Molecular convergence and positive selection associated with the evolution of symbiont transmission mode in stony corals

Heritable symbioses are thought to be important for the maintenance of mutually beneficial relationships (1), and for facilitating major transitions in individuality, such as the evolution of the eukaryotic cell (2, 3). In stony corals, vertical transmission has evolved repeatedly (4), providing a unique opportunity to investigate the genomic basis of this complex trait. We conducted a comparative analysis of 25 coral transcriptomes to identify orthologous genes exhibiting both signatures of positive selection and convergent amino acid substitutions in vertically transmitting lineages. The frequency of convergence events tends to be higher among vertically transmitting lineages, consistent with the proposed role of natural selection in driving the evolution of convergent transmission mode phenotypes (5). Of the 10,774 total orthologous genes identified, 403 exhibited at least one molecular convergence event and evidence of positive selection in at least one vertically transmitting lineage. Functional enrichments among these top candidate genes include processes previously implicated in mediating the coral-Symbiodiniaceae symbiosis including endocytosis, immune response, cytoskeletal protein binding and cytoplasmic membrane-bounded vesicles (6). We also identified 100 genes showing evidence of positive selection at the particular convergence event. Among these, we identified several novel candidate genes, highlighting the value of our approach for generating new insight into the mechanistic basis of the coral symbiosis, in addition to uncovering host mechanisms associated with the evolution of heritable symbioses.\n\nDATA ARCHIVAL LOCATIONRaw sequencing data generated for this study have been uploaded to NCBIs SRA: PRJNA395352. All bioinformatic scripts and input files can be found at https://github.com/grovesdixon/convergent_evo_coral.