One plus one is better than two: genome doubling in flowering plants

Published on December 1, 2013   36 min

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Hi, I'm Doug Soltis from the University of Florida. And today we're going to talk about polyploidy, or genome doubling, in flowering plants and show that one plus one is better than two.
So let's start with some background. We'll start with definitions: what is genome doubling or polyploidy, define autopolyploids versus allopolyploids, the importance of polyploidy, and then a bit about the formation of polyploids.
After that background, we want to overview some recent discoveries in the field of genome doubling. We'll reconsider some of the traditional tenets of polyploid evolution, looking at the frequency of polyploidy, polyploidy and its role in diversification, the importance of autoployploidy. We'll talk about the multiple origins of polyploids, and then finally, discuss polyploids as genomically dynamic organisms, looking at genomic expression, chromosomal changes, and what we can learn from the study of recently formed polyploid species.
So let's get started. Are you ready?
So let's begin with some definitions as background, and then from there, we'll move quickly into the importance of polyploidy.
As a simple definition, a diploid nucleus contains two copies of each chromosome, but in a polyploid, the nucleus contains three or more copies of each chromosome. Generally, two types of polyploids are recognized, autopolyploids and allopolyploids. Autopolyploids generally form from a single species. You could imagine two diploid populations of the same species. Perhaps they hybridize, chromosome doubling occurs, and you end up with an autopolyploid. In an allopolyploid, now we're dealing with two closely related species, diploid species A and B. Imagine that they hybridize. And here the chromosomes are even color coded so that you can tell them apart. Again, the chromosome number is doubled, and we end up with an allopolyploid with 20 chromosomes in this example.
Polyploidy is of major importance in all eukaryotic organisms. Flowering plants, for example, all are of ancient polyploid origin. Seed plants are of polyploid origin. Perhaps 95% of all ferns may be polyploid. Most of the world's worst weeds are polyploids, and many of our major crops are recent polyploids, including wheat, corn, sugar cane, cotton, potato, coffee, and many more.
So this major economic importance of polyploidy is something to think about the next time you go to the grocery store. Almost every product you see that's plant related is of polyploid origin.
So a topic of great interest that we do not have much time to discuss today is, why are polyploids so successful. Well, this is a case where, oftentimes, size matters. Here we compare some tetraploids that have formed compared to their diploid parents or progenitors, and you can see that in all cases, looking at aboveground biomass, the tetraploids are significantly larger than their diploid progenitors. Now this is not always the case, but it's a good example of the importance of polyploidy.
Now polyploidy is of importance beyond the plant world. It's of very broad systematic importance. For example, all vertebrates are the product of two episodes of polyploidy. The genome of yeast was anciently duplicated. Salmonids are ancient tetraploids, and many other fish are also of ancient polyploid origin.
Some amphibians are of polyploid origin. Polyploidy is very important in shrimp, and also in flatworms.
It's also useful to look at the extent of polyploidy in some lineages. These are several examples here. In the angiosperm genus, Sedum, the stonecrop's chromosome numbers range from 16 to 640. In Saxifraga, another flowering plant, they range from 10 to 200, and in one species, Claytonia virginica, the spring beauty, chromosome numbers range from 12 to 192. But the record is in ferns and the fern relatives. In Ophioglossum, the species have chromosome numbers ranging from 200 to over 1,000,
and here is the present world record as we know it, the highest known chromosome number, a result of polyploidy with a diploid number of 1,262 in a species of Ophioglossum. That's a lot of chromosomes.

One plus one is better than two: genome doubling in flowering plants

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