Hello!

Thinking about the Prag Citsuma, a putative chimera, I got a question today: When we double the chromosome number of old well known citranges how does that effect the fruit quality?

It is pure speculation that it helps producing better fruits. But I think we should try that. I do not know what exactly to do but colchicine, a poison from meadow saffron (Colchicum autumnale) is used for doubling the chromosome number. It suppresses cell division so that growing plants treated with it get cells with more chromosomes. These have to form a twig (should be more thick) the fruits of which we must test.

A positive or negative aspect - depending on the standpoint - of 4 chromosome sets is that such is hybrid regains full fertility, i. e. gets seeds as pure species. The problem with this new fertility is, however, that the gametes will have 2 chromosome sets so that they produce triploid plants if crossed with normal diploid plants. But well, some like triploid plants because they have less seeds and are pollen sterile. So the triploid offspring of tetraploid hybrids could be interesting for other people than myself who want full fertility.

What do you think? Any biologists around who know what must be done?
If it works that would be the fastest way of producing new varieties: You could start with mature plants and get mature twigs.
The procedure is not uncommon in plant breeding.

A got a hot tip from hardyvermont: I should look for "'daylily tetraploid conversion". And yes, I have found this manual here: http://www.patriotdaylilysociety.org/colchicine05.htm

Another page says that the mortality rate of treated daylilies is very high. Well that must not be the case with citrus. And if so you would only sacrifice a the tip of a shoot.

The overall question remains if tetraploidity of common hybrids leads to better fruits.

I should also name one disadvantage for breeding: A tetraploid hybrid will probably not mix up its Poncirus part with the its citrus part but keep both chromosome sets apart when producing gametes. That means self-pollination or crossing two tetraploids will probably result in hybrids with 50% Poncirus. This too is of cause not totally sure unless experiment has proven it true.

There are a few (spontaneous) tetraploid Citrus varieties, ie giant key lime.
When you double the chromosomes, you double the expression of characteristics. Citranges will still be citranges, maybe some change in the balance of fruit characters, but hard to imagine a massive improvement???
Maybe a triploid citrange, with an extra set of sweet orange genes will improove fruit, but who knows if significant hardiness will be lost.

Not sure why the tetraploids chromosomes wouldn't randomly segregate in the gametes?

Till,
Up to 5% of citrus seedlings are tetraploids. They have a deep green color and grow less rapidly than diploids. They are widely used in modern selection for the production of seedless triploids by crossing with diploid plants. Not sure that they are more resistant to the freeze. Fortunella hindsii (Hong Kong kumquat is tetraploid.

That is what I suspect. But as you write there is at least the chance that the fruit improves and that hardiness is not lost.

I have red that tetraploid hybrids regain fertility because the plant can divide the chromosomes in two equal halves, each for one gamete. If that is true then it would also mean that the plant can recognize how similar chromosomes are and that it gives each gamete an equal set which means that each gamete gets a full set of Poncirus and of Citrus chromosomes so that they are not mixed. But it might be - I don't know - that all or some Poncirus chromosomes are so similar to the respective Citrus ones that the plant mixes them up. I could imagine that this is not the case because many Poncirus hybrids at least lack fertility.

Till,
Many citruses are able to form fruits without pollination, so fertility (in terms of fruits) is not determined by chromosomes pairing during pollen or ovum formation. Zygotic or nucellar embryogenesis are also much independent of chromosome conjugation.
So I do not think you can improve seed formation and germination by working with tetraploids. I know that triploid seeds are smaller than diploid and need artificial media for their germination.

I agree, Ilya. What I meant was the development of ZYGOTIC embryos. I should have said that more clearly. I know that also the development of zygotic seeds can be suppressed if a variety is strongly polyembryonic. But even in that case a tetraploid would have the greater chance to produce zygotic seeds because it has less embryos that are weak already by the fact that the gametes were deficient. And if there are more strong embryos then the chance of their survival is greater even in a strongly nucellar variety.
This at least is my reasoning.

You may remember my experiment with colchicine
link

Mike/Citrange

One thing for sure, colchicine sure quickly cures an attack of gout. - Millet

What were your results, Mike?

Results were mentioned in that thread - but actually there were no results.
Or rather, there were no observable differences between Poncirus seedlings germinated with different amounts of mashed Colchicum bulbs added to the growing medium.
I concluded that I should have planted the bulbs and enjoyed the flowers!
However, I've no idea how much colchicine is contained in one Colchicum bulb. Perhaps it's a tiny amount and needs concentrating to have any effect.

That tetraploids are slower growing is indeed a problem that I should consider.
Maybe this whole idea of doubling the chromosomes leads to nothing else than a play. The advantages of normal plants are greater it seems.
Yet, I am curious...

Citrange, This spring I am going to redo your experiment but with Oryzalin witch is much less dangerous than colchicine.
I already bought Surflan.

Dangerosity of colchycine is exaggerated, it is actually FDA approved drug for three medical indications.
Again, artificial chromosome doubling is worth to do only for the production of tetraploids for zygotic citruses. Nucellar tetraploids are rather common, just look for very green rather stunted seedlings. (The Biology of Citrus
by Pinhas Spiegel-Roy,Eliezer E. Goldschmidt)