An intermediate euglena?

You might remember euglena from your biology days peering at tiny critters under the microscope. Euglena was that weird one-celled organism that could swim around like an animal but photosynthesize like a plant. Turns out there are a number of euglena-like organisms (euglenids) that are also photosynthetic but appear to have evolved from one-celled organisms that are "phagotrophic." Phagotrophic is a fancy word that basically means that the non-photosynthetic euglenids get their energy from eating other stuff. The question is how do you evolve from one to the other? Seems like a big change in lifestyle. Imagine if I suddenly stopped eating, turned green, and started laying out in the sun in my underpants. OK, maybe you don't want to imagine that, but that's the kind of change we're talking about.

In a recent paper from BMC Evolutionary Biology, Yamaguchi et al. report the discovery of a euglenid named Rapaza viridis that appears to be able to get energy in two different ways. It's capable of eating, and it needs to eat a particular kind of algae in order to survive. Rapaza cells also possess the ability to photosynthesize, and they cannot survive longer than a week in the dark even when supplied with plenty of algae to eat. What's even weirder is that Rapaza appears to have two types of chloroplasts, the cellular structures where photosynthesis takes place. One chloroplast from Rapaza looks like a regular photosynthetic euglenid chloroplast, but the other looks like a chloroplast from the algae that it eats. I'm sure there's a very interesting story going on at the molecular level, regulating the appropriation of chloroplasts and possibly other items from the algae that Rapaza eats. Hopefully, future research will illuminate this fascinating relationship.

Rapaza makes a fascinating intermediate form between the eating and photosynthesizing euglenids. It's like the photosynthesizing euglenids because it needs to get its energy from photosynthesis, but it's like the eating euglenids because it can eat algae. What makes it even more interesting is that a molecular phylogeny puts Rapaza on a branch between the eating and the photosynthetic euglenids (that means it looks very much like a "transitional form"). Like some other intermediate forms, though, Rapaza has traits from two different groups rather than intermediate traits (I should note that that's not true for all intermediates): it has euglenid and algal chloroplasts, not something "in between." Does this mean we should view all euglenids as members of the same created kind? Time will tell, but Rapaza definitely stands as a great example of the kind of complicated, symbiotic relationships found among one-celled organisms.

Yamaguchi et al. 2012. Morphostasis in a novel eukaryote illuminates the evolutionary transition from phagotrophy to phototrophy: description of Rapaza viridis n. gen. et sp. (Euglenozoa, Euglenida). BMC Evol Biol 12:29.

(photo from wikipedia)

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