Assessment of Local Adaptation and Outbreeding Risks in Contrasting Thermal Environments of the Giant Kelp, Macrocystis Pyrifera

Understanding the scope of local adaptation is critical to the definition of restoration strategies as it may affect individuals used as breeders or transplants. The assessment of such risks is, however, challenging for large seaweeds such as kelps, because their experimental manipulation is difficult in marine coastal environments. We investigated the consequences of local adaptation on reproductive traits of gametophytes, and growth and survival of sporophytes of the giant kelp, Macrocystis pyrifera . Over a thousand juvenile sporophytes were obtained from controlled crosses using 2 to 3 parental sporophytes from each of 18 populations from 5 different regions (i.e. Magellanes. Southern Chile, Atacama, Peru and California), to assess fertility and fecundity of female gametophytes when exposed to males of different habitats, and to produce juvenile sporophytes that were used to test for local adaptation sensu stricto in common garden experiments. By comparing sympatric and allopatric combinations of source populations and the average temperature of the sampled regions, we detected a significant pattern of local adaptation. We quantified the risks associated with different combinations of population/habitat for the performance of juvenile sporophytes typically used as transplants in kelp restoration actions. We found evidence of reduced reproductive success associated with outbreeding for high latitude populations, while low latitude populations favored hybrid crosses, suggesting a shift between outbreeding depression and hybrid vigor across different habitats (i.e., temperature). We also showed that a heatwave equally increased mortality of sporophytes across all regions except for Peruvian inbred crosses, suggesting these low latitude populations hold key genetic resources for future actions under ocean warming. Altogether, these experiments provide a framework for assessing risks associated with the choice of source populations in any kelp species.