Question regarding 2n, 3n and 4n classifications

[Kawarthapine]

Sorry for the newbie questions but can someone explain and/or refer me to an explanation of what the 2N, 3N or 4N classifications mean.

Also, is 2N better or worse than 4N in plant breeding?

Does _N affect plant culture or breeding potential?

Thanks,

 

[Kawarthapine]

I found this answer on Orchidtalk:

In normal reproduction, the sex cells are haploid (n) meaning the offspring gets half the set of one parent and half the set of the other.
In plants you can have a full set of those sex cells passed on, they're called diploid (2n) gametes.*
So say you have an ovary full of diploid egg cells (2n) and a regular haploid(n) sperm cell, the result will be offspring with one full set of chromosomes from one parent and half the set from the other is called triploid (3n).* Triploids have usually larger flowers, good growth, number of flowers can be increased but they are usually sterile and cannot be used reliably for breeding. So now if you have an egg cell that is diploid (2n) that encounters a sperm cell that is diploid (2n) you get a tetraploid (4n). Tetraploids produce larger flowers with great shape that last longer, the plant can adapt to a wider range of growing conditions, the plant is overall stronger.

That's kind of a general answer but I am sure you get the idea.

 

[SlipperFan]

That's a good explanation. Thanks!

 

[eteson]

Tetraploid crossed x diploid gives triploid (usually sterile).

I do not agree about the sentence "...(4N are) overall stronger": Some 4N plants are stronger but in my experiments most part of the polyploids are weak growers and some of them very prone to rot. I do think that a big % of the 1st generation polyploids do not reach mature stage. Guess that once selected , the second generation F2 plants are much more stronger.

 

[PaphMadMan]

It isn't really a newbie question, since many experienced orchid growers, even breeders, are a little foggy on what it means. The basics have been answered, but there is always another way of saying something.

Most organisms have 2 matched sets of chromosomes, one from the egg and one from the sperm. This makes them diploid, or 2N.

A normal egg or sperm is haploid, 1N. In very rare cases it is possible to get a 1N plant.

Rarely, the complete set of chromosomes gets duplicated. This results in a tetraploid, 4N. It is also possible to use chemicals that inhibit normal cell division to deliberately make 2N plants become 4N plants. It isn't 100% effective, but often enough to be a useful tool sometimes.

Eggs and sperm from a 4N plant will usually be 2N.

If you cross a 2N plant and a 4N plant the offspring will be triploid, 3N.

4N plants are often physically larger, more robust, thicker broader leaves, heavier larger flowers with broader petals, etc. It is those changes in the flowers that sometimes make 4N orchids desirable. Basic cultural needs don’t change but if they are healthy, 4N plants may be more vigorous, more resistant to cold, etc. But sometimes the extra sets of chromosomes cause problems and the plants may be slow growing or stunted, or more susceptible to disease.

2N plants, and usually 4N plants too, have high levels of fertility. They are easy to breed with in that sense.

3N plants often have some of the advantages of 4N plants, sometimes without the disadvantages, but they tend to have significantly reduced fertility because of the mismatched sets of chromosomes.

In breeding, 4N plants have advantages if they are healthy – bigger flowers, broader petals. There are also limitations. Unless you have another 4N plant for breeding you are limited to selfing or a cross with a 2N plant that leads to infertility in the next generation, almost a dead end to breeding. And because of the doubling of all genes the genetics get more complicated. If you’re looking for a recessive color to come out it is harder to get, for instance.

What is better? It depends on your breeding goals, the health of your plants, and more. For a typical hobby grower, you have to decide for yourself if the possible problems of 4N plants, and sometimes greater expense, are worth the benefits.

 

[NYEric]

Umm, some posts from this thread have disappeared!

 

[Drorchid]

I have a plant genetics and plant breeding background, and something has always bugged me in the orchid world. Everyone else (that I know of) never uses the terms "4N" or "3N", but rather 2n = 4x or 2n = 3x (so tetraploid will be "4x" and triploid will be "3x").

"n" stands for the gametic number or haploid number, and is the number of chromosomes in a gamete (in the case of an orchid, the number of chromosomes in a pollen cell, or in a egg cell).

n = gametic or haploid number
2n = zygotic or diploid number

There is NO 3n or 4n because an organism/cell can only be a gamete or a zygote.

"x" stands for the basic number of unique chromosomes to the genome of the species

x = monoploid (1 set of chromosomes)
2x = diploid (2 sets of chromosomes)
3x = triploid (3 sets of chromosomes)
4x = tetraploid (4 sets of chromosomes)
5x = pentaploid (5 sets of chromosomes)
6x = hexaploid (6 sets of chromosomes)
etc

Can anyone explain why in the orchid world all of a sudden we talk about 2N for diploid, 4N for tetraploid and 3N for triploid (it should be 2n = 2x for diploid, 2n = 4x for tetraploid and 2n = 3x for triploid)? (I have to say, even I use the terms 2N, 3N and 4N now for diploid, triploid and tetraploid...)

some more info (if you are interested):
http://en.wikipedia.org/wiki/Ploidy

Robert

 

[gonewild]

Can anyone explain why in the orchid world all of a sudden we talk about 4N for tetraploid and 3N for triploid (it should be 2n = 4x and 2n = 3x)? (I have to say, even I use the terms 3N and 4N now...)

Because x was already used to mean "crossed with".
If you wrote " I like Phrag DrOrchid 4x" instead of "I like Phrag DrOrchid 4n" people would think you forgot to write what DrOrchid 4 was crossed with.

 

[Drorchid]

Because x was already used to mean "crossed with".
If you wrote " I like Phrag DrOrchid 4x" instead of "I like Phrag DrOrchid 4n" people would think you forgot to write what DrOrchid 4 was crossed with.

good answer!

Robert

 

[PaphMadMan]

The use of 3n, 4n etc. isn't unique to the orchid breeding world. I definitely heard it used in this way in horticulture and agronomy classes (2n, 4n, 6n wheat) over 30 years ago. In this case it conveys the information that is truly important, the haploid multiple relative to the original diploid. Biologically, evolutionarily, the monoploid (x) vs. haploid (n) distinction is important. Over a few generations of controlled breeding it complicates things unnecessarily. Considering how little most orchid breeders follow traditional plant breeding paradigms I am sometimes surprised they even make the distinction, except that it works as a marketing ploy.

 

[cnycharles]

Someone probably used it in an ad, someone else copied and a 'convention' was born! sort of like how the flowering plant fuchsia ended up being pronounced fyusha instead of feuck sia like the persons name is pronounced

 

[gonewild]

In the 60s, 4x was a condom not an orchid....

 

[Kawarthapine]

Robert, Eric, Lance et al.

Thank you for expanding on the rudimentary explanation I found on the web.

Like anything in life, there are simple answers and then there are more complex answers.

The more I learn the deeper this rabbit hole appears to become.

Can you guys suggest a good book on orchid propagation that would help me better understand this very interesting topic and get me started with breeding techniques and post germ culture.

Note. I do have some tree and tallgrass prairie propagation experience so I should be able to at least understand the forward and credits...

Thanks.

Duncan