Certain groups of eukaryotes evolved by a series of endosymbiotic processes. Chlorarachniophytes and cryptomonads’ ancestors underwent two endosymbiosis events.
In the above diagram taken from Gilson’s et al paper in PNAS, you can see the process that led to what is known as the nucleomorph of the chlorarachniophyte (uf…).
In a nutshell, an eukariotic cell (Euk 1) engulfed a prokaryotic photosynthesising cell establishing a symbiotic relationship (1 endosymbiosis), the common one among plants and algae. A second eukaryote (Euk2) swallowed this first endosymbiont leaving a cell with two nucleus (Nu1 and Nm) and a prokaryotic chromosome (within Pl). Well, what was Nu1 is the nucleomorph (Nm) and has undergone massive genome reduction.
In the aforementioned paper, they sequenced the Nm of Bigelowiella natans (a chlorarachniophyte), and they found that it was only 373,000 bp. which is smaller than the smallest sequenced prokaryote. The funny thing is that this tiny genome belongs to what once was a free-living organism, now most of the genes have been either lost or transfered to the main nucleus.
It is hypothesised that with the time the main nucleus (Nu2) will end up acquiring all the genes needed to maintain the plasmid, which is really what matters!. By that time the nucleomorph might end up being lost and we could find plastids with three or four membranes. This might explain why some groups such as dinoflagellates, euglenoids, haptophytes, diatoms, brown algae, chrysophytes and apicomplexan have these “multimembraned” plastids; maybe they underwent secondary endosymbioses and “got rid” of the nucleomorph.
So, we are now, to our evolutionary delight, in an intermediate stage of this reductive process.