Roosterlorn said:Auratum. I have a question. What percentage of conversion to 4n qualifies a plant to be called or defined as a tetraploid? Is there ever anything like 100% 4n conversion?. I suspect not because all the test results I've seen, whether by flow cytometry or leaf stomate count, the test results are always substantially less than 100%. Such as in a typical cyto analysis: tetraploid= 78%, triploid= 12%, diploid=10%.
Also, in you thinking, do you believe these numbers change over time as a plant ages and if so, might this have a similar affect as what's called clonal drift?
Lorn – great questions! For me if it shows the traits of a tetraploid (stomata, pollen, leaves, flowers, etc) and breeds like a tetraploid then it is a tetraploid. The % thing is just confusing – let me explain more on where I think the % come from.
We need to be cautious as we try to interpret the data from Flow Cytometry analysis. This testing looks at relative amount of genetic material. I like the carrot analogy where we are trying to figure out how many carrots went into a certain amount of shredded carrots or carrot juice. If you look at the printout from a flow cytometry analysis, it looks like a bell curve (histogram). To really interpret the data, it is best to see the histogram. The tables I have seen published listing % for 2N, 3N, & 4N are intending to represent the data from the histogram but they need to be interpreted carefully. The histogram is a graphical display of the count of cells that are picked up by the detector on a given channel. The machine is supposed to be set-up with the 1N channel number for the reference material so if the 1N (haploid) for the reference hits channel 40, then diploid nuclei should center around channel 80 (double the channel number but some hits fall slightly lower and slightly higher) from this number. Triploids would hit centered on channel 120 and tetraploids would hit centered on 160. There is always some variation in the data and one thing they look at is the coefficient of variation (or spread) for a peak around a given channel. I have only seen a few histograms but there is also a noise level or baseline number of hits across all channels. It is possible to look at where a 3N peak should be and see nothing but baseline/noise signal hits and report the baseline in this area as a % using the standard coefficient of variation. Also, due to cell division there are likely some cells that are going through various stages of cell division so it would be expected on a 2N plant to see a small peak around 4N as some cells would have doubled their chromosomes prior to dividing and that would get picked up on the analysis. The bottom line is the data needs to be interpreted properly with appropriate reference material to draw correct conclusions.
There are a couple of articles in the 1990 NALS yearbook that go along with this discussion. The first is on doing chromosome counts by inspection under a microscope and the other is on flow cytometry and how it works. It is a good read on this topic and likely better explained than what I have done here. Also did some searching and found an article that may help show with a picture the different techniques - http://www.academicjournals.or...
. Here is a picture from the article showing comparison of different cultivars and what the stomata, flow cytometry, & microscope count look like:
As for things changing over time, there is very little solid evidence to this for lilium. Can it happen – yes. Does it happen – maybe. How much does it happen – I don’t know. If you look at plants like hosta – it is a big deal and happens all the time but for lilies, they seem much more stable. The concept of clonal drift comes from changes in the plant over time due to errors or mutations in the cell division process. In most cases those errors don’t result in chromosome counts but more subtle things like flower color or leaf color. The only mutation I have heard of for this is with Judith’s Scheherazade producing Silver Scheherazade.
Australis said:My understanding was that ploidies above 4N (tetraploid) weren't stable and the plants don't normally survive.
Apologies for not being clear, @Protoavis. I understood Liliums with ploidy > 4N don't survive. I know other genera can survive at higher ploidies - Cymbidiums, for example, will survive up to 6N (8N just dies in the flask), although 4N seems to be optimal for them (and 2N is their natural state).
Australis - yes – ploidy levels about tetraploid are possible. There are some reports of hexaploids in lilium but they are not common. There are also reports of octoploids in tissue culture from chromosome doubling work but no evidence the plants were grown outside of tissue culture. A more recent research article from the NALS Yearbook talks about pentaploids.
Many people don’t realize the number of polyploids around us. Most of the apple cultivars we eat today are triploids. As already mentioned, bread wheat is a hexaploid and commercially grown strawberries are octoploids.
I had the opportunity to discuss this at length with Bob Griesbach. I had the same question as you – are tetraploids the limit? He shared that he did not feel that tetraploids were the limit but thought it would be possible to develop normal growing hexaploid (6N) lilies. He also shared that he did not think octoploid (8N) lilies would grow normally in the garden. He shared that the fewer number of base chromosomes a species has, the higher the level of polyploidy that would theoretically be possible to grow normally. He gave a very specific example from irises where they were playing with this and has done many conversions to tetraploid and then there was an attempt to further double to octoploid. They successfully produced and octoploid cultivar that was able to grow in the garden but it grew really slowly and was a dwarf when compared to the tetraploids. In lilium the concerns shared with polyploidy are more brittle stems and flower tepals along with difficulty for the flowers to open normally due to the increased substance of the tepals. Also tetraploids grow more slowly than triploids and diploids. The Dutch breeders aim for triploids as they have the balanced benefits of some increase in ploidy (somewhat larger flowers, thicker & more lush foliage) with not too excessive detractors (brittle stems & flowers, slow growth rate) from the higher genetic content.