Plants, polyploidy and producing new species

Modern wheat. A product of polyploidy.
CC Wheat field / Weizenfeld II | Christian Schnettelker

When I talk about my career and my interest in evolutionary biology, I often get asked, “How do you actually get new species?”. It’s not a stupid question; for people without a background in biology it really is very hard to imagine how the diversity of life we see today has formed from the types of ancient creatures we find in the fossil record. I normally look to my favourite fish, the Mexican blind cavefish, or point out the variety that can be produced in the single species of dog, or mention horizontal gene transfer to confer antibacterial resistance in bacteria, to show how even small changes can result in quite big differences in a species. Add to that vast amounts of time and it becomes a little easier to imagine the “hedge” of life taking shape.

But this is only part of the story. I usually forget what I learnt (a long time ago now) about plant speciation. I forget the fact that huge explosions in plant diversity and speciation appear to have coincided with ancient genome duplications, or polyploidy. (Plants often get the rough end of the stick in biology; I’m as guilty as most for thinking that Plant Sciences was boring at university and didn’t pay much attention – shame on me). Polyploidy is an important mechanism for the generation of new species, speciation, and has been vitally important in the production of viable new crop species (e.g. wheat) during human history.

Usually polyploidy occurs between two closely related species whose gametes fuse and produce new baby plants, which show the characteristics of the parent plants. This is like normal sexual reproduction, where sperm + egg = embryo, just that in (allo)polyploidy the parent plants can be of different species. Imagine that!

However, according to a new piece of research this month, polyploidy can also occur by an asexual mechanism, meaning that genomes can be transferred from plant to plant without the fusion of gametes. This is a bit like the example of horizontal gene transfer in bacteria, only this time it’s horizontal genome transfer. Horizontal genome transfer, like regular polyploidy, imparts tremendous capacity to the new generation to diversify, allowing them to take on new niches and environments, rapidly becoming reproductively isolated from the parent plants and becoming new species.

Fuentes et al. show this rather simply and elegantly, by first grafting plants of the same species as a proof of concept, before performing grafts between different plant species. The first experiment verified that polyploidy occurs using a simple antibiotic selection experiment: to do this they use transgenic parents which have been given one of two genes that provides resistance to a certain antibiotic. Each parent is only resistant to a single antibiotic; neither parent can grow when both antibiotics are present. When Fuentes et al. put both antibiotics in the plant growth medium, only the cells which have undergone horizontal genome transfer, and therefore now have both antibiotic resistance genes, survive to grow. After being satisfied with this result, they move on to grafting parent plants of different species and examining the appearance of the offspring. This second experiment confirms that, despite there being no sexual reproduction, new types of plants are indeed created. Fuentes et al. go as far as to give the resulting plant a new species name, Nicotiana tabauca and illustrate how its inherited some of the characteristics of both parents. And like that they demonstrate a novel process of generating new species. (The mechanism by which the genomes are transferred is not fully answered in the paper, though their strongest evidence suggests the nuclei migrate from one cell to another at the graft site, joining the two genomes in a single cell containing one set of parental organelles.)

The paper was published in Nature, one of the ‘top’ journals (though unfortunately not fully Open Access) and goes to show that a good idea, well executed, and with some well designed experiments, you can get very important results without too much highly complicated research.

Featured research

Fuentes et al., 2014, Horizontal genome transfer as an asexual path to the formation of new species, Nature.

Fuentes et al. 2014. Horizontal genome transfer as an asexual path to the formation of new species
Fuentes et al. 2014. Nature

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