Having very recently tweeted that “Life, we have discovered, is modular, with functions bolted on to existing ones” I stumble on confirmation of just how this process works in a paper on horizontal gene transfer (HGT) in Science magazine. HGT is one way in which large chunks of genetic material are incorporated into a different organism. In the Science paper the organism is the flowering shrub Amborella trichopoda, believed to be one of the earliest flowering plants.
Amborella is found only on the Polynesian archipelago of New Caledonia. Its genome was sequenced in an attempt to understand the evolution of flowering plants. Amborella, it seems, has practised HGT on an enormous scale. The HGT concerns its mitochondria, the energy producing organelles in every cell of multicellular organisms. Of course, this arrangement itself, which gave rise to all multicellular creatures, was a result of the most important act of HGT ever: the engulfment of the proto-mitochondria by other cells, where they eventually settled down to their role as universal energy providers.
The mitochondria of Amborella show multiple transfers of genetic material from a variety of algae, mosses, and other flowering plants. In some case, whole mitochondrial genomes have been engulfed. In this case, the foreign genetic material seems to be inactive, with no function, but in some crucial cases HGT has provided important new functions, the most striking being the syncytin gene active in the human placenta, which derives from a retrovirus gene.
How did Amborella acquire its hitchhikers? Perhaps in the simplest way. Algae and mosses often festoon the plants. It is thought that genetic material is incorporated when the plants sustain wounds.
Science, 2013, Vol 342, pp. 1468-73