Nuclear genome sequencing reveals the highly intron-rich architecture of the chlorarachniophyte alga Amorphochlora amoebiformis.

Chlorarachniophyte algae possess complex plastids derived from endosymbiosis between a cercozoan protist and green alga. As evidence of this event, remnant nucleus of the endosymbiont, nucleomorph, is present in the plastid intermembrane space. Chlorarachniophytes are excellent models to study genome evolution via endosymbiosis. Although the three organelle genomes of mitochondrion, plastid, and nucleomorph have been sequenced in several chlorarachniophyte species, nuclear genome information is currently limited to Bigelowiella natans. To gain insights into the genome diversity and evolution of chlorarachniophytes, we sequenced the nuclear genome of another chlorarachniophyte, Amorphochlora amoebiformis. Its size is approximately 214 Mb, which is more than twice that of B. natans. Remarkably, three-quarters of the nuclear genome encodes spliceosomal introns, indicating its highly intron-rich structure compared to other known eukaryotic genomes. Single nucleotide polymorphism analysis revealed that A. amoebiformis possessed a diploid nuclear genome, unlike the haploid genome of B. natans. Additionally, we identified organellar DNA fragments within the nuclear genome, suggesting recent DNA migration from the three organelles to the nucleus. Overall, our findings reveal that chlorarachniophyte nuclear genomes differ substantially in size, structure, and ploidy across species, and provide evidence of ongoing endosymbiotic gene transfer.