The transcriptional co-factor Yorkie (Yki; YAP in vertebrates) is a key effector of the Hippo signalling pathway and has been implicated in growth control and patterning, as well as stem cell regulation and regeneration, in flies and vertebrates. Now, on , Alexander Lin and Bret Pearson investigate the role of Yki in planarian flatworms. The researchers report that planarians have a single orthologue of yki and, using RNAi, they show that yki carries out pleiotropic functions. For example, they report that yki is required for homeostasis of the planarian excretory system, which is analogous to the vertebrate kidney. The researchers also show that, in contrast to its role in flies and vertebrates, yki functions to limit stem cell proliferation and hence the size of the stem cell population. In addition, yki plays a role in axial patterning, where it acts synergistically with Wnt signalling to supress head formation. These, together with other findings, demonstrate that yki plays diverse yet non-overlapping roles in planarian biology.
