Fertile heteroallelic combinations of mutant alleles of the otu locus of Drosophila melanogaster.

This paper describes the ovarian pathologies observed when 108 different heteroallelic combinations were made involving 17 independent mutations at the ovarian tumor (otu) locus. Most of the mutant phenotypes can be explained as graded responses by individual germ cells to different levels of functionally active otu gene product (OGP) synthesized by the mutant cells themselves. The lowest and highest levels of OGP appear to be produced by otu and otu , respectively. In most heteroallelic ovaries the alleles have additive effects, and hybrid germ cells reach a developmental stage more advanced than the "weaker" homozygote but less advanced than the "stronger" homozygote. However, examples of both positive and negative complementation also have been found, and these suggest that the products encoded by different mutant alleles can combine to form dimers or multimers which may be superior or inferior to the homodimers. In flies homozygous for otu most ovarioles contain tumors, but some germ cells are able to develop further than those in otu homozygotes. This suggests that, while otu produces intermediate levels of OGP, it also produces a second product (which otu cannot make) that is utilized at the period in oogenesis when development in cells homozygous for otu is blocked. When otu is combined with any one of eight specific alleles, it allows oocyte/nurse cell syncytia to differentiate that can complete development and undergo embryogenesis, if fertilized. The endopolyploid nurse cells of these hybrids have giant polytene chromosomes, and the presence of GPCs in functionally active, germ-line derived cells provides an interesting new system for experimental study. Analysis of the characteristic ovarian pathologies produced by flies of different genotypes leads to the conclusion that the products of the otu gene are utilized during at least six different periods in Drosophila oogenesis.