Climate warming reduces the speed and predictability of polygenic adaptation to salinity decline

Climate change exposes populations to multiple stressors simultaneously, yet our understanding of how the addition of one stressor alters adaptation to another remains poor. As a result of climate change, high-latitude coastal habitats are experiencing rapid salinity decline, resulting in serious impacts on food webs and ocean circulation. Here, we examine how temperature increase impacts adaptation to salinity decline, in terms of the speed, genomic response, and repeatability of adaptation. We performed replicated Evolve-and-Resequence experiments over 20 to 25 generations in the model copepod Eurytemora carolleeae (Atlantic clade of the E. affinis species complex). Under salinity decline alone, replicate selection lines exhibited a polygenic response involving 66 selected haplotype blocks, with increasing parallelism among the replicate lines through Generation 20. Fitness (egg number) declined sharply over the first four generations but underwent full Evolutionary Rescue, recovering to ancestral levels by Generation 10. In contrast, imposing temperature increase on the salinity decline lines resulted in significantly lower parallelism among the selection lines, along with delayed and incomplete Evolutionary Rescue. Only 14% of selected SNPs were shared between the two selection regimes, and Gene Ontology analyses revealed largely distinct functional categories of genes under selection. These results show that adding warming to salinity decline can alter the genomic trajectory of salinity adaptation, slowing and impeding Evolutionary Rescue, and reducing the repeatability of polygenic responses. Our findings have direct implications for predicting evolutionary responses to realistic, multi-stressor climate change in high-latitude coastal ecosystems experiencing simultaneous ocean freshening and warming.