There is a group of what I call “postmodern evolutionists” who loudly claim that, for one reason or another, the modern synthetic theory of evolution has to be thrown out the window. Epistasis, modularity, adaptive mutation, genetic assimilation—these are the buzzwords that will supposedly dismember modern Darwinism. On closer examination, though, one always finds that the theory remains pretty much intact: that these phenomena, while interesting, hardly constitute a Kuhnian revolution in evolutionary thought.
One of the main claims of postmodern biology is that “horizontal gene transfer” (HGT), the movement of genes between distantly related species via ingestion, viral vectors, and so on, can not only serve as a previously unknown source of genetic variation, but will also, by blurring the true pattern of ancestry and descent, efface the tree of life. The former claim is correct. The latter claim, while probably wrong, is still quite popular, as evidenced by the cover of New Scientist last February:
Why was Darwin “wrong”? As I noted at the time, New Scientist claimed that HGT is so pervasive among species that “the tree of life is not a fact of nature.” New Scientist’s claim was palpably wrong, since species do have ancestors, whether or not we can recover that ancestry, and their evolutionary history, via gene sequencing. Too, there’s no evidence that HGT is pervasive in nature. It’s fairly common in bacteria, and has been seen in a few multicellular species, including rotifers and even Drosophila, but it’s so rare in metazoans that in those groups it hardly constitutes a challenge to the tree of life.
HGT has now been found in fungi, previously thought to be largely immune to the phenomenon since fungi can’t digest entire cells—usually the way genes are passed among species. The new issue of Current Biology has a dispatch on the topic by Thomas H. Richards, “Genome evolution: horizontal movements in the fungi“, summarizing recent work in that group, including a nice new paper in the journal by Slot and Rokas (reference below).
As Richards notes, HGT is usually detected by reconstructing evolutionary trees using different genes. Most of the trees will coincide, but occasionally the use of one gene gives a tree that’s wildly disparate, showing “relatedness” of species that we know from other data are pretty unrelated. That’s a clue that that gene has moved horizontally among the unrelated species, giving a false signal of close evolutionary ancestry.
Using this technique, Slot and Rokas found that an entire cluster of 23 genes, those involved in the metabolic pathway for making sterigmatocystin (a toxic compound that is a precursor for the deadly aflatoxins), had been horizontally transferred from Aspergillis to the distantly related Podospora. That’s a lot of genes, and since genes involved in the same pathway are often physically linked on the DNA of fungi, there’s a potential for widespread transfer of entire metabolic pathways between species (metabolic genes appear to be horizontally transferred among all species much more often than “informational” genes involved in DNA replication and transcription).
How did this happen? Well, Aspergillis and Podospora often occupy the same niche: both are “saprotrophs“, or species that break down dead animals and plants. Living cheek by jowl, some of the DNA of an Aspergillis could have been ingested by a Podospora in the stew of organic matter, and incorporated into its genome. But we don’t know exactly how this happened.
Richards gives a nice diagram summarizing cases of HGT among fungal species. HGT events are shown as colored arrows superimposed on the family tree of species determined from sequencing other genes. (The Slot and Rokas finding is the blue arrow.) Note that transfers often involve species that are quite distantly related (click to enlarge):
But Richards hastens to add that this transfer is not common enough to efface the fungal tree of life, which can easily be discerned by concordant patterns of ancestry among the many non-transferred genes:
The inventory of HGTs identified represents a relatively small fraction of any of the fungal genomes studied. Consequently, the scale of transfer is unlikely to prevent the accurate resolution of a fungal phylogeny, as some have suggested to be the case for the prokaryote phylogeny , but will provide an interesting source of future research as investigators compare trait evolution with the fungal species phylogeny.
Even with these fungi, then, Darwin wasn’t really “wrong,” But what is exciting is the manifestly non-orthodox idea that adaptive genetic change can involve not just mutations within a species, but the wholesale movement of genes between different species—often very different ones. As Richards notes, fungi have acquired genes not just from other fungi, but from bacteria and even plants.
Look for more examples of bizarre and wide HGT as DNA sequencing becomes more common. But don’t expect it to demonstrate that there’s no tree of life!
Richards, Thomas A. Genome evolution: horizontal movements in the fungi. Current Biology 21:R166-R167.
Slot, J.C., and Rokas, A. 2011. Horizontal transfer of a large and highly toxic secondary metabolic gene cluster between fungi. Curr. Biol. 21, 134–139.