Improved Genome Assembly Reveals Features of a Facultative Coral Symbiont and the Complex Evolutionary History of Dinoflagellate Genes.

Dinoflagellates of the family Symbiodiniaceae are crucial photosymbionts in corals and other marine organisms. Of these, is one of the most dominant symbiont species in the Indo-Pacific. Here, we present an improved genome assembly of combining new long-read sequence data with previously generated short-read data. Incorporating new full-length transcripts to guide gene prediction, the genome (1.2 Gb) exhibits a high extent of completeness (82.4% based on BUSCO protein recovery) and better resolution of repetitive sequence regions; 45,322 gene models were predicted, and 327 putative, topologically associated domains of the chromosomes were identified. Comparison with other Symbiodiniaceae genomes revealed a prevalence of repeats and duplicated genes in , and lineage-specific genes indicating functional innovation. Incorporating 2,841,408 protein sequences from 96 taxonomically diverse eukaryotes and representative prokaryotes in a phylogenomic approach, we assessed the evolutionary history of genes. Of the 5246 phylogenetic trees inferred from homologous protein sets containing two or more phyla, 35-36% have putatively originated via horizontal gene transfer (HGT), predominantly (19-23%) via an ancestral Archaeplastida lineage implicated in the endosymbiotic origin of plastids: 10-11% are of green algal origin, including genes encoding photosynthetic functions. Our results demonstrate the utility of long-read sequence data in resolving structural features of a dinoflagellate genome, and highlight how genetic transfer has shaped genome evolution of a facultative symbiont, and more broadly of dinoflagellates.