Genome assembly of provides evidence of host nucleus overthrow by the symbiont nucleus during speciation.

The species of the genus are unique in their maintenance of a nucleus-plastid continuum throughout their cell cycle, non-motility and asexual reproduction. These characteristics should have been endorsed in their gene assemblages (genomes). Here we show that has a genome of 29.3 Mb consisting of 32 pseudochromosomes and containing 7,330 protein-coding genes; and the host nucleus may have been overthrown by an ancient red alga symbiont nucleus during speciation through secondary endosymbiosis. In addition, has lost its flagella and abilities to undergo meiosis and sexual reproduction, and adopted a genome reduction strategy during speciation. We propose that emerged through the active fusion of a host protist and a photosynthesizing ancient red alga and the symbiont nucleus became dominant over the host nucleus while the chloroplast was wrapped by two layers of endoplasmic reticulum. Our findings evidenced an alternative speciation pathway of eukaryotes.