Abstract

The homeodomain protein Bicoid establishes embryo polarity and anterior patterning in the fruit fly Drosophila melanogaster via unique DNA binding affinity, but how it acquired this fundamental role in axis specification is unknown. We found that in moth flies, which lack Bicoid, a maternal isoform of odd-paired is localized at the anterior pole of eggs and specifies head-to-tail polarity while a zygotic isoform retains the ancestral function in segmentation. Other dipterans lack a maternal odd-paired function in axis specification although their odd-paired genes can also drive embryo polarity and head development in the moth fly Clogmia albipunctata. These findings indicate that Odd-paired acquired a Bicoid-like function via alternative transcription rather than protein evolution. A survey of maternally localized transcripts in the eggs of other fly species suggests that this function evolved repeatedly via AT. We may thus overestimate the role of protein divergence as primary drivers for new gene functions.