells in the wing disc; however, their viability in the abdominal histoblasts is nearly normal. The possibility of inducing, at any developmental age, cells with different cell-cycle lengths and clones that are larger or smaller than normal provides a powerful new technique with which to approach develop- mental problems. INTRODUCTION A set of mutants of Drosophila melano- gaster known generically as “Minutes” have an interesting phenotype. They are homozygous lethal with an early phenocrit- ical phase; when heterozygous, they ex- hibit shortened bristles and sometimes ab- normal morphogenesis of other structures. Some of them are reported to increase the rate of mitotic recombination (Stern, 1936; Kaplan, 1953). All of them lead, in varying degrees, to a slowdown in development. Stern and Tokunaga (1971) showed in genetic mosaics that bristle differentiation and lethality of some Minutes are cell autonomous in their expression. For a de- scription of the Minute mutants and of those mutations and special chromosomes used in this work, see Lindsley and Grell (1968). In this report we show that the develop- mental delay of Minutes is also reflected in a cell autonomous decrease in mitotic rate. We have studied the proliferation dynam- ‘Present address: MRC Laboratory of Molecular Biology, University Postgraduate Medical School, Hills Road, Cambridge, CB2 2QH England. its of cells changed by mitotic recombina- tion (Stern, 1936) from a Minute to a non-Minute condition, and vice versa, in two developing cellular systems with dif- ferent and well-analyzed growth parame- ters: The imaginal wing disc (Bryant, 1970; Garcia-Bellido and Merriam, 1971a) and the dorsal abdominal histoblasts (Garcia- Bellido and Merriam, 1971b; Guerra et al., 1973). MATERIAL AND METHODS Two basic experimental procedures are employed. To examine cell division of M+ cells in a M background, heterozygous (and therefore phenotypically) Minute larvae were X-irradiated to induce a mitotic ex- change between the M locus and the cen- tromere. Following the proper mitotic seg- regation, sister M/M and M+/M+ cells are produced: the homozygous M condition is cell lethal so that only a M+ clone will result from the induced exchange. These M+ cells proliferate, of course, in an indi- vidual most of whose cells are still pheno- typically M. The recombination event it- self is monitored by use of appropriate cell markers (see Fig. 1, parts I, II, and III). 211 Copyright Q 1975 by Academic Press, Inc. All rights of reproduction in any form reserved.