The surprising finding suggests a new mechanism of evolution.
Friday, August 31, 2007
By Emily Singer
Wolbachia in yellow with host cells in red.
Credit: University of Rochester and J. Craig Venter Institute |
In a strange genetic version of the Russian doll, scientists have discovered the genome of a bacterial parasite nestled inside the genome of its host. The findings, published today in the journal Science, suggest that organisms might quickly acquire new genes and functions through the large-scale transfer of genes.
The parasite, known as Wolbachia, invades the eggs and sperm of many different types of insects, ensuring that it is passed down to the host's offspring. In this case, scientists discovered the bacterium's genome within the chromosome of its fruit-fly host. While microbiologists have previously seen cases of gene swapping between microbes or between parasites and their hosts, this is the first example of such an extensive exchange.
According to a press release from the University of Rochester,
"This study establishes the widespread occurrence and high frequency of a process that we would have dismissed as science fiction until just a few years ago," says W. Ford Doolittle, Canada Research Chair in Comparative Microbial Genomics at Dalhousie University, who is not connected to the study. "This is stunning evidence for increased frequency of gene transfer."
"It didn't seem possible at first," says [Jack] Werren, professor of biology at the University of Rochester and a world-leading authority on the parasite, called Wolbachia. "This parasite has implanted itself inside the cells of 70 percent of the world's invertebrates, coevolving with them. And now, we've found at least one species where the parasite's entire or nearly entire genome has been absorbed and integrated into the host's. The host's genes actually hold the coding information for a completely separate species."
A similar phenomenon may have happened in our own distant past.
"In our very own cells and those of nearly all plants and animals are mitochondria, special structures responsible for generating most of our cells' supply of chemical energy. These were once bacteria that lived inside cells, much like Wolbachia does today. Mitochondria still retain their own, albeit tiny, DNA, and most of the genes moved into the nucleus in the very distant past. Like Wolbachia, they have passively exchanged DNA with their host cells. It's possible Wolbachia may follow in the path of mitochondria, eventually becoming a necessary and useful part of a cell. In a way, Wolbachia could be the next mitochondria," says Werren. "A hundred million years from now, everyone may have a Wolbachia organelle."
Fruit fly ovaries showing Wolbachia infection within.
Credit: University of Rochester and the J. Craig Venter Institute |
Comments
dmm on 09/10/2007 at 3:50 PM
135
In any case, the theory is that mitochondria and primitive cells formed a symbiotic relationship, not a parasite-host relationship.
So the question arises: is there any evidence that Wolbachia provide a survival advantage to their hosts? If not, then this seems to be evidence for the Second Law of Thermodynamics, rather than for evolution. That is, the information content of invertebrates is being degraded rather than improved, unless Wolbachia is providing a critical benefit to its hosts.
rajnz on 09/11/2007 at 9:06 AM
23
What exactly are the difficulties and objections that you are referring to? The difficulties and objections have been dealt with, if you read "Power, Sex, Suicide: Mitochondria and the Meaning of Life" by Nick Lane
The evidence that mitochondria have bacterial origins is overwhelming. Their DNA, just like bacterial DNA, is circular. They have a cell wall as you have in bacteria and their method of energy generation is exactly the same.
The birth of the Eukaryotic cell and the hence the evolution of all the complex life as we know it was made possible with the assimilation of the mitochondria, about 400 million years ago. Before that there was only bacteria for 3 billion years which did not and has not evolved into anything more complex.
dmm on 09/12/2007 at 5:48 PM
135
What exactly are the difficulties and objections that you are referring to? The difficulties and objections have been dealt with, if you read "Power, Sex, Suicide: Mitochondria and the Meaning of Life" by Nick Lane
Response:
Haven't read that book. Thanks for the tip. Will be interested to see how it explains everything. I suspect I will be disappointed. Although I love biology, my experience is that biologists have an unfortunate tendency to sweep counter-arguments under the rug. They seem to be jealous of the certainty of the more mathematical sciences.
You said:
The evidence that mitochondria have bacterial origins is overwhelming. Their DNA, just like bacterial DNA, is circular.
Response:
Actually, most of the DNA that mitochondria need to function is in the nucleus of the cell, not in the mitochondria. There is no proof that this DNA "moved into" the nucleus. That is an assumption. (Circular reasoning doesn't count.) The corresponding proteins are fabricated just like every other cellular protein. Then they are transported to the mitochondria and through the mitochondrial membranes/walls (all using special "transport proteins"). That is not at all "bacteria-like." The DNA segments in mitochondria are fairly short, and they are translated into proteins on mitochondrial ribosomes. Admittedly that is bacteria-like. But is it because mitochondria used to be bacteria, or is it simply because that's the best way of doing things?
You said:
They have a cell wall as you have in bacteria and their method of energy generation is exactly the same.
Response:
First of all, similarity does NOT prove a common origin. Do I have to bring up squid eyes? Second, I don't know where you get the idea that mitochondria have a cell wall. They don't. Apparently they lost that, along with pillae, flagella, and other bacterial structures during the same time that they gained their unbacterial cristae and their unique membrane transport proteins. [note sarcasm] Third, their method of energy generation is NOT exactly the same as bacteria. Do bacteria make excess ATP and spew it into the environment? And do you realize that mitochondria are species-specific, not interchangeable (as they would be if your overstatements were true)?
Summary:
I don't have a problem in principle with the endosymbiotic theory. Like I said, it's an interesting idea. But I get tired of hearing it talked about (along with a lot of other theories) as established fact when it still has many holes in it. Scientists need to stop being so arrogant about their opinions.