Bristles and hairs on the wing margin of a pkpk mutant fly. Image credit: Bomsoo Cho (CC BY 4.0)
Our right and left hands are mirror images of each other and cannot be precisely superimposed. This property, known as chirality, is vital for many tissues and organs to form correctly in humans and other animals. For example, fruit flies have hair-like sensory organs on the edges of their wings known as bristles. One of the cells in each bristle forms a shaft that generally tilts away from the main body of the fly and is anchored in place by another cell known as the socket.
A signaling pathway known as PCP signaling controls the directions in which many chiral tissues and organs in animals form. The pathway contains two signaling modules: the global module collects “directional” information about the orientation of the body and sends it to the core module, which interprets this information to control how the tissue or organ grows.
Fruit flies have two different versions of one of the core module components – known as Prickle and Spiny legs – that are thought to alter the direction the core module responds to the information it receives. Mutant flies known as pkpk mutants are unable to make Prickle and their wing bristles tilt in the opposite direction compared to those in normal flies, but it was not clear exactly why this happens.
To address this question, Cho et al. studied PCP signaling in the wings of normal and pkpk mutant flies. The experiments showed that Prickle directed the bristles on the right wing of a normal fly to grow in left-handed corkscrew-like patterns in which the emerging shaft and socket of each bristle twisted around each other. As a result, the bristles tilted away from the bodies of the flies. In the pkpk mutants, however, Spiny legs substituted for Prickle, causing the equivalent bristles to grow in a right-handed corkscrew pattern and tilt towards the body.
The findings of Cho et al. show that PCP signaling controls the direction fly bristles grow by selectively using Prickle and Spiny legs. In the future, this work may also aid efforts to develop effective screening and treatments for birth defects that result from the failure of chiral tissues and organs to form properly.
