Epithemia sp. SFrail-2023a isolate:GazosCreekCA Genome sequencing and assembly
Endosymbiont gene transfer with import of host-encoded gene products are considered hallmarks of organelles necessary for permanent partnerships. However, newer endosymbiotic models have challenged the source and timing of such genetic integration in organellogenesis. Epithemia diatoms contain diazoplasts closely related to recently described nitrogen-fixing organelles and show comparable metabolic and cellular integration. We report genomic analyses of two Epithemia species, freshwater E. clementina and marine E. pelagica, which are highly diverged but share a recent ancestor at the origin of the endosymbiosis. 3 host-encoded proteins of unknown function were identified in the diazoplast proteome, far fewer than in other recently-acquired organelles. We detected fragmented, non-functional diazoplast-nuclear DNA transfers. The Epithemia diazoplast is a valuable counterpoint to existing organelle models, demonstrating that endosymbionts can be maintained over millions of years of speciation absent significant genetic integration. Identifying the non-genetic interactions that facilitated diazoplast integration with Epithemia has important implications for bioengineering.