DIVISION CHLORARACHNIOPHYTA

Chlorarachniophyte algae contain a complex, multi-membraned chloroplas t derived from the endosymbiosis of a eukaryotic alga. Phylogenetic tr ees indicate that the host is closely related to filose amoebae and sa rcomonads whereas the endosymbiont is most closely related to green al gae. The endosymbiont is greatly reduced retaining only the plastid, p lasmamembrane, a modicum of cytoplasm, and the nucleus. The vestigial nucleus of the endosymbiont, called the nucleomorph, contains three sm all linear chromosomes with a haploid genome size of 380 kb and is the smallest known eukaryotic genome. The overall gene organisation of th e nucleomorph genome is extraordinarily compact making this a unique m odel for eukaryotic genomics.