Mechanistic Regulation of Planarian Shape During Growth and Degrowth

Abstract

Adult planarians can grow when fed and degrow (shrink) when starved while maintaining their whole-body shape with correct proportions. Different planarian morphogens are expressed at the anterior-posterior poles, medio-lateral border, and midline to provide positional information signals for the specification of different tissues at the right locations. However, it is currently unknown how these signals are coordinated together during feeding or starvation and how they modulate the differential tissue growth or degrowth necessary to form correct whole-body shapes. Here we investigate the dynamics of planarian shape during growth and degrowth together with a theoretical study to evaluate the mechanisms that regulate whole-body proportions and shape. We found that the planarian body proportions scale isometrically following similar linear rates during growth and degrowth, but that fed worms are significantly wider than starved worms. By combining a descriptive model of planarian shape and size with a mechanistic model of anterior-posterior and medio-lateral signaling calibrated with a novel machine learning methodology, we demonstrate that the feedback loop between these positional information signals and the shape they control can regulate the planarian whole-body shape during growth. Furthermore, the model can predict the correct shape and size dynamics during degrowth due to an increase in apoptosis rate and pole signal during starvation. These results offer mechanistic insights into planarian shape and size dynamics and the regulation of whole-body morphologies.