Gene Dmel\otu

Homozygous females defective in proliferation, differentition, or maturation of the germ line, depending on the level of activity of the particular allele. So-called quiescent alleles (QUI) produce ovarioles lacking in germ cells; oncogenic alleles (ONC) produce cystocytes that continue dividing and form tumors; differentiated alleles (DIF) produce chambers containing only "pseudonurse" cells (PNCs) or nurse cell/oocyte (NC/O) syncytia. In these, transport of nurse cell cytoplasm to the oocyte is inhibited and chambers are arrested at a pseudo-12 stage [Bishop and King, 1984, J. Cell Sci. 67: 87-119 (fig.)]. Mutant nurse cells that fail to pump their cytoplasm into the oocytes are also unable to form a system of actin microfilament bundles in their cortical cytoplasm during stage 10B (Storto and King, 1988, Dev. Genet. 9: 91-120). The proportions of ovarioles with the different phenotypes appear to reflect the level of function of the particular allele; homozygotes are less severely affected than hemizygotes (80% of ovarioles of females carrying otu1, otu4, otu5, or otu7 in combination with an otu deficiency lack germ cells, whereas 5% of the ovarioles of homozygotes lack germ cells); similarly, the levels of function of certain alleles decline as the developmental temperature is raised. Thus otu1 behaves like a DIF allele at 18, an ONC allele at 23, and a QUI allele at 28. The ovarian tumors which give the mutant gene its name are made up of large numbers of single cystocytes and small numbers of clones of 2-4 interconnected cells [King, 1979, Int. J. Insect Morphol. Embryol. 8: 297-309 (fig.)]. Most cystocytes undergo complete cytokinesis, and there are defects in the construction and functioning of the polyfusomal system during the cycles of cystocyte divisions [Storto and King, 1989, Dev. Genet. 10: 70-86 (fig.)]. Drosophila nurse cells normally undergo nine or ten cycles of DNA replication (Mulligan and Rasch, 1985, Histochemistry 82: 233-47), and the chromatids dissociate so that each nucleus is filled with a jumbled mass of oligotene threads. In otu PNCs, the chromatids remain in register, generating banded polytene chromosomes [Dabbs and King, 1980, Int. J. Insect Morphol. Embryol. 9: 215-29 (fig.)]. Homologues pair and rearrangement configurations can be discerned [King, Riley, Cassidy, White, and Paik, 1981, Science 212: 441-43 (fig.)]. The largest polytenes have undergone 12 cycles of endonuclear replication (Rasch, King, and Rasch, 1984, Histochemistry 81: 105-10). The banding pattern of PNC polytenes is similar to that of the polytenes from larval salivary gland cells (Sinha, Mishra, and Lakhotia, 1987, Chromosoma 95: 108-16; Heino, 1989, Chromosoma 97: 363-73). At 25, otu11 behaves as an ONC allele, the cells dividing to form tumors, but at 18, homozygous females produce oocytes that reach a pseudo-14 stage, contain beta yolk spheres and can undergo early embryogenesis. In the case of DIF alleles such as otu4, females generate pseudo-12 eggs which lack beta yolk spheres and never initiate development. When otu11 is combined with alleles from the QUI class such as otu2, the heteroallelic females are sterile. Heteroalleles between otu11 and certain DIF alleles show various degrees of fertility [Storto and King, 1987, Roux's Arch. Dev. Biol. 196: 210-21 (fig.)]. otu11/otu14 females are fully fertile although the nurse cells, unlike those of wild-type females, contain banded chromosomes (Storto and King, 1988). Oocyte differentiation is destabilized in certain otu alleles; for example, the presumptive oocytes in about 20% of otu7 homozygotes resemble nurse cells in their polytenization, although they lag behind the remaining nurse cells by at least one replication cycle [King, Rasch, Riley, O'Grady, and Storto, 1985, Histochemistry 82: 131-34 (fig.)]. Germ line autonomy has been demonstrated for otu3, otu4, and otu7 (Wieschaus, Audit, and Masson, 1981, Dev. Biol. 88: 92-103; unpublished work cited in King et al., 1986).