A Cross-Climate Comparison of Molecular Phenology in Three Tropical and Temperate Trees

Latitudinal gradients in temperature seasonality shape the evolution of thermal tolerance and acclimation capacity in organisms. According to Janzen's climate variability hypothesis, tropical species experiencing stable temperatures evolve narrower thermal niches than temperate counterparts. To test whether this principle extends to gene expression dynamics, we compared annual transcriptome profiles of the tropical tree Rubroshorea leprosula and the temperate evergreen trees Lithocarpus edulis and Quercus glauca under natural field conditions. Time-series RNA-seq analyses revealed that R. leprosula exhibited sporadic transcriptional shifts triggered by slight cooling events (minimum temperatures of 21°C-22°C), whereas the temperate species showed clear annual cycles characterized by winter-specific expression patterns. Mild temperature decline in the tropical tree triggered widespread down-regulation of photosynthesis-related genes and activation of stress-response and jasmonate-associated signaling pathways, suggesting coordinated responses even to mild temperature declines. Cross-species comparison of 3793 single-copy orthologs showed that the sensitivity to temperature and dynamic range of gene expression were substantially larger relative to the narrow dynamic range of temperature in the tropics, indicating amplified transcriptional responses per unit of temperature variation. Conversely, temperate species displayed broad but proportionate transcriptomic shifts that paralleled large seasonal temperature fluctuations. These results demonstrate that transcriptional sensitivity to temperature is exaggerated in tropical species and buffered in temperate ones, extending Janzen's climate variability hypothesis from physiological tolerance to its molecular basis.