Some of those critics who see neo-Darwinism as grossly insufficient assert that lots of adaptations result not from mutations that arise within a species, as evolutionary biology posits, but from “lateral gene transfer” (LGT) the capture of genes from one species by a different one. This transfer can occur by either hybridization (mating with a different species) or assimilation (eating or absorbing foreign DNA).
This attack on neo-Darwinism is misguided on two counts. First, neo-Darwinism doesn’t really require that genetic variation originate by mutation in the species in which that variation is selected. The dynamics of natural selection will work on adaptive genetic variation no matter what its source. Second, while bacteria often acquire genes from different species (much of antibiotic resistance, for example, is carried on plasmids—small circular pieces of DNA—that one species acquires from another), there are only a handful of cases in which nonbacterial “eukaryotes” (animals, plants, and fungi) get genes from a different species (Wikipedia has a nice summary of these cases).
One reason we think that LGT is fairly rare in eukaryotes is because we’d detect it by making DNA based phylogenies. Genes captured from a different species, especially one that is quite different, would stick out of these phylogenies like sore thumbs.
Indeed, that is the method used by two researchers in a really nice demonstration of LGT between fungi and aphids, reported by Nancy Moran and Tyler Jarvik in this week’s Science.
Moran and Jarvik were studying a color polymorphism in the pea aphid (Acyrthrosiphon pisum): some individuals are red, and others are green (Fig. 1). This polymorphism behaves as if it were controlled by a single gene, with red color dominant to green. Assays of differently colored aphids showed that they contain different kinds of the pigment carotene: green aphids have three types of carotenes, while red ones have those three and two additional ones.
Fig. 1. Clones of red and green aphids (from Moran and Jarvik).
Further, the polymorphism is thought to be maintained by natural selection: ladybugs preferentially pick off the red aphids, while green aphids are more often destroyed by parasitoid wasps. This may cause frequency-dependent selection, in which the color morphs are kept polymorphic because the rarer forms are eaten/parasitized less frequently. This kind of selection can, theoretically, maintain both genes—and colors—in the population.
This puzzled Moran and Jarvik, because all carotenes in animals have been thought to come solely from diet, since no animal species is known to make the pigments with its own metabolic machinery. (In WEIT, I discuss how male house finches become more attractive mates if they have redder feathers; the red pigment derives from carotenoids in the seeds that the finches eat, and is a sign to a female of a healthy, well-fed male who would be a better father [see photos below].) But it’s unlikely that aphids get carotenoids from plant sap (the aphids’ food), because those pigments are not soluble in sap, and the carotenoid polymorphism appears, as I said, to act as if it were produced by genes in the aphids themselves. Indeed, DNA sequencing—the aphid genome was just sequenced completely—revealed that aphids do indeed carry carotenoid-synthesizing genes in their genome. There were seven of them, coding for both carotenois desaturases and carotenoid synthases.
This led Moran and Jarvik to hypothesize that somehow the aphids acquired genes for making carotenoids from another species, presumably bacteria. They thus compared the aphid carotenoid genes to those from other species in the genome databank. And they got a surprise. Yes, the aphid genes did come from another species, but not a bacterial one. They were closely related, instead, to genes from fungi.
Figure 2 gives a phylogenetic tree of the carotenoid desaturases, and shows clearly that the aphid genes nest within the group of desaturases from fungi. This tree (and the tree for synthases as well) also show that all seven of the aphid genes were acquired from fungi in a single capture event between 80 and 30 million years ago. We don’t know how this happened, but it’s possible that an ancestral aphid infected with a fungal disease captured some of the fungus DNA.
Moran and Jarvik also showed that the red-versus-green polymorphism is based on a mutation that presumably happened after the genes were captured: green aphids derive from a “mutation” in one of the carotenoid desaturase genes, a mutation that deleted about 30,000 base pairs of the DNA. Presumably the red color was ancestral, and the green resulted from an error in DNA replication. (Moran and Jarvik also studied a mutation from red to green that spontaneously arose in the lab, and found that the new green form was based on a single amino-acid change, from glutamic acid to lysine, in the same carotenoid desaturase gene.)
This is a remarkable use of an acquired gene in an adaptive way, for the captured fungus DNA is the basis for the color polymorphism presumably maintained by natural selection. It’s clear, then, that evolution in one species can be based on the acquisition of genetic information from a distantly related species. Now that different species’ genomes can be sequenced quickly and reasonably cheaply, we’re bound to find more cases like this. I don’t think they’ll be that common, simply because we don’t see evidence of LGT from existing gene trees in animals and plants. Nevertheless, Moran and Jarvik have shown that nature still has the capacity to surprise us. And a good thing, too, because it makes our jobs as evolutionary biologists even more interesting.
Fig. 3. Male house finches (Carpodacus mexicanus) showing color variation due to diet. Finch at bottom has had a lot more carotenoids. Photos from Project FeederWatch.
Moran, N. A. and T. Jarvik. 2010. Lateral transfer of genes from fungi underlies carotenoid production in aphids. Science 328:624-627