Prickle isoform participation in distinct polarization events in the Drosophila eye

Planar cell polarity (PCP) signaling regulates several polarization events during development of ommatidia in the Drosophila eye, including directing chirality by polarizing a cell fate choice and determining the direction and extent of ommatidial rotation. The pk(sple) isoform of the PCP protein Prickle is known to participate in the R3/R4 cell fate decision, but the control of other polarization events and the potential contributions of the three Pk isoforms have not been clarified. Here, by characterizing expression and subcellular localization of individual isoforms together with re-analyzing isoform specific phenotypes, we show that the R3/R4 fate decision, its coordination with rotation direction, and completion of rotation to a final +/- 90 degrees rotation angle are separable polarization decisions with distinct Pk isoform requirements and contributions. Both pk(sple) and pk(pk) can enforce robust R3/R4 fate decisions, but only pk(sple) can correctly orient them along the dorsal-ventral axis. In contrast, pk(sple) and pk(pk) can fully and interchangeably sustain coordination of rotation direction and rotation to completion. We propose that expression dynamics and competitive interactions determine isoform participation in these processes. We propose that the selective requirement for pk(sple) to orient the R3/R4 decision and their interchangeability for coordination and completion of rotation reflects their previously described differential interaction with the Fat/Dachsous system which is known to be required for orientation of R3/R4 decisions but not for coordination or completion of rotation.