Ploidy is a term that you may have heard bandied about in orchid circles. It comes in several flavors — haploid, diploid, triploid, tetraploid (and more). It’s actually a very simple concept, once you understand two things:
- chromosomes
- normal complement (i.e., number of) chromosomes in an organisms
Chromosomes
Some people may have only vague (and bad) memories of high school biology and discussions of chromosomes, DNA, cells, etc. Simply put: a chromosome is a physical grouping of genetic material, i.e., DNA. DNA is composed of four basic units (Adenine, Thymine, Guanine, and Cytosine) that are strung together like beads on a string. The pattern of A’s, C’s, G’s, and T’s contains the actual information of heredity. Genes are organizational units of these A/C/G/Ts that typically tell the cell how to build a certain other type of thing in the cell called a protein. Chromosomes are long strings of connected A/C/G/Ts. If you know where to look in the seemingly random string of A/C/G/Ts, you’ll find patterns of these letters are the genes themselves.
Normal complement of chromosomes
Every organism has what is a standard number of chromosomes. In diploid organisms (e.g., humans, dogs, cats, most orchids), half of these chromosomes come from one parent, and a near-mirror-image half comes from the other parents. In other words, we all have two copies of each chromosome, one from each parent. These copies are not identical, however, but contain slight variations that make up the difference between individuals, and enable everyone (save identical twins) to have their own unique genetic composition.
Ploidy Explained
What if you only have a copy from one parent, not two? Well, then you’re haploid (and not human, since humans cannot be haploid). Many microorganisms exist in either haploid or diploid states. A shorthand for referring to the haploid complement of chromosomes is to write it as “n”. A diploid state is referred to as “2n”.
Now, what if by some quirk of genetics, you end up with one set of chromosomes from one parent,but TWO identical, duplicate sets from the other parent? Then you’re triploid (”3n”).
Now, what if you start off as a diploid, but through some screw-up of the genetic machinery, each set of chromosomes is doubled? Then you’re a tetraploid. (Yep, “4n”).
Plants are surprisingly tolerant of ploidy. Wheat, for example, can become hexaploid (6n)!
Why this matters for orchids
Breeders of orchids often seek to induce tetraploidy at the orchid seed stage, or even earlier for certain genera that can be tissue cultured (i.e., grown up from cells in a laboratory — not the same as flasking!). The idea is the age-old “more is better.” In other words, if you increase the number of genes that determine, say, size or color intensity, the better the flower will turn out.
Does it work?
That’s a subject for another post!