An explanation of ploidy

by Robert G. Halgren, PhD
(with a few phal comments by Howard S. Ginsberg)

Humans happen to have 22 pairs of chromosomes and a weirdo, XY or XX, which determines the sex of the human. Plants can have a wide range of chromosome numbers. Through the miracle of meiosis (a word to use in crossword puzzles meaning cell division), gametes have half of the chromosomes of the parent type (i.e. one of each kind). One would think that that would be called "ploid", but instead, it is "haploid" (half of the normal ploidy), or 1N. In the normal case, two gametes combine to form a zygote, which has, once again, a diploid (2N) set of chromosomes. Polyploid is just a fancy word for any multiple of the normal (diploid) ploidy. So 3N, 4N, 5N, 6N, etc. are all polyploid. Note that 3N is 1.5 times 2N, in other words, the multiplier can be fractional.

So, as Steve mentioned, in plants we can screw things up royally with chemicals (such as colchicine) and create tetraploid cells (tetra = four, or 4N), or octaploid cells (octo = eight, or 8N), etc. In animals, this doesn’t happen much in a normal case. I think fish can be polyploid. I’m certain mammals can’t be.

So far we have discussed even ploidy, since we can only double the chromosome number in a plant. So, diploid (normal), tetraploid (2x diploid), and octoploid (2x tetraploid) are possible.

What happens when we cross a tetraploid plant to a diploid plant? Each parent gives one half of their chromosomes in the gametes, which combine to form a zygote. 2+1 = 3, so the progeny are 3N, or triploid. So far so good, but what happens if we want to breed with a triploid? Half of three chromosomes is 1.5, that number doesn’t work too well! Triploids are generally not particularly good breeders because they are not particularly fertile. To complete the cycle, you can double the ploidy of a triploid with chemicals to make a hexaploid (6N). This is fertile, as half of 6 is three, a nice round number.

All this screwing around with chromosome number does have limits.... A cell can only contain so much DNA. So don’t go looking around for 20N plants any time soon. Also, crossing between species or genera of orchids isn’t exactly trivial. In paphs, for example, different species have different chromosome numbers. Which is why primary hybrids are easy to make, but these hybrids don’t always breed well (perhaps the plant is 2.2N or 1.9N - not ideal for dividing in half and getting a nice even number!).  Phals, on the other hand, generally all have the same number of chromosomes but there are apparently other problems in breeding species (and hybrids) with large chromosomes to those with small chromosomes.

Why make polyploids at all? Well, in orchids at least, natural polyploids can occur. Mistakes happen sometimes. People have been selecting these out of the "el grande plant progeny pool" for a long time. A polyploid can be (but isn’t always), larger flowered, more intensely colored, etc. Paph. Maudiae "The Queen" (perhaps the best Maudiae ever) is widely assumed to be a triploid. However, it isn’t particularly (if at all) fertile. It has more copies of the genes that make things good (also more of the ones that aren’t so good...). Often this works out spectacularly well. Sometimes you get a crippled, mutated mess. So, since we have seen over the years that polyploid plants can be superior, it is a natural instinct to use the technology we have developed and chemically alter the chromosome counts.

Anyway, I hope this helps. If nothing else, when you are buying plants, and see a lovely tetraploid (4N) plant crossed with an equally lovely diploid (2N) plant, don’t buy that flask with the hopes of getting the world’s best breeding stock. We now know that the progeny should all be triploid (3N), which may be a glorious flower, but not a particularly good breeder.