Cell size and ploidy modulate heat tolerance in <i>Daphnia</i>

The theory of optimal cell size postulates that cell size imposes constraints on oxygen delivery in larger cells owing to their smaller surface-area-to-volume ratios. Smaller cells, with a higher surface-area-to-volume ratio, have an increased capacity to take up oxygen, which is therefore expected to increase their heat tolerance (CTmax). However, the precise mechanisms linking cell size, body size and heat tolerance remain unclear. We obtained contrasts in cell size by raising diploid and triploid juvenile Daphnia at different temperatures. Daphnia raised at lower temperatures had larger cells than those raised at higher temperatures. Triploid clones had larger cells than diploid clones at 16°C, 20°C and 24°C. CTmax increased with acclimation temperature and was negatively correlated to both cell and body size. Triploid clones had lower CTmax values than both subarctic and temperate diploid clones at 16°C and 20°C. Heat shock significantly increased the expression of Hsp70 and catalase, but these were not correlated with CTmax or ploidy, suggesting that heat tolerance was not directly linked to heat-shock proteins or oxidative stress responses. These findings highlight the role of cell size and polyploidy in shaping the heat tolerance and geographic distribution of ectotherms.