The Tofu Mutation Restores Female Fertility to Drosophila with a Null BEAF Mutation

Background: Compensatory mutations offer clues in deciphering the role of a particular protein in cellular processes. Here, we investigate an unknown compensatory mutation, present in the BEAFNP6377 fly line, that provides sufficient rescue of the defective ovary phenotype caused by null BEAF alleles to allow the maintenance of fly stocks lacking the chromatin domain insulator proteins Boundary Element-Associated Factors BEAF-32A and BEAF-32B. We call this dominant mutation Tofu. Methods: We employ both classical genetics and genomic sequencing to attempt to identify the mutation. Results: We find evidence that points to a mutation in a predicted Polycomb response element (PRE) upstream of the ribbon transcription factor gene. This may lead to aberrant rib expression, which is otherwise not expressed in adult ovaries. BEAF and Rib colocalize to a set of promoters, suggesting overlap in gene regulation. Conclusions: Tofu could be a PRE mutation leading to the aberrant activation of rib in the ovaries. This could allow Rib to compensate for a lack of BEAF to activate one or more coregulated genes necessary for egg production in flies.