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

<b>Background</b>: 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 <i>BEAF^NP6377</i> fly line, that provides sufficient rescue of the defective ovary phenotype caused by null <i>BEAF</i> 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 <i>Tofu</i>. <b>Methods</b>: We employ both classical genetics and genomic sequencing to attempt to identify the mutation. <b>Results</b>: We find evidence that points to a mutation in a predicted Polycomb response element (PRE) upstream of the <i>ribbon</i> transcription factor gene. This may lead to aberrant <i>rib</i> expression, which is otherwise not expressed in adult ovaries. BEAF and Rib colocalize to a set of promoters, suggesting overlap in gene regulation. <b>Conclusions</b>: <i>Tofu</i> could be a PRE mutation leading to the aberrant activation of <i>rib</i> 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.