Small heat shock protein responses of a closely related pair of desert and coastal Encelia

Evolutionary variation for accumulation of small heat shock protein (sHsp) may contribute to thermal niche differentiation between species. Here we examine temperature and time-course-dependent variation for sHsp accumulation in a recently diverged pair of Encelia raised in a common environment: Encelia farinosa, common in the Mojave desert, and Encelia californica, which is found along the cool coastal bluffs of southern North America. Both species exhibit peak sHsp accumulation at 42degreesC. Encelia californica accumulated greater levels of sHsp at temperatures below 42degreesC, while E. farinosa had greater levels above 42degreesC. Encelia farinosa accumulates sHsp at temperatures up to 45degreesC, while E. californica does not synthesize sHsp above 44degreesC. Both species accumulated significant levels of sHsp while maintaining photosynthetic electron transport (F-v/F-m), but above the temperatures that elicited peak sHsp expression, levels of sHsp and F-v/F-m declined in parallel to zero. Encelia californica accumulated greater levels of sHsp more rapidly than E. farinosa following a 15 min, 42degreesC heat treatment; however, E. farinosa maintained greater F-v/F-m at all time points. Our results indicate that there are significant differences between Encelia species for sHsp accumulation but that these results depend on the duration, magnitude, and recovery time following temperature stress.