Wednesday, 19 February 2014

Darwin's Flowers: Unraveling an 'Abominable Mystery'

One of the most famous branches in plant's evolution is the difference between gymnosperms and angiosperms, which puzzled Darwin.

Rob Martienssen: It's difficult for any evolutionary biologist to understand some of the arguments against evolution that are in the public domain. I mean, the evidence for evolution is not only overwhelming, but it drives everything that we do.

Goldschmidt famously -- who is a famous geneticist once said that nothing in biology can be understood except in the light of evolution. And that's true. I think we wouldn't understand as much as we do about the pathways that I described without the evolutionary side of it that makes it all real. Especially in plants. We can rewind the clock of recent evolution by remaking a hybrid, for example, and then seeing how it evolves in the laboratory or in the field and compare that to what we know happened even in recent history. And so evolution is an experimentally tractable topic. I mean, it's very real.

One of the most famous branches in plant's evolution is the difference between gymnosperms and Angiosperms. And let me explain what they are. They're both seed plants so they have seeds, but Angiosperms also have flowers, so whereas gymnosperms have a different sort of reproductive structure or cone. So people will be very familiar with, for example, with cones on pine trees, which are gymnosperms and flowers on, you know, their favorite garden plant. The difference between gymnosperms and angiosperms is very important in evolution because angiosperms were able to somehow radiate into thousands and thousands and thousands of species, whereas, gymnosperms are rather narrow in their species. Also, gymnosperms have very, very large genomes that are largely a component of transposons.

Transposons are pieces of DNA that can move around the genome. And so they, they can move potentially and cause genetic as well as epigenetic changes without having to go through a sexual cross. And most genomes are actually composed mostly of transposons. So, for example, the human genome is about 50 percent transposons and only about five percent genes. The maize genome is even more extreme, it's about 80 percent transposons and only two or three percent genes. So the bulk of the genome is actually prepared, if you like, for epigenetic change. And this is the area that we work on the most looking at the way in which transposon control, and especially epigenetic transposon control, has influenced the phenotype, the properties of plants over the generations.

Darwin famously referred to the radiation of angiosperms, the generation of thousands of species, as an abominable mystery because it didn't fit with his gradual change hypothesis of evolution. And we think that maybe some of the explanation lies in this ability to control transposons. But that's very speculative, but it's an interesting, interesting thought.

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