Transgenerational and developmental plasticity interact to shape heat stress responses in a splash pool copepod

Understanding how organisms adapt to variable environments is critical in the face of rapid global change. Acclimation through phenotypic plasticity is one way organisms keep pace with change and maintain fitness long enough for genetic adaptation to occur. Phenotypic plasticity takes multiple forms from rapid, physiological adjustments, to carryover effects from exposure to stress during development, to transgenerational effects from parental experiences. These forms of plasticity may interact to shape phenotypes and ultimately affect fitness. Here, we used Tigriopus californicus copepods in a fully factorial, split-brood design to examine how acute, yet sublethal heat shocks experienced at different life stages across generations influenced thermal tolerance. This design allowed us to evaluate the effects of parental heat exposure on offspring (transgenerational plasticity), larval heat exposure on later-life responses (developmental plasticity), adult heat exposure (acclimation), and their interactions on heat tolerance and transcriptomic profiles. We found evidence for transgenerational plasticity in the form of a negative carryover effect, as offspring from heat shocked parents exhibited lower thermal tolerance and distinct gene expression patterns, particularly when this adult shock was accompanied by a larval shock. Gene expression data indicate that one possible mechanism producing these transgenerational effects is a carryover of increased metabolic demand. However, we also found that a heat shock experienced in adulthood increased thermal tolerance regardless of prior history as larvae or in parents. This finding suggests that reversible physiological adjustments in adults may be a more adaptive strategy in this system than relying on cues from parents or from early life. The ability to adapt dynamically to environmental stress may mean a greater chance of adaptation in the face of climate change, but past exposures to stress interact to shape physiological responses and ultimately survival. Accounting for this complexity is essential to making accurate predictions about population and species persistence in the future.