evolutionary theory, transgenic organisms

Natural Chimeras: Speciation through Horizontal Genome Transfer

A while back I posted about a crackpot scientist who speculates that humans could be hybrids of chimpanzees [sic] and pigs. What a crock! Everyone knows that the barriers to reproduction between such remotely related species are way too high at the chromosomal level for such a hybrid, even were it ever so briefly conceived, to be viable.

Except it turns out plants do just that—and in more ways than one. Not only does sexual hybridization happen between plant species; now it turns out that plants of different species exchange not just genes, but whole genomes, at graft sites, creating new species that are “allopolyploid” without the need for sex.

This was just published in no less than Nature, so it is not heresy. It has received science’s rabbinical kosher stamp.

Did you know grafting was a spontaneous natural process? I didn’t, but it makes perfect sense. If a growing tree trunk can incorporate a fence or an abandoned bicycle, plants could surely merge with each other where their trunks press together or their stems or branches intertwine. (Humans may well have gotten the idea of grafting from observing this natural occurrence.) It turns out they don’t just physically surround one another’s substance, but get promiscuous on the organelle and molecular scales. Like sexual alloploidy in plants, or even more so, this process might enable the formation of new species “from more distantly related progenitor species belonging to different genera or even different tribes.”

Polyploidy is far rarer in animals than in plants, but some evolutionary theorists have hypothesized that it may have played a pivotal role at key points in the evolution of vertebrates. In prokaryotes, horizontal gene transfer, at least, is accepted as a mechanism of speciation, and I found at least one paper that ends with the speculation that eukaryotes, up to and including yours truly, could have diversified in part with the help of this mechanism:

It can therefore be concluded that eukaryotes possess the same capacity and similar mechanisms for effective HGT as prokaryotes do, and laboratory experiments have shown that these mechanisms are functional. Given the instruments and the opportunities, is it possible that they are not being extensively used? It is only 98% true that the genomes of humans and other primates are 98% identical – they are almost 100% identical in almost every gene, and the process of speciation probably consisted of the acquisition of one or several sets of new genes by HGT. To mention just a single example, Alu sequences are very successful transposable elements that entered the ancestral primate germ line ~60 million years ago (by HGT?). They might have played a pivotal role in the speciation of primates. Which are the ‘human-specific’ genes? Are there humanization gene islands? Our prediction is that they will be found.

It’s an impossibly big jump to pigs and apes. Vertebrate animals have much higher barriers to genome-mingling—skins, immune systems, membranes—and now we’re talking about not just different tribes but different orders, a taxonomic gulf that (as far as I know) not even plants are known to cross. And at first glance, no asexual mechanism comparable to grafting exists that could ever bring mammalian genomes into such close contact with each other.

Except possibly . . .

Parasites.

I’m not arguing for porcine ancestry. Just pondering the increasing evidence that no species, and no individual, is an island, and that boundaries—lines drawn in the cytoplasm—are more provisional and permeable than we supposed.

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