Autosomal P [ ovo D 1 ] dominant female-sterile insertions in Drosophila and their use in generating germline chimeras

Formation of the Drosophila egg involves the contribution of two different ovarian cell types, the nurse cell-oocyte complex of germ-line origin derived from the embryonic pole cells, and the follicle cells derived from the embryonic mesoderm (King, 1970). The early separation of these two cell types makes it possible to generate chimeras that have germ-line and follicle cells of different genotypes. Generation of such chimeras has been important for two sets of experiments. First, they allow the determination of the tissue specificity of recessive female-sterile (fs) mutations (Wieschaus et al., 1981; Perrimon and Gans, 1983). Saturation screens have provided us with a large collection of fs mutations involved during oogenesis and embryonic patterning (Gans et al., 1975; Mohler, 1977; Komitopoulou et al., 1983; Perrimon et al., 1986; Schupbach and Wieschaus, 1989). Due to the dual origin of the cell types that form the egg and their complex interactions in oogenesis, it is critical to determine the tissue specificity of these fs mutations by generating germ-line mosaics. Second, germ-line chimeras allow one to analyze the maternal effect of zygotic lethal mutations (Perrimon et al., 1984, 1989). Approximately 85% of the genes in Drosophila are zygotic lethals when mutated and one direct way to determine the maternal function of these mutations is to analyze the phenotype of eggs derived from mosaic mothers that carry an homozygous mutant germ line. Two techniques are currently used to generate germ-line chimeras: pole cell transplantation in which mutant pole cells are transferred into an otherwise wild-type donor (Illmensee, 1973) and mitotic recombination (Wieschaus and Szabad, 1979). A critical aspect of both techniques is the ability to distinguish the eggs derived from homozygous mutant cells from the otherwise wild-type eggs. In the case of pole cell transplantation, this can be easily done by injecting the mutant pole cells into an animal that does not develop germ cells. Females lacking germ cells can be generated by using recessive or dominant female-sterile mutations that prevent pole cell formation or affect germ cell development. In the case of mitotic recombination, the most successful approach has been the use of germ-linedependent DFS that do not lay eggs (the ‘DFS’ technique; Wieschaus, 1980; Perrimon and Gans, 1983). A mitotic recombination event occurring in the germ cells of females heterozygous for DFS will result in recombinant germ-line daughter cells that have eliminated the DFS mutation and thus can develop into normal eggs. If the female is transheterozygous for both a specific mutation (fs or zygotic lethal) and the DFS mutation, the mitotic exchange will result in simultaneous loss of the DFS mutation and homozygosity of the mutation. The tissue specificity of a fs mutation or the maternal effect phenotype of a zygotic lethal mutation can then be analyzed by examining the phenotypes of eggs and embryos derived from these germ-line clones. The most widely used DFS mutation is the X-linked ovoD1 mutation (or Fs(1)K1237, Busson et al., 1983; Perrimon, 1984). Females heterozygous for ovoD1 never lay 1359 Development 119, 1359-1369 (1993) Printed in Great Britain © The Company of Biologists Limited 1993