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What happens if you try to breed Triploid citrus

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SoCal2warm:
Some of you may be aware that Oroblanco and Melogold grapefruits are triploid, meaning they have 3 sets of chromosomes rather than the normal 2.
So a natural question that arises is what would happen if you tried to breed a triploid citrus with something else? What would you get.
Although Oroblanco is normally considered seedless, I found several fully formed large seeds inside one the fruits I just bought. I'm assuming all/most of these seeds are nucellar (clones of the parent) but it got me thinking.

I've been doing a lot of research, and wanted to present to you what I've found so far, in an attempt to answer the question. I'm no expert in this area.

Basically, when a triploid like Oroblanco pollinates another citrus variety (generally a monoembryonic variety) it induces the formation of seed. These seeds will be haploid, having only a single set of chromosomes from the female fruit parent. Most of the gametes produced from a triploid will have unevenly split chromosomes during meiosis, but a smaller portion will also be diploid and haploid. That means that pollen from a triploid parent could contribute either one or two sets of chromosomes. Assuming that the female parent is diploid, this will result in the seed either having 2 or 3 sets of chromosomes. So you could get a normal zygotic seedling, or you could get another triploid. The triploids will have a lower probability of having fully formed seeds, so the majority will not survive. (Embryo rescue techiques are usually employed to be able to grow these underdeveloped seeds, but in a small number of cases the seeds will be well formed enough to be able to germinate on their own)

It is also possible for a triploid parent to produce an unreduced gamete but this is comparatively much more rare, so in that case the gamete would be able to contribute 3 sets of chromosomes.

Haploid citrus plants are normally feeble and slower growing, generally sterile, and right now I cannot find any references to a haploid citrus plant ever having produced fruit. But for breeding purposes, they can be reconstituted back to normal diploids by treating the growth on bud shoots with colchicine, and then later taking a graft from that offshoot.

This still leaves so many questions but it's a start to understanding.


--- Quote ---Triploid plants can and do go through meiosis (cite). Of course, their fertility is lowered with most gametes being aneuploids (uneven number of chromosomes), however, triploid plants also produce small numbers of euploids (1x, 2x, 3x)(Fig 1 of the above cited source).

This should make you wonder why we don't often observe seeds in triploid fruit since they can make viable gametes (1x, 2x, 3x). This seems to be purely driven by the number of female gametes (ovules) produced vs. the number of male gametes (pollen grains) produced. Plants produce many more pollen grains than ovules making it much more likely that a euploid pollen grain will be produced. Contrasted by the small number of ovules produce making it very unlikely that a euploid ovule will be produced.

The fertility of triploid plants is the foundation for one of the most used mechanisms for polyploid formation called the triploid bridge. The triploid plant produces 1x, 2x, and 3x gametes that combine with the 1x gametes produced by a diploid plant to form 2x, 3x, and 4x offspring
 
--- End quote ---

I could well be wrong about most of the seeds in Oroblanco being nucellar. That would be interesting.

Marcin:
I used to plant Sweetie seeds (Sweetie is a triploid cross similar to Oroblanco). All seeds were monoembryonic, zygotic. Some grew into nice, vigorous plants, but some seedlings were weak and kind of defective. So my idea is you can use a triploid as a mother plant, you will just need to screen the weak seedlings out, and keep the healthy ones.

An interesting paper:
https://ejournal.sinica.edu.tw/bbas/content/2009/3/Bot503-06.pdf

Ynk88:
I also found some seeds in a triploid hybrid. But it was a Tacle mandarin and not an Oroblanco or Melogold grapefruit.
I found 2 seeds after eating about 25 Tacle   ;D

So I have sown them at beginning of february this year, because I love this hybrid, for me it's the best tasting mandarin I have eaten so far.
The first seed was monoembryonic and the second was polyembryonic.
The monoembryonic seed was a nice big one and the poly was not fully formed.

The plant from the monoembryonic seed is groing much faster than the two plants from the other seed ,and I wonder if it will give something good to taste. I'll have to wait a long time before tasting some fruits, if they survive till there  ::)

Marcin:
Ynk88, congrats on finding these seeds :D

This year I've also found my first seed in a Persian lime (a triploid too). The seed was small but luckily sprouted and gave this one seedling. It's about 1 month old now. I hope it will get more vigorous soon.

SoCal2warm:
After several hours of research I came across something.
P. Ollitroult found half diploid and half triploid among the seeds of 'Oroblanco'.
Citrus genetics, breeding and biotechnology, Iqrar A. Khan, p205

Only diploids were found in the progeny of clementine fertilized by Oroblanco pollen, the same source says. But I wonder if they checked to see if the next generation were even hybrids, since very likely the triploid Oroblanco pollen might just have been inducing parthenocarpy. Edit: oops, if that was the case, one would expect haploid offspring (see gynogenesis). Another potential explanation, I did read in some citrus cultivars (particularly many oranges) the surrounding nucellar tissue in the embryo is highly sensitive to triploidy and will abort.

Also, while it is true triploid seeds are most often found in shrivelled up smaller seed (which also causes low viability), this is not necessarily true when the fruit parent happens to also be a triploid. The poor seed development is caused by differences in ploidy between the developing megagametophyte and surrounding nucellar tissue.

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