Well, I have decided to start a new award. For years I have been fighting against the tide on the tendency for people doing genomics work to resort to silly adaptationist arguments for observations. The argument goes something like this. We sequenced a genome (or did some type of genomics). We made an observation of something weird being present (take your pick – it could be a gene order or a gene expression pattern or whatever). We conclude that this observation MUST have an adaptive explanation. We have come up one such adaptive explanation. Therefore this explanation must be correct.
Gould and Lewontin railed against this type of thing many years ago and others have since. Just because something is there does not mean it is adaptive (e.g., it could be neutral or detrimental). And even if something is adaptive, just because you can think of an adaptive explanation does not mean your explanation is correct.
And this is so common in genomics I have decided to invent a new word – Adaptationomics. And I am giving out my first award in this to Jack
Warren Werren and colleagues for their recent press release on their new study of lateral transfer in Wolbachia (plus it lets me plug their new study which is pretty ^$%# cool).
Basically, in their study (led by a past colleague of mine from TIGR, the brilliant up and coming Julie Dunning Hotopp) they showed that there have been multiple lateral transfers of DNA from Wolbachia (which are intracellular parasites that can infect germ cells) into invertebrates. Furthermore they showed that that the DNA transfered to the host genome is not completely transient and that in many cases it is passed on to future generations. This is interesting because it is the first report of strong evidence for such “stable” transfers from bacteria into multicellular species. Of course, one could say that this finding is not that surprising given that Wolbachia infect germ cells and given that DNA transfer from organellar genomes to nuclear genomes is quite common. But Wolbachia are not organelles and since it appears that their DNA can readily move into genomes of multicellular species, this opens up a new window into our understanding of gene transfer.
This of course does not mean that the DNA is anything but “junk” in terms of functions in the host genome. And this is where the adaptationomics comes in. One of the press releases associated with the paper has a bit of an outrageous adaptationomics claim that I would like to counter. In response to their finding of a nearly complete Wolbachia genome in the nuclear genome of a fly
Warren Werren says
And Dunning Hotopp in a Nature article says:
The discovery also hints that the bacterial genome must have provided some sort of evolutionary advantage to its host. “You’re talking about a significant portion of its DNA that is now from Wolbachia,” says Julie Dunning Hotopp, a geneticist at the J. Craig Venter Institute in Rockville, Maryland, who led the study. “There has to be some sort of selection to carry around that much extra DNA.”
This notion that the DNA MUST have an beneficial function is pure adaptationomics. Consider the movement of DNA from organellar genomes into the nucleus. Such movement occurs at an incredibly high rate and the DNA seems to be maintained in the host genomes for millions of years. For example, when we were sequencing the Arabidospsis genome, we found at least one if not more whole copies of the mitochondrial genome embedded in the nuclear genome and we concluded this was likely a non adaptive event. That is, the mt DNA was not conferring some advantage on Arabidopsis plants. There is extensive work on what are called “numts” (nuclear mitochondrial DNA) in humans and other species that makes similar conclusions – the mtDNA in the nucleus is basically junk but it is maintained for long periods of time. Sure, occasionally, the DNA confers some selective advantage. But this is a very rare event and one cannot infer that some DNA is advantageous simply because it is present. This is especially the case for eukaryotes which are generally more able than bacteria to maintain DNA that confers little selective benefit.
So I would argue that
WarrenWerren and colleagues appears to have fallen for the adaptationomics trap – they want to believe these Wolbachia DNA inserts confer some selective advantage. But there is no need to assume this simply because the DNA is there.
Anyway – as is the case more and more. Read the paper. Ignore the interviews and the press releases.