Genomes of nitrogen-fixing eukaryotes reveal a non-canonical model of organellogenesis.

Endosymbiont gene transfer and import of host-encoded proteins are considered hallmarks of organelles necessary for stable integration of two cells. However, newer endosymbiotic models have challenged the origin and timing of such genetic integration during organellogenesis. Epithemia diatoms contain diazoplasts, closely related to recently-described nitrogen-fixing organelles, that are also stably integrated and co-speciating with their host algae. We report genomic analyses of two species, freshwater E.clementina and marine E.pelagica , which are highly divergent but share a common endosymbiotic origin. We found minimal evidence of genetic integration: nonfunctional diazoplast-to-nuclear DNA transfers in the E.clementina genome and 6 host-encoded proteins of unknown function in the E.clementina diazoplast proteome, far fewer than in other recently-acquired organelles. Epithemia diazoplasts are a valuable counterpoint to existing organellogenesis models, demonstrating that endosymbionts can be stably integrated and inherited absent significant genetic integration. The minimal genetic integration makes diazoplasts valuable blueprints for bioengineering endosymbiotic compartments de novo .