Transcriptional dynamics following freezing stress reveal selection for mechanisms of freeze 3 tolerance at the poleward range margin in the cold water intertidal barnacle Semibalanus 4 balanoides 5 6 Running title: Transcription following freezing in S. balanoides 7 Abstract 22The ability to survive freezing has repeatedly evolved across multiple phyla. This suggests that 23 the mechanisms of freeze tolerance must be readily evolvable from basal physiological traits.
24While several biochemical correlates to freeze tolerance have been described, the mechanism that 25 confers freeze tolerance is still not well understood. To understand both the basic biochemical 26 mechanisms of freeze tolerance as well as their role in local adaptation at the poleward range 27 edge, we conducted a transcriptomic study on two populations (one from the poleward range 28 margin in the White Sea, Russia, and one from the central coast of British Columbia, Canada) of 29 the cold water acorn barnacle Semibalanus balanoides on a time series following a freezing 30 event. We found that the British Columbia population (at the equatorward range margin) was 31 significantly less freeze tolerant than the White Sea population (at the poleward range margin).
32After assembling and annotating a de novo transcriptome for S. balanoides, we found that the 33 patterns of differential transcript expression following freezing were almost entirely non-34 overlapping between the two populations, with the White Sea population expressing a series of 35 heat shock proteins in response to freezing stress as well as several aquaporins, while the British 36 Columbia population expressed a series of proteases instead, indicating severe protein damage. 37 We found strong evidence of purifying selection on the significantly upregulated transcripts in 38 the White Sea population, suggesting local adaptation to freezing threat. Taken together, this 39 shows the importance of freeze tolerance to population survival at the poleward range margin, 40 and highlights the central roles of aquaporins and heat shock proteins to the trait of freeze 41 tolerance across taxa. 42