RobLaffin said:Thanks very much, Maurice, for taking the time to lay all that out.
You are quite welcome.
I have to assume that FCM must offer some advantage of traditional chromosome counting - easier or faster?
Yes.
If one had a plant and was unsure of its ploidy and wanted to know, then a FCM test might indicate ploidy, but the reliability of that determination would be subject to question unless the tester A) followed the specific protocols you quoted above, or B) confirmed through chromosome counting. Is that fair?
Even if the tester followed all the protocols and was an expert at FCM there would still need to be a chromosome count. FCM is not a field that I am competent in but my understanding is that it is not yet very reliable in detecting differences in small numbers of chromosomes (or small chromosome size differences).
So a tetraploid daylily might have 45 (or 46, or 47, etc.) chromosomes and FCM would be unable to identify it as abnormal. It is also possible for two plants to have the same amount of DNA but different numbers of chromosomes or have the same number of chromosomes but substantially different amounts of DNA.
If tet conversions have some tendency to throw odd ploidys, that would provide at least one explanation for the fertility problems users of conversions sometimes report. I assume these problems be avoided by waiting for the next generation?
Daylilies are naturally diploid and so the mechanisms for producing gametes, pollen and egg are able to divide the chromosomes evenly into the gametes successfully at a very high percentage of the time. Then tetraploids were created. With four chromosomes the methods that work well with a pair become less effective. A pair of chromosomes can align with each other end to end and can be evenly divided into two new equally balanced daughter sets [diploids have 11 pairs of chromosomes, each pair is different, each gamete must get one copy of each pair to total 11 chromosomes so that when the two gametes join the embryo once again has 11 pairs or 22 chromosomes].
When chromosomes align (or try to align) in tetraploids there can be a mess. They may align as a quadruplet. They may align as a triplet and a single. They may align as two pairs. There are eleven sets that have to align. Each set can align in a different way. If all 11 sets aligned as two pairs then tetraploids would be equally as fertile as diploids (more or less) and there would be no problems as long as they always aligned in that pattern. But they don't and there is nothing to make them do so in new tetraploids. So what happens is that there are gametes that should have had 22 chromosomes that have 21 or 23 or 20 or 24, etc. The further from a balanced count of 22 the less viable the gamete. The more gametes with non-22 counts the less fertile the daylily.
New conversions would have the most fertility problems because of the above (but also because many are not complete tetraploids and may have large amounts of diploid tissue still where it counts in the cells that make gametes).
With the passage of generations, as long as hybridizers select for better fertility, the number of tetraploid daylilies that form gametes with perfect sets of 22 in high proportions (and those proportions) will increase (although it may take the millions of generations that natural tetraploid species have had for fertility to be approximately normal).
