Sometimes, evolution leads to life's equivalent of Russian dolls. The chloroplast, a cell organelle capable of photosynthesis, was once an independent microbe, which was engulfed--enslaved, some say--by another microbe, an event that paved the way for plants and algae to evolve. Eons later, in a process called secondary endosymbiosis, predatory protozoa gulped down the green algae. Now, the sequence of a chloroplast has shown that the latter process occurred not once but at least twice.
The research focused on the chlorarachniophytes, flagellated, amoeboid cells that can link up with one another by fusing threadlike projections into a web that traps prey, such as other protozoa. (Their name is Greek for "green spider.") Chlorarachniophytes can also carry out photosynthesis; they are one of two groups of protozoa that, at some point in their history, have acquired chloroplasts by engulfing a green alga. The other group is called the euglenids.
Researchers disagree as to how both groups acquired their green algae. Some argue that this happened in a common ancestor, while others believe they were independent acquisitions. Chlorarachniophytes are particularly interesting study objects, because these microbes still retain some of the green alga's nucleus, suggesting that the process of secondary endosymbiosis is still ongoing.
Patrick Keeling, an evolutionary biologist at the University of British Columbia in Vancouver, Canada, and his colleagues sequenced the chloroplast genome of one chlorarachniophyte, Bigelowiella natans. The genome is just 69,200 bases long, the smallest known for chloroplasts, and contains 57 protein-coding genes, Keeling reports in the January Molecular Biology and Evolution. A comparison of the sequence to that of chloroplasts from green algae and euglenids showed that chlorarachniophytes acquired their chloroplasts independently, presumably from pond scum. Thomas Cavalier-Smith of the University of Oxford estimates that event may have been 135 million to 380 million years ago; if this endosymbiosis had occurred just once, then these microbes may have been harnessing sunlight as far back as 540 million years ago.
"These are really ancient events and it's challenging to extract the evolutionary [history]" of endosymbiosis, says John Archibald, a comparative genomicist at Dalhousie University in Halifax, Canada. The new analysis "seems to indicate quite strongly" that green algae were scooped up at least twice by predatory protozoans, he says.