The noncanonical Frizzled/planar cell polarity (PCP) pathway regulates establishment
of polarity within the plane of an epithelium to generate diversity of cell fates,
asymmetric, but highly aligned structures, or to orchestrate the directional migration
of cells during convergent extension during vertebrate gastrulation. In Drosophila,
PCP signaling is essential to orient actin wing hairs and to align ommatidia in the
eye, in part by coordinating the movement of groups of photoreceptor cells during
ommatidial rotation. Importantly, the coordination of PCP signaling with changes in
the cytoskeleton is essential for proper epithelial polarity. Formins polymerize linear
actin filaments and are key regulators of the actin cytoskeleton. Here, we show that
the diaphanous-related formin, Frl, the single fly member of the FMNL (formin related
in leukocytes/formin-like) formin subfamily affects ommatidial rotation in the Drosophila
eye and is controlled by the Rho family GTPase Cdc42. Interestingly, we also found
that frl mutants exhibit an axon growth phenotype in the mushroom body, a center for
olfactory learning in the Drosophila brain, which is also affected in a subset of
PCP genes. Significantly, Frl cooperates with Cdc42 and another formin, DAAM, during
mushroom body formation. This study thus suggests that different formins can cooperate
or act independently in distinct tissues, likely integrating various signaling inputs
with the regulation of the cytoskeleton. It furthermore highlights the importance
and complexity of formin-dependent cytoskeletal regulation in multiple organs and
developmental contexts.
