The role of cell size in shaping responses to oxygen and temperature in fruit flies

Abstract

The body size of an animal is a key driver of ecological traits such as fecundity, mortality and growth. Ectotherms mature at smaller body sizes in warmer conditions and under low oxygen availability (hypoxia). Whether these size reductions are driven by a decrease in cell size or cell number (which together determine body size) and how such cellular responses may help ectotherms cope with thermal challenges and hypoxia is poorly understood. The theory of optimal cell size postulates that cell size impacts metabolic costs and the capacity of cells to take up oxygen via differences in surface-to-volume ratio. In this way, cell size has consequences for growth rate and body size. Here, using inbred lines of fruit flies Drosophila melanogaster, which exhibits differences in cell size, we investigate how body size reductions due to warming or hypoxia are modulated by and result from differences in cell size. Temperature-size responses were stronger under hypoxia and in lines of flies exhibiting large cell sizes, supporting the idea that larger cells constrain organismal growth by their inherently lower capacity for oxygen uptake. Reductions in body size were driven by reductions in cell size, whereas cell number increased to compensate partially. Thus, a cellular perspective offers a complete understanding of changes in body size in response to temperature and oxygen.