Plants with 3 parents
When we reproduce, a single sperm is permitted to fertilise a single egg, bringing together two half-sets of chromosomes to produce a cell containing the full 23 pairs. And if anything goes wrong with this process - such as two sperm fertilising the same egg and contributing too many chromosomes - the result is usually lethal. But plants, on the other hand, seem to be able to tolerate this quite well and may even rely on the process to drive their evolution. When a flower is pollinated, the male pollen grows down the flower stigma towards the egg. When it gets there, it contributes DNA to both the nourishing tissue that surrounds the egg, and the egg itself. The nourishing tissue inspects the DNA and can abort the fertilisation if something is wrong. But, rarely, two pollen tubes, from two separate fathers, can try to fertilise simultaneously. When this happens, the second one doesn’t need to interact with the nourishing tissue and can directly add DNA to the egg, producing a plant with 3 parents. This has significant implications for plant breeding. As she explains to Chris Smith, Rita Gross-Hardt proved how it happens in an ingenious series of experiments...
Rita - Because we were expecting a really rare event, we thought it would be good to have something which is easy for us to score. And the best case scenario, we thought, is that all the seedlings which have just one father dropped dead and only the ones which have two fathers survive. So we decided to use a gene which confers a herbicide resistance. And we kind of "separated" this gene. We put one part into father one, and we took the other part into father two. The rationale behind this is that, if only one sperm from one father is inherited, the gene is not complete and is not expressed, so the plants are herbicide sensitive. By contrast, if an egg inherits the sperm from the two different fathers, the gene would be complemented and the tri-parental offspring would be herbicide resistant.
Chris - And what fraction, when you did this actually survived, in other words, had inherited both halves of the gene, one from each dad and therefore reconstituted a functional gene in the offspring?
Rita - Yeah, that is actually quite fascinating. We had to look at 10,000 seedlings to find one which is herbicide resistant.
Chris - And did you check, when you looked at the seedlings that appear to be resistant and therefore would appear to have three parents, did you go in and genetically confirm that was definitely the case?
Rita - Yes, we felt that was a very important step because, just getting a herbicide resistant plant did not feel comfortable for making such a strong claim that plants can have three parents. So we, in addition, checked whether we could find back the two halves of the gene, and then we also said that we would assume that such plants would have additional chromosomes. So a typical arabidopsis plant gets five chromosomes from the mother and five chromosomes from the father. What we found in these herbicide resistant plants was 15 chromosomes. So that fitted quite well to the idea that actually a second father had contributed its entire genome.
Chris - How does this actually happen though? What's the mechanism?
Rita - Actually there was a remarkable difference between what the first inheritance step was from father one, and the fusion that we observed with father two. The first father delivered its sperm not only to the egg cell but also to the adjacent nourishing tissue. This is normal and quite characteristic for flowering plants. Importantly, this nourishing tissue has a DNA checkpoint. So the first father's genetic material went through this genetic checkpoint. However, to our surprise, in many cases, genetic material from father two was only delivered to the egg cell but not to the nourishing tissue, thereby bypassing this DNA checkpoint.
Chris - Given that you've shown that this can happen, albeit with a lowish frequency, this must therefore be an important way in which plants can share genes, trade genes, gain additional genes from multiple parents. It must be, have been going on in evolutionary terms like this and have played quite an important role in the plants that we see around us today?
Rita - Yes, that is an absolutely correct point. So, during evolution, the increase in chromosome numbers is assumed to be really having played a major role in speciation. Why is that? Well, having additional chromosomes and duplications of chromosomes is kind of like a playground for evolution, so these genes can either be newly functionalised, they can be maintained in their function, or they can be deleted.
Chris - Why doesn't this totally screw up the ability of the plants to reproduce themselves though? Why don't they end up with what we call aneuploidy - the wrong numbers of chromosomes in their own gametes, their own eggs and sperm - when they want to reproduce subsequently?
Rita - Yeah, that's a very good question and I wish I could give you a good answer on that. The point is indeed, we have plants which have now three sets of chromosomes. That implies that, when it comes to the formation of germ cells, that these three sets need to be distributed. This can occur in an even fashion, but typically it occurs in an uneven fashion. Saying that, for example, chromosome number one is represented by one copy in a germ cell, whereas chromosome number two is represented by two copies. And this gives different ratios for gene products and that can cause turmoil and developmental problems. However, we saw that some 40% of these plants which had three chromosome sets were able to give rise to fertile offspring.
Chris - Was that just by chance do you think; just by luck they happen to select just one copy of everything in the one particular gamete that was successful, and so that's the one you saw, or do you think there's a mechanism at play here - nature has made provision for the fact that it's allowing this extra set of chromosomes to sneak through from time to time, so it's got a mechanism to deal with it when it happens?
Rita - There has been speculations, for example, that shutting down chromosome copies might do the job here, but, to my knowledge, this is not a clarified how plants actually manage to tolerate these extra chromosomes in comparison to animals, which are highly intolerant for this scenario.
Chris - Now, what are the implications of this? We've touched on why evolution might want to live with this because the benefits are obvious. Is there any implication in terms of where we see ourselves going from a plant breeding perspective?
Rita - Yes. We think that this has implications for plant because the combination of beneficial traits from three parents in i think a cross can speed up breeding processes. That is one aspect. A second aspect which we consider important is that plant breeders often have the problem that they cannot combine plants because they are not related enough, and this is actually detected in the nourishing tissue, which checks for the DNA quality and quantity. And then if the test is not past, the seed, simply aborts. If we now have a means that the second father is not going through this quality check, then there would be maybe an option to combine plants which we could previously not combine in such three parent crosses.