Interspecific and interhabitat variation in hsp70 gene expression in native and invasive kelp populations

This study characterized the response to thermal stress in 3 kelp species to contribute to the understanding of the role of the heat shock response in species distributions and in native-invasive species interactions. We sampled the invasive kelp Undaria pinnatifida in its native range in Japan and its introduced range in California, USA, to investigate small- and large-scale differences in its response to temperature stress. We then conducted similar experiments on native kelp species in different habitats in California to investigate differences in the response among species and habitats. We examined temperature response by measuring the induction of the gene (hsp70) that encodes for heat shock protein 70 (Hsp70), which protects cellular proteins from mis-folding and degradation by environmental stress. Individuals of U. pinnatifida, and the native California species Egregia menziesii and Pterygophora californica were heat-shocked at a range of temperatures, and mRNA was extracted and analyzed for expression of hsp70. Significant differences in the timing and magnitude of hsp70 induction were observed between intertidal and harbor populations of U. pinnatifida within a few meters of each other in Japan, indicating environmentally driven variability in this response. Similarly, intertidal and subtidal populations of E. menziesii showed different responses, with subtidal E. menziesii populations responding more like subtidal P. californica populations. Native California species showed similar magnitudes of expression across all population, while U. pinnatifida collected from California harbors exhibited a more robust hsp70 response than native California species but was similar in magnitude to Japanese populations sampled.