Broad Complex isoforms have unique distributions during central nervous system metamorphosis in Drosophila melanogaster.

Broad Complex (BRC) is a highly conserved, ecdysone-pathway gene essential for metamorphosis in Drosophila melanogaster, and possibly all holometabolous insects. Alternative splicing among duplicated exons produces several BRC isoforms, each with one zinc-finger DNA-binding domain (Z1, Z2, Z3, or Z4), highly expressed at the onset of metamorphosis. BRC-Z1, BRC-Z2, and BRC-Z3 represent distinct genetic functions (BRC complementation groups rbp, br, and 2Bc, respectively) and are required at discrete stages spanning final-instar larva through very young pupa. We showed previously that morphogenetic movements necessary for adult CNS maturation require BRC-Z1, -Z2, and -Z3, but not at the same time: BRC-Z1 is required in the mid-prepupa, BRC-Z2 and -Z3 are required earlier, at the larval-prepupal transition. To explore how BRC isoforms controlling the same morphogenesis events do so at different times, we examined their central nervous system (CNS) expression patterns during the approximately 16 hours bracketing the hormone-regulated start of metamorphosis. Each isoform had a unique pattern, with BRC-Z3 being the most distinctive. There was some colocalization of isoform pairs, but no three-way overlap of BRC-Z1, -Z2, and -Z3. Instead, their most prominent expression was in glia (BRC-Z1), neuroblasts (BRC-Z2), or neurons (BRC-Z3). Despite sequence similarity to BRC-Z1, BRC-Z4 was expressed in a unique subset of neurons. These data suggest a switch in BRC isoform choice, from BRC-Z2 in proliferating cells to BRC-Z1, BRC-Z3, or BRC-Z4 in differentiating cells. Together with isoform-selective temporal requirements and phenotype considerations, this cell-type-selective expression suggests a model of BRC-dependent CNS morphogenesis resulting from intercellular interactions, culminating in BRC-Z1-controlled, glia-mediated CNS movements in late prepupa.