Natural variation in cold and heat survival among temperate and tropical Caenorhabditis briggsae.

We developed a simple assay to quantify natural variation in adult thermal stress tolerance in the nematode Caenorhabditis briggsae . Selfing hermaphrodites from wild strains were exposed to cold (0°C, 5°C) or heat (29°C, 33°C, 35°C) for 15 hours during early adulthood. Cold exposure revealed clear clade-level differences: temperate strains showed high survival, while tropical strains were more variable and generally less tolerant. Heat exposure at 35°C caused high mortality across strains from both clades, with only slightly lower mortality in tropical strains. At 29°C, all strains showed full survival. By 33°C, mortality rose modestly, with temperate strains showing slightly greater sensitivity. This assay captures natural variation in thermal tolerance and provides a simple and efficient tool for studying thermal adaptation in C. briggsae and other Caenorhabditis nematodes.