Symbiodinium microadriaticum (coral microalgal endosymbiont)

Photosynthetic microalgae, from the family Symbiodiniaceae, engage in endosymbioses with marine invertebrates, including stony corals. More generally, dinoflagellates are ubiquitous protists and the main primary producers in the oceans. Despite their ecological and economic importance, their biology remains enigmatic. Here we assembled 94 chromosome-scale scaffolds of the ancestral Symbiodiniaceae species Symbiodinium microadriaticum. Contrary to the random order of genes typically found in eukaryotic cells, genes are enriched toward the ends of chromosomes in alternating unidirectional blocks that are sometimes enriched for genes of specific biological processes. These gene blocks are coexpressed and separated by structural boundaries where transcription converges. These structural domains, in turn, comprise the transcription-dependent basic building blocks of the chromosomes that fold as linear rods. Such a highly ordered structure linking gene orientation, transcription, and spatial organization of chromosomes is exceptional.