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Identifying Ploidy

This is a discussion on Identifying Ploidy within the Breeding & Hybridization forums, part of the Orchid Propagation category; Originally posted by lja I have these questions: How is tetraploidy expressed in an orchid's ...

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  1. #1
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    Default Identifying Ploidy

    Originally posted by lja

    I have these questions: How is tetraploidy expressed in an orchid's phenotype? In other words, is there some way to tell, just by looking at a plant, that it's 4N and not 2N? Do any differences make themselves apparent if a 4N and a 2N of the same grex are placed side by side? And if so, what would those differences look like, typically?
    There is no way to tell whether a plant is 2n, 3n, or 4n, or whatever.

    I have, however, heard that it is generally true that 4n plants grow more slowly, but tend to be larger in plant size, as well as often in bloom size and fragrance, while 3n plants tend to grow rather quickly and largely, with bigger brighter smellier blooms. But triploids are mules.

    I'll start putting together some FAQ files. Including one on ploidy, for those who aren't sure what this question was, not to mention the answer.

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    Default Re: Identifying Ploidy

    Originally posted by Sue
    There is no way to tell whether a plant is 2n, 3n, or 4n, or whatever.

    I have, however, heard that it is generally true that 4n plants grow more slowly, but tend to be larger in plant size, as well as often in bloom size and fragrance, while 3n plants tend to grow rather quickly and largely, with bigger brighter smellier blooms. But triploids are mules.

    I'll start putting together some FAQ files. Including one on ploidy, for those who aren't sure what this question was, not to mention the answer.
    Yes, that is so true the 4N plants are slow growers, larger in size more rigid leaf structures, larger blooms but fewer in numbers than the "normal" ones. Yes, most triploids are sterile.

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    Oh, I forgot to mention. If somebody wants to buy me the microscope required, along with the centerfuge for the seperation of the DNA, I'd be glad to check ploidy for you.

    It would only cost around $2000. What a deal!

    I would bet some labs could do it for you for a reasonable fee, but I don't have anywhere to recommend. Except my house. And that's only if you buy me a really really nice microscope.

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    So, given that, without a microscope to count chromosomes with, if there's no real way to tell if a certain plant has 2 sets of chromosomes, or 3, (or 4, for tetraploidy), there must be no way to breed just any two plants for tetraploidy specifically, and get those bigger blooms, etc... Am I understanding that correctly?

    Let's say you *did* do a count, and crossed two plants that were both 4N. Will their progeny all be 4N, or is there no way to predict that either?

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    Grrr.

    Well, almost all of the progeny of a 4n x 4n crossing will be 4n. Just like almost all of the progeny of a 2n x 2n crossing will be 2n.

    More specifically: what "2n" means is that the cell has a chromosome count being twice that of its sex cells (i.e., the ova and pollen). So, the sex cells go out and meet each other, pair up, and replicate, forming new plants. So we use "2n" because the chromosome counts of normal (2n) plants are always even, becuase the division of these chromosomes in half is what allows for sexual reproduction.

    Every so often, something odd happens.

    Sometimes when the blastocyst is dividing, in the early life of the new plant, it freaks out and produces too many chromosomes. The most common such mutation is when it doubles its chromosome count, producing a tetraploid (4n). This type of mutation can be encouraged by use of colchecine, a mutagenic chemical derived from crocus.

    Tetraploids, in turn, divide their chromosome counts in half when producing their ova and pollen. But, since they have twice the chromosomes, their sex cells are actually diploid (2n) rather than haploid (1n). So, when a 4n breeds with a 4n, you get a 2n pollen cell with a 2n ovum, producing a 4n offspring. Unless random mutation occurs.

    Meanwhile, if you cross a 4n with a 2n, you get a 2n pollen cell with a 1n ovum, producing a triploid (3n) offspring. Which is a mule; i.e., can't breed. Because what do you get when you divide 3n in half? You get a big mess, and at least one chromosome missing, extra, or broken.

    So, all in all, if you cross a tetraploid (4n) with another 4n, you'll get tetraploid offspring. Except very occasionally. . .

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    Grrr.
    LOL!! (No, really: I'm not laughing at your dog.....)

    Thanks for explaining it!

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    I've come across mention of wrinkled leaves (along the longitudinal axis) as a possible indicator of polyploidy. Of course, I think it's more probably due to bad environmental conditions, but I do have this one Maudiae-type vinicolor with slightly wrinkled leaves that I wonder about. It has thick leaves and grows about as slow as a Galapagos tortoise...I sent off the pollen to someone a while ago, and the cross took, so maybe this plant is a 4n?

    Some people also say that polyploid plants generally have thicker and possibly wider leaves, although other people say wider leaves means better form. I says:

    I think I can shake the couch up for about $1.25 in loose change for the "ploidy microscope" fund.

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    This is all very interesting! Thank you!

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    I think I needed a good 2 months of basics to actually finally start to understand all this! but I cannot yet *recite it back*!! LOL! Thanks again!

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