The longstanding question of how animals and their organs reach a defined size is fundamental to developmental biology. Planarians offer an intriguing model, as they can dramatically change their body size throughout life, in response to nutrient availability or when regenerating. This body size regulation depends on balancing cell proliferation and cell death, but its molecular regulation remains incompletely understood. Now, Teresa Adell and colleagues identify a novel gene family that regulates body size in the planarian Schmidtea mediterranea. In a screen for genes involved in eye regeneration, the authors identify Blitzschnell (bls; ‘quick as a flash’ in German), the loss of which causes faster regeneration. bls belongs to a gene family encoding putatively secreted peptides that are found only in the planarian order Tricladida. Three bls subfamilies are expressed in secretory prepharyngeal cells, and their coincident knockdown after dsRNA treatment promotes faster regeneration via increased cell division and decreased mitosis. During starvation-induced body shrinkage, bls knockdown does not change overall body size, even though total cell number is increased; rather, cell size is reduced in this condition. Finally, bls expression is downregulated following nutrient intake, and genetically interacts with the evolutionary conserved insulin/Akt/mTOR metabolic network. The de novo evolution of the bls gene family in the Tricladida lineage may have provided an additional mechanism to restrict cell number in these very plastic animals.

• © 2020. Published by The Company of Biologists Ltd