A mdg4 Retrotransposon Screen for X-linked Female Sterile Alleles and its Relationship with the Transcription Factor OVO

Abstract

In the germline, the mdg4 retrotransposon integrates in close proximity to the location of OVO DNA binding motifs, suggesting that insertion bias is driven by the OVO transcription factor. A classical genetic example of this is the reversion of the dominant female-sterile allele, ovoD1, by the transposition of mdg4 into the ovo promoter where OVO protein binds. We wanted to take advantage of this relationship and determine if we could recover female sterile alleles along the X chromosome due to mdg4 insertion, with the hypothesis that these would be genes that OVO binds and transcriptionally regulates in the germline. We mobilized the mdg4 retrotransposon with the use of mutants for the lncRNA gene flamenco (flam) and recovered 17 recessive female sterile alleles out of a total of 1,192 chromosomes screened. We identified 11 complementation groups, for which a mdg4 insertion was responsible for female sterility in 7 groups. Notably, a complementation group consisting of 6 alleles was found to be the result of a Doc transposable element insertion into the gene Grip91 and is potentially evidence for a Doc insertional hotspot in the genome. Our screen also uncovered that 7/17 recessive female sterile chromosomes contained multiple transposable element insertions indicating that flam- females derepress numerous transposable elements that can lead to multiple transposon insertions along a single chromosome, as has been suggested previously. Altogether, we found that mdg4 did have an insertion bias into OVO bound regions of the genome that can result in female sterility, however, this was the case for a minority of the female sterile alleles recovered with this method. Article SummaryThe retrotransposon mdg4 preferentially inserts near binding sites of the female germline transcription factor OVO in Drosophila melanogaster, most notably at the ovo locus itself. We leveraged this relationship to screen for X-linked recessive female-sterile mutations generated by mdg4 mobilization in flamenco mutant females. From 1,192 chromosomes, we recovered 17 female-sterile alleles across 11 complementation groups. mdg4 insertions were significantly enriched in OVO-bound regions but accounted for only a subset of sterility phenotypes, revealing substantial background mutagenesis by other transposable elements. These results refine the OVO-mdg4 relationship and highlight both the promise and limitations of transposon-based genetic screens.