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Post by melenduwir on May 3, 2022 15:55:08 GMT -5
I know this is going to be somewhat controversial. There are certain advantages to nucellar reproduction from the perspectives of both plants and human growers. But I'm convinced that high rates of cloning are the doom of a variety or even a species. And it makes crossing different varieties/species much, much more difficult.
Is it possible to select against this trait? I think it's notable that the major ancestors of commercial citrus mostly seem to lack it, with the exception of some strains of mandarin. Citron is so highly self-pollinating that it's almost as uniform as a self-cloning variety, but it can still be bred with effort - I'd be willing to settle for that.
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kumin
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SE Pennsylvania, 45 miles north of Chesapeake Bay, Zone 6b
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Post by kumin on May 3, 2022 16:26:34 GMT -5
There are both species and cultivars that have high percentage, as well as 100% zygotic embryony. Pomelos are an example of a zygotic species. Temple tangor, Meyer lemon, and Clementine mandarin (sometimes referred to as a tangor),etc.are exclusively zygotic. Zygotic X zygotic crosses should produce zygotic progeny. US 1279, 1281 and 1282 citrandarins are essentially 100% zygotic rootstock cultivars out of favor, due to lack of nucellar seedlings.
I prefer to use the terms zygotic and nucellar in preference to monoembryonic and polyembryonic, as it more precisely addresses the genetic aspect, rather than a simple embryo count.
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Post by melenduwir on May 3, 2022 17:39:58 GMT -5
I was thinking more of identifying and selecting for, say, C. trifoliata specimens that are zygotic. (I've heard PoncirusPlus is zygotic, but can't confirm.)
Alas, I can't rely on picking zygotic parents. The Mexican lime is a cross between two zygotic parents, but it ended up mostly nucellar. I note that the Darcy mandarin is supposedly 100% nucellar, or very close to it, and sweet oranges vary from year to year but can be up to 97%. That means it's difficult-to-impossible to use them as seed parents, and using them for pollen still means their offspring will have the same problem.
If all citrus were like that, citrus breeding would come to a complete stop, except possibly for labs capable of embryo rescue.
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kumin
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SE Pennsylvania, 45 miles north of Chesapeake Bay, Zone 6b
Posts: 113
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Post by kumin on May 3, 2022 18:20:42 GMT -5
Provided nucellar cultivars are pollen fertile, they can be used as male parents. However, as you noted their progeny may well again produce nucellar seedlings. My Poncirus+ is flowering for the first time (at three years of age). I self pollinated a few flowers, but also pollinated others with Meyer lemon and segentrange pollen. Perhaps I'll be able to determine the type of embryos produced.
By using exclusively zygotic plants as parents, then continuing to select for zygotic seedlings, nucellar progeny expression should be reduced, or eliminated.
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Post by ilya11 on May 4, 2022 2:37:03 GMT -5
It seems that one dominant principal gene is responsible for the nucellar embryogenesis. But the ratio of zygotic/nucellar embryos in polyembryonic varieties is determined by several other genes.
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Post by melenduwir on May 4, 2022 15:21:36 GMT -5
It seems that one dominant principal gene is responsible for the nucellar embryogenesis.
That's encouraging. But, how do nucellar varieties arise from zygotic parents? (I believe the micrantha is zygotic, the original wild founder populations all seem to be so.) Mexican lime derives from citron and micrantha, and citron hybrids are noted for preserving at least partial zygotism. I note that wild mandarins are mostly completely dominated by nucellarism. Which is why the finding of the isolated island population was so exciting, because it was completely zygotic. Being able to precisely duplicate a successful individual is a huge advantage in the short-term, but in the long term... I find it reassuring that some of the varieties I'm interested in aren't completely nucellar, and are zygotic enough that I can try to hybridize them even with my very limited resources. I wish I knew more about what environmental factors combine to determine whether zygotic or nucellar seeds are produced, but it seems no one knows for sure. Identifying and possibly eliminating nucellar plants is hard except by letting them fully mature and breeding them in turn. Even then it will take several years of seed production to be certain of identifying nucellars.
