Defined diets for freshwater planarians

Planarian flatworms are popular invertebrate models for basic research on stem cell biology and regeneration. These animals are commonly maintained on a diet of homogenized calf liver or boiled egg yolk in the laboratory, introducing a source of uncontrolled experimental variability. Here, we report the development of defined diets, prepared entirely from standardized, commercially sourced ingredients, for the freshwater species Schmidtea mediterranea, Dugesia japonica, and Girardia dorotocephala. These food sources provide an opportunity to test the effects of specific nutritional variables on biological phenomena of interest. Defined diet consumption was not sufficient for growth and only partially induced the increase in stem cell division that normally accompanies feeding, suggesting these responses are not solely determined by caloric intake. While our defined diet formulations do not support long-term planarian maintenance, they do enable delivery of double-stranded RNA for gene knockdown in a manner that provides unique advantages in some experimental contexts. We also present a new approach for preserving tissue integrity during hydrogen peroxide bleaching of liver-fed animals. These tools will empower research on the connections between diet, metabolism, and stem cell biology in the experimentally tractable planarian system.