Horizontal transfer makes worms photosynthesize?

Horizontal transfer is cool. This is the process whereby cells pick up DNA from other cells that are totally unrelated to them. Bacteria do this all the time. They pick up DNA from viruses and other bacteria and integrate this DNA into their genomes. If it turns out to be useful, it remains in the genome. If not, it's slowly lost over time.

People have always assumed that horizontal transfer doesn't happen in larger organisms. When was the last time you met a person who had picked up a little cabbage DNA. This has historically been one of the complaints about anti-genetically modified food campaigners. Putting genes into a plant that come from a bacterium is somehow "unnatural". Well, turns out horizontal transfer is very natural (in the sense that it’s starting to look like it might be quite common in nature).

In the last few months, there has been more and more data suggesting that horizontal transfer *does* occur in larger organisms. A group at the J. Craig Venter Institute recently published a paper in Science showing large chunks of the Wolbachia genome has been transferred into some fruitfly genomes. Wolbachia is a bacterial obligate intracellular parasite in insects. It grows inside the germ cells of insects, and transfers itself to the following generation inside the egg or sperm of the infected parent (usually through the egg). Well turns out that in at least one detectable case, the Wolbachia genome has actually been incorporated into the fruitfly genome. Very cool! This shows at least 2 things:

1) Horizontal transmission in fruitflies (and possibly other metazoans) is probably not astronomically improbable. If it were, this discovery would represent an incredibly unlikely find. (Of course, until someone finds more examples of this, we could wave this away as just a bizarre anomaly, but I doubt it - more in a bit).

2) This demonstrates an early step in the endosymbiont hypothesis. The endosymbiont hypothesis suggests that organelles that contain their own DNA come from previously endosymbiotic bacteria (like mitochondria and choloroplasts). These organelles contain their own DNA, but not all the proteins in them are come from their own genomes. Some of the proteins come from the host genome. This new Wolbachia finding is a demonstration of an early step in moving gene products from an endosymbion into the host genome. Very cool!

Well, today I heard a talk by a gentleman named Debashish Bhattacharya who had recently sequenced parts of several protist genomes. These are organisms like Paramecium, that are single-celled organisms that feed on bacteria (they’re quite a bit larger than bacteria, so they swim around and gobble them up). They don’t photosynthesize. However! He found the remains of several hundred genes from photosynthetic bacteria in their genomes. This shows yet another example of horizontal transfer in eukaryotes.

The coolest part of his story comes last. Dr. Bhattacharya hypothesized (and is looking for supporting evidence) that he would find genes from photosynthetic algae within the genome of a particular worm Placida cf. dendritica. This worm does something pretty cool. It eats photosynthetic algae, but it preserves the plastids within its own body, and uses them to generate energy from light. Holy photosynthesizing worms, Batman! But how do they maintain these complex organelles in a viable state that can provide energy? He hypothesizes that he will find hundreds of genes from photosynthetic algae that are involved in keeping the plastids functioning. Stay tuned!