Rapid thermal adaptation in coral photosymbionts draws on standing variation and recombination

Corals' ability to withstand higher temperatures, known as bleaching tolerance, relies partly on the thermal resilience of the microalgal symbionts living within them. Scientists have managed to increase the tolerance of the symbiont Cladocopium proliferum through laboratory evolution over approximately 120 generations, but the genetic mechanisms behind this adaptation remained unclear.

Researchers analyzed the transcriptomes of three heat-evolved C. proliferum strains and one wild-type strain, all originating from the same progenitor. They discovered 15,640 single nucleotide polymorphisms (SNPs) in the transcriptomes, yet found no mutation that was exclusive to a specific strain and consistent across all replicates. This suggests that new mutations played a minimal role in the observed increase in thermal tolerance.

Instead, the study revealed that allele frequencies at 350 specific loci differed significantly between the strains. Linkage patterns further indicated that recombination events had taken place within scaffolds both before and after the strains were isolated from one another. These findings show that both natural selection and recombination of pre-existing variations in the progenitor strain were crucial in driving the rapid adaptation of C. proliferum to higher temperatures.

The authors recommend that experimental evolution efforts for coral reef restoration should begin with genetically diverse cultures rather than relying solely on single cell isolates. This approach would provide a broader range of genetic material, potentially accelerating the adaptation process and increasing the likelihood of identifying beneficial traits for enhancing coral resilience in the face of climate change.