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Understanding geographic variation in traits and the genetic dynamics underlying it can be important for our ability to understand future species' responses to climate change. Increasingly, genomic techniques are used to investigate the evolutionary process and underlying genetic architecture of population differentiation. Insights into phenotypic and genomic differentiation in marine macrophytes, which are foundation species and the basis of many intertidal communities, will be critical for predicting species and ecosystem response to anthropogenic warming. Ascophyllum nodosum, a marine macrophyte in the Gulf of Maine, has gametes with limited dispersal and inhabits sites with consistently different environments. Both factors should favor the evolution of local adaptation, but this has yet to be investigated in the species. To study geographic variation in temperature adaptation in A. nodosum, we employed experimental and genomic techniques. We collected ten individuals of A. nodosum from six sites, three in the southern Gulf of Maine (average summer temperature = 20 ºC) and three in the northern Gulf of Maine (average summer temperature = 15 ºC), for a total of sixty genotypes. After acclimation to common garden conditions at the Schiller Coastal Studies Center, Harpswell, ME, we conducted a thermal ramping experiment and constructed thermal performance curves based on maximum photosystem II efficiency. We then estimated thermal optimum and critical maximum for the A. nodosum at each site. We found evidence of site level differentiation in thermal optimum, but not exactly as would be expected by region. Although the highest thermal optimum of 34 ºC was in the south, and the lowest thermal optimum was 2 ºC lower in the north, there was also overlap between in the thermal optimums of a northern and southern site. There was less variation and differentiation for critical maximum. Surprisingly, we found little site level genomic differentiation and evidence for significant admixture between regions. We hypothesize that the phenotypic differentiation between sites is either due to long term acclimation to home conditions or to local adaptation through a small number of isolated genomic islands of differentiation.
Available for download on Thursday, May 18, 2028