Monday, October 5, 2009
Selfish DNA: Östergren
The lack of posts lately has been due to several things: lab work, laziness, having to input and organize my collection of literature into a new program, and trying to find certain references so I can do my Selfish DNA posts in the (almost) correct chronological order. The labwork remains, as always, the laziness has subsided somewhat, all of the literature I've read has been inputted into Mendeley, and I finally think I've reached the very first mention of selfish DNA in the literature.
This is an oldie, but a goodie is seems.
While not talking about transposable elements I do feel that this is one of those papers that every transposon biologist should be required to read. I'll probably say that about every piece of literature featured in these Selfish DNA posts but, nonetheless. Östergren's paper deals with the presence of extra, or B, chromosomes in plants. B chromosomes are superfluous chromosomes found in a number of different taxa that have acquired through mutation a means of ensuring that they are passed down to offspring even if they are not essential for survival. These chromosomes were the first kind of selfish DNA to be discovered but it was this paper, to the best of my knowledge, that first proposed that they were molecular parasites.
Östergren does consider that they could be beneficial but then proposes what most would assume to be is favourable alternative:
"Selection brings about an increase in frequency of favourable genes and chromosomes. In the case of the normal complement the chromosomes cannot have a positive selection value except by being favourable to the individuals carrying them. The differential behaviour of the extra fragments, however, makes their 'response' to selection quite different from that of the normal complement. If there occurred a constitutional variation in their mechanism of differential behaviour (their accumulation mechanisms) caused by structural or mutational changes within them, selection would accumulate such fragments as have the most efficient power of spreading, even if they were quite neutral in effect on the plant or even unfavourable....The decisive point in the selection of fragments with a differential behaviour is how favourable their properties are for their own existence rather than how favourable they are to the plant."
And there we have the first articulation of the selfish DNA hypothesis, over thirty years before Dawkins published The Selfish Gene. It's amazing what you can find in some of the older literature. I myself was rather surprised at some of the things Östergren articulates in this paper. He was really ahead of his time. Or we're behind in ours :P
He touches on the seemingly inevitable Red Queen molecular arms race that must occur between selfish DNA and its host:
"The relation between such extra fragments and their carrying plants obviously bears a strong resemblance to the relation between a parasite and its host. Selection will favour such changes in the parasite as give it an increased power of spreading from host to host. On the other hand, selection will also favour such changes in the host as enable it to get rid of the parasite. A similar antagonism...should be expected between parasitic fragment chromosomes and the plants carrying them."
He also makes the crucial parallel between parasitic organisms and selfish DNA which I think is one of the most important things to keep in mind when thinking about any selfish genetic elements. To me, so many bad interpretations have arisen because people tend to forget things like transposable elements are, to a certain extent, separate evolutionary entities from their host.
Östergren mentions one final thing that was once again ahead of his time but I think I'm going to save that for a future non-Selfish DNA post.
Östergren, G. 1945. Parasitic nature of extra fragment chromosomes. Botaniska Notiser 2: 157-163.