Wednesday, September 19, 2007

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:

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!



makita said...

Nice post. I just linked to it. Now, it's pretty obvious is that the next step would be to find the algae genes in the worm and voila!

The Factician said...

Yep, I sort of think the guy was being coy. It's an easy experiment to do, and to just put it out there in a talk like that suggests to me that he's already done the experiment, but isn't totally ready to publish.

Very cool, and I suspect it will be published soon.

Rob said...

How long do you think it will take before creationists jump on this as the latest example of evolution being a Theory In Crisis{TM}? Eg:

"Photosynthesizing worms are something biologists didn't predict, thus evolution is wrong. Somehow."
"How amazingly and irreducibly complex: clearly the work of an intelligent designer who for legal reasons can't be called God."
"The probability of algae DNA ending up in worms by random chance is impossibly small. Now of course evolutionists *assume* it got there because the worms ate algae but they're overlooking the possibility that God did it."

Irving Washington said...

That's too cool! Thank you.

Felicia Gilljam said...

Rob, the creationist argument is most likely to be this: "Evilutionists thought that horizontal gene transfer didn't happen in larger organisms but it does. Hence they were wrong about something. Therefore they are wrong about everything!"

Factician; There's something I've been wondering about when it comes to obligate endosymbionts. I've pestered just about every teacher and lecturer I've had and none have given me a good answer. My question is: Why are they still called endosymbionts and not organelles? Where do we draw the line? If these bacteria cannot possibly live outside the cells of their hosts, why do we still count them as separate organisms? Just something I'm curious about. Maybe you have an idea?

The Factician said...


Honestly, I'm not sure where to draw the line, though in the case of Wolbachia and mitochodria, I think where the lines are currently is pretty good. Keep in mind that an awful lot of naming has more to do with the history of how things are discovered than to do with what we know about them today.

I think part of the distinction to think about is genetic content. The vast majority of the hundreds of genes involved in mitochondria formation and metabolism are in the nucleus. For example, in humans, only 37 of those hundreds of genes for the mitochindria are actually in the mitochondrial genome. Whereas with Wolbachia most, if not all of the genes involved in Wolbachia metabolism are still in the Wolbachia cell.

The other part is that Wolbachia are not required for viability. You can *cure* them out by treating the flies with an antibiotic, and they will be fine (indeed, in many cases it will restore some of the balance in production of male and female progeny).

I tend to think that the more we look, the more we are going to find intermediate forms of bacteria that have genes that are required for viability that are in the genome of their hosts, and these lines are going to get considerably more blurred. Exciting times for those folks involved in endosymbiont work...

Anonymous said...

No, the propoer creationist argument is that this undermines the entire double-nested hierarchy claim which is one of the strongest supports for evolution. This statement would be wrong for a more interesting reason, because even if horizontal evolution is "common" compared to what we thought it was, the overall shape of the hierarchy is still nearly identical. Of course, since most creationists either don't talk about or don't understand the double-nested hierarchy, I doubt that we'll hear this argument from them.

Eric Michael Johnson said...

Great post! I linked to it at The Primate Diaries.

The Factician said...

Unlike bacter, horizontal transfer is still going to be a fairly small contributor to genome makeup in metazoans. That said, it's starting to look like it's a lot bigger than anyone ever thought it would be (i.e. a few years ago, people would have postulated that it would have made up nearly none of the genes in metazoans, now it may turn out to be merely very few of their genes).

It's certainly going to turn out to be a pretty cool story.

The Factician said...

Make that "bacter" bacteria.