Decreased expression of lethal giant larvae causes ovarian follicle cell outgrowth in the Drosophila Scutoid mutant

Snail, a zinc-finger transcription factor, controls the process of epithelial-mesenchymal transition, and ectopic expression of this protein may produce cells with stem cell properties. Because the effect of Snail expression in ovarian epithelial cells remains unclear, we generated Drosophila ovarian follicle stem cells (FSCs) with homozygous Scutoid (Sco) mutation. The Sco mutation is a reciprocal transposition that is known to induce ectopic Snail activity. We found that Sco mutant FSCs showed excess proliferation and high competitiveness for niche occupancy, and the descendants of this lineage formed outgrowths that failed to enter the endocycle. Surprisingly, such phenotypes were not rescued by suppressing Snail expression, but were completely restored by supplying Lethal giant larvae (Lgl). The lgl allele is a cell polarity gene that is often mutated in the genome. Importantly, Sco mutants survived in a complementation test with lgl. This result was probably obtained because the Sco-associated lgl allele appears to diminish, but not ablate lgl expression. While our data do not rule out the possibility that the Sco mutation disrupts a regulator of lgl transcription, our results strongly suggest that the phenotypes we found in Sco mutants are more closely associated with the lgl allele than ectopic Snail activity.