[Aroid-l] What is Aneuploidy?

From: pjm at gol.com (Peter Matthews) on 2008.01.14 at 03:19:41(16950) Dear Ted, >1. How reliable are chromosome counts, statistically? How many cells do >you need to count before you are satisfied that you have the right number? Some kinds of plants are more stable than others, but many plants (like Colocasia eesculenta - taro) are very stable - i.e. one clear count on one cell from one tip is likely to be the full story. Ferns with hundreds of tiny similar-looking chromosomes are hard to count accurately, and may also be more unstable in terms of number. >2. Is it possible to have a nuisance cell or two that show a different >count from others from the same plant? I mean, what do you do if you >count, say 20 cells with 2n = 40 and one with 2n = 42? Count more cells, confirm, and report! In my experience, this sort of thing is a reflection of poor preparations with spilled chromosomes and difficult, inaccurate counting. If the counts are all clear and accurate, and the unusual number is real, then there could be some interest in counting many more cells to establish a frequency of occurrence, and measure the instability in cell divisions. +More Another way to get at this is to look at chromosome spreads from hundreds of seedlings in an experimental breeding cross, to learn what the natural frequency of aneupleoidy or polyploidy is. Such insanely eye-straining work has actually been done for some plants (including taro, Colocasia esculenta - by a Japanese research group). In the case of taro, it was possible to use this approach experimentally to confirm very low rates of polyploid formation by the rare production of unreduced gametes. I do not remember if aneuploids were also found. >3. How easy is it to break a chromosome in the process of performing the >preparation and root tip crush? It is easier to break the cell membranes, and spill chromosomes across the field of view on a slide - sometimes with the result that it is unclear which cell the chromosome comes from. The individual, fully-condensed chromosomes at metaphase are very resilient in the cell solution, and under the pressures applied a slide preparation, so they do not break. Metaphase chromosomes are what we target, usually, when making chromosome counts. Certain chemicals can be used to arrest cells at this stage in cell division, in order to increase the chance of being able to make a good count. Best regards, Peter Matthews (Kyoto) ****

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