Field of Science

Mostly Unicellular

"Unicellular organisms are so successful, so numerous, and so diverse that an unbiased description of life on Earth could be summed up with just two words: mostly unicellular." This is the 1st sentence from the 3d chapter of the book the Phactor is supposed to be finishing, soon. This of course paraphrases the Hitchhiker's Guide to the Universe's description of Earth: mostly harmless. As a big conspicuous organism living at a macroscopic level, it's very hard to understand just how many unicellular organisms there are, but just as a hint, your body harbors more unicellular organisms than you have cells making up your body, and both are really big numbers. The other thing about this is that organisms seem to have become big, and this happened because lots of small cells teamed up to produce big organisms, rather quickly, at least in geological terms, which means over millions of years. Recent research has shown that under selection pressure, yeast, usually a unicellular organism, becomes multicellular rather quickly. Although this seems to be getting a lot of attention, it doesn't actually surprise the Phactor very much for two reasons. One yeast undoubtably has a multicellular ancestry among filamentous fungi, in other words, it was reduced to unicellularity and it isn't unreasonable to think that some of its multicellular genetic heritage still resides within. When dividing quickly, yeast cells divide by budding, an asymmetrical division, that can produce short chains of cells although evetually they separate. Second, research with other unicellular organisms, in this instance a unicellular algae called Chlorella that lives free-floating in its aquatic environment, has shown similar tendencies. If predators are in the environment, the selection pressure upon unicells (getting eaten), selects for larger multicellular organisms where several cells hang together after division rather than separating. This makes them larger and not so prone to predation, and therefore more successful in reproducing. This is just what evolution is about, non-random reproduction. If anyone says they don't understand how random processes can produce biological diversity they have demonstrated that they have no idea at all of what they speak. What is being altered by the selection is developmental timing, the onset of cell wall synthesis, which if it begins prior to cell separation effectively "glues" the two daughter cells together. Note that no new genes were needed, no new genetic information, just a bit of inheritable variation in developmental timing. In the absence of predators, the selection pressure shifts back and unicellular types again dominate because the bigger multicelled algae have a faster sinking rate and aren't as successful as free-floating algae under these conditions. No big surprises here although certainly a very nice piece of research, and when the actual publication is released, we shall see if the algae work is cited in their literature or not.


The Lorax said...

Im not touching this paper for political reasons, but you may want to compare it with this paper
from Andrew Murray's lab.

The Phytophactor said...

Interesting, very interesting! Well, this wouldn't be the 1st time a PNAS publication was over hyped.