The Potential and Evolutionary Fireworks of Cross-Kingdom Genetic Transfers.

By Dave Armstrong - 16 Apr 2016 11:20:0 GMT
The Potential and Evolutionary Fireworks of Cross-Kingdom Genetic Transfers.

Deep in the mists of time, something bit a conifer and transferred arthropod DNA into plant DNA. This story will produce new research into how our genomes are full of material that can greatly affect evolution without the involvement of any previously known mutational mechanism. Pine image; Credit: © Gannet Halla

Not on the surface a mind-boggling event, but the discovery of plant genes in animals and vice versa is potentially mind-blowing.

This type of transfer of information without sexual reproduction is known in animals and plants as horizontal transfer (HT) and can involve transposons or transposable elements (TE), similar to virus infections, that fill up our chromosomes with many unnecessary repeats and useless information. Ancient species especially seem prone to millions of this kind of HT, in many Angiosperms, for example.

While viruses infect species, the TE are more like gene parasites. They can spread into new genomes but are rare. The main problem is that they persist for millions of years, so don’t wait up for them! Their major success story is in the TE that became our retroviruses (like HIV) many generations ago. The authors of a paper Genome Biology and Evolution last week revealed how we could retain TE over millions of years, even from plant material. Xuan Lin , Nurul Faridi and Claudio Casola from Texas A&M University and the USDA Forest Service in Mississippi, USA, perform the analyses, Bayesian and otherwise!

TE were past infections in mammals in some cases, now outmoded and simply replicating as about 8% of the human genome. However, 340 million years ago, some pine (such as the American Loblolly pine) and spruce DNA received (plant-eating) insect DNA from an ancestor. Other gymnosperms have no TE but these conifers have gigantic amounts of DNA in their recently studied genomes with half made up of TE. Such animal DNA is so significant that they have been called Dryads, after mythical Greek tree nymphs. The Greek names spread to the use of Penelope in the paper title, An Ancient Trans-Kingdom Horizontal Transfer of Penelope-like Retroelements from Arthropods to Conifers.

It has to be said that Penelope like elements (PLE) was a genetic phrase invented for these TE, found in Drosophila (fruit flies) and shrimps. Their interest lies in the heavy hand they play during many different classes of mutation. They could be one of the major influences on our evolution and diversity.

How the TE influence modern trees is the question. DNA templates make TE RNA that can readily synthesise more DNA, explaining the large amounts of TE in the conifer genomes. In animals, theses retroelements are known as I can’t help comparing the situation to useless information that replicates on the internet! The related TE Dryads and PLE can currently be said to influence arthropod genomes as well as vertebrates such as the lamprey and Anolis lizards. The name we may have to remember as this line of research continues is at present called the CA (Conifers + Arthropods) clade, with the obvious inclusion of vertebrates and possibly sea-urchins (specifically, Strongylocentrotus purpuratus) yet to be made.

What can be said about the distribution of TE? They are widespread in plants animals, fungi and protists, with many transfers taking place over the past few million years. More ancient transfers were rare or have been lost, probably through decay of the TE sequences. In similar fashion, most transkingdom transfers have also probably been lost, apart from this significant one. As an example, none have been yet discovered in humans or domestic animals, where genome research has been very active in for example sheep and goats.

Arthropods gave this present of TE to an ancestral conifer 340 mya. Animals themselves host a couple of more ancient TE, while there do exist thousands (1928 genomes) of species that apparently have no similar TE. Human DNA can contaminate other animal species, so no doubt they can also have added to our 46 chromosomes!