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Cell size and ploidy modulate heat tolerance in Daphnia
France Dufresne1, Sarah-Béatrice Bernier1, Alexis Gagnier-Michel1
1Dept de Biologie, chimie et geographie, Université du Québec à Rimouski, Rimouski, QuebecG5L 3A1, Canada.
Proceedings. Biological Sciences
|January 20, 2026
Summary
Smaller cell size in Daphnia correlates with higher heat tolerance (CTmax), challenging optimal cell size theories. Polyploidy and acclimation temperature also influence heat tolerance, impacting ectotherm distribution.
Area of Science:
- Physiology
- Ecology
- Genetics
Background:
- The theory of optimal cell size suggests smaller cells have higher oxygen uptake due to larger surface-area-to-volume ratios, potentially increasing heat tolerance.
- However, the exact links between cell size, body size, and heat tolerance are not fully understood.
Purpose of the Study:
- To investigate the relationship between cell size, body size, and critical thermal maximum (CTmax) in Daphnia.
- To explore the influence of polyploidy and acclimation temperature on heat tolerance.
Main Methods:
- Juvenile Daphnia (diploid and triploid) were raised at varying temperatures to induce differences in cell size.
- CTmax was measured, and gene expression of heat shock proteins (Hsp70) and catalase was analyzed.
Main Results:
- Larger cells were observed in Daphnia acclimated to lower temperatures and in triploid clones.
- CTmax was negatively correlated with both cell and body size, increasing with acclimation temperature.
- Triploid Daphnia exhibited lower CTmax than diploids at 16°C and 20°C.
- Heat shock increased Hsp70 and catalase expression, but these were not linked to CTmax or ploidy.
Conclusions:
- Cell size, influenced by ploidy and environmental temperature, plays a significant role in determining the heat tolerance of ectotherms.
- Findings suggest cell size, rather than heat-shock proteins or oxidative stress, is a key factor in heat tolerance and may influence geographic distribution.
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