RNA-seq of Drosophila embryos with P218 mutation in the faint sausage (fas) gene against wild-type controls to study the role fo fas in pre-mRNA splicing during embryogenesis

Morphogenesis requires the dynamic regulation of gene expression, including transcription, mRNA maturation and translation. Dysfunction of general components of the splicing machinery can cause surprisingly specific phenotypes, but the basis for these cell-type specific effects is not clear. Here we show that the faint sausage (fas) locus, implicated in epithelial morphogenesis and previously reported to encode a secreted immunoglobulin domain protein, in fact encodes a subunit of the Prp19 complex that is essential for efficient pre-mRNA splicing. Loss of zygotic fas function impairs the efficiency of splicing, associated with widespread retention of introns in mature mRNAs and dramatic changes in gene expression. Surprisingly, despite these general effects, zygotic fas mutants show specific defects in tracheal cell migration. Zygotic fas function becomes essential during late embryogenesis when maternally supplied splicing factors decline. We propose that tracheal branching, which relies on dynamic changes in gene expression, is particularly sensitive for efficient spliceosome function. Our results provide an entry point to study functions of splicing during organogenesis and provide a better understanding of specific disease phenotypes associated with mutations in general splicing factors.