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Post by pagnr on May 5, 2022 1:04:40 GMT -5
The highly nucellar rootstock varieties are the basis for the Citrus industry. Basically they are virus free propagation material and highly uniform. Avocados are fairly seedling variable, and there are methods to grow desired clonal rootstocks to overcome this. Grapes are propagated on clonal cutting rootstocks, but these are not virus free.
In the case of "The Mexican lime is a cross between two zygotic parents, but it ended up mostly nucellar".
This would seem to be a way of stabilising a successful hybrid progeny from two zygotic parents. Otherwise there would be a cascade of endless variation in the progeny, from every hybridisation event in nature, and every next generation. Even a highly successful zygotic hybrid plant would produce variable offspring. Since Citrus seed dispersal is probably by fruit eating animals, highly variable fruit types are possibly confusing targets.
Possibly only the Finger Lime has endlessly variable fruit colours, fruit pulp colour and sizes in nature, but not shape so much.
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till
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Post by till on May 16, 2022 16:20:15 GMT -5
The number of zygotic embryos can also depend on the pollen parent.
I have often thought about Eremolemon, a hybrid of Citrus glauca and Citrus meyeri. Both parents have zygotic seeds but the hybrid is 100% nucellar.
Could it be that the presence of nucellar seeds has also something to do with the hybrid nature of a plant? My though is this: If a nucellar plant produces a greater number of zygotic seed when the right pollen is applied and a hybrid of two zygotic species / varieties produces only nucellar seeds then isn't it likely that nucellar embryogenesis is the more developed the more difficult the production of vital zygotic embryos is? So it is basicly the plan B of a plant that has proplems to produce vigorious zygotic seeds? I would assume that a certain gene must be present that nucellar embryogenesis is possible and likely (assumedly inactive in pure Citrus glauca). But other factors may play a role to what extend that gene is activated. Admittingly, this is only my speculation. But what do you think? Any pros or cons for the idea?
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Post by ilya11 on May 17, 2022 14:39:48 GMT -5
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Post by pagnr on May 20, 2022 17:08:06 GMT -5
So it is basicly the plan B of a plant that has proplems to produce vigorious zygotic seeds?
I think we can assume that the original Citrus types that propagate via nucellar polyembryony have been doing so for millions of years ?? There must be a strong advantage to this, otherwise it would fade away. Also since they exist in nature as "species" which we can recognise, zygotic variants must be uncommon in the wild ?? The Citrus hybrids produced by people are interesting artefacts, both the recent deliberate hybrids and historic types in cultivation. In a stable unchanging rainforest environment, there may be little advantage to producing highly variant adaptive offspring. There may be more advantage in producing uniform offspring that can spread into openings in the understory. Citrus are mainly understory plants. It is interesting that both the zygotic and nucellar systems are present simultaneously in these Citrus species. Is this an artefact of previous ancestors or a useful ongoing part of the plants method of reproduction?
The Pummelo is considered a wild species and monoembryonic, but when grown from seed basically produces another Pummelo, not the highly variable offspring seen in cultivation. Even though it can produce variable offspring, it also conservative in what is produced ??
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Post by melenduwir on May 20, 2022 18:22:03 GMT -5
Oh, if there's a lot of variation in traits, making clones of a particularly-fit individual is very efficient. But in the long term, it can be fatal, since a strongly-nucellar strain can't recombine its genes or adapt to changing circumstances. And once the nucellar trait becomes common in a population, it tends to dominate in the short term and convert entire populations to cloning strains.
There are reasons why this particular method of reproduction is only really found in Citrus and isn't common throughout the plant world.
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till
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Posts: 160
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Post by till on May 22, 2022 16:56:08 GMT -5
Pagnr, a lot of questions you pose and I am not sure which of them are rhetoric and which real questions. I agree to you that nucellar embryony is a very old trait in citrus, at least in part also used by pure species as poncirus. It certainly has its advandages. My thought that it is essentially a plan B was not to claim that it is detrimental in all respect or that it does not have its advantages. To the contrary, the advantage of this plan B would exactly what you point to: that highly fit clones secure the success of a cultivar were variant seedlings would not chance to be equally fit. We should, however, be very carefull not to confuse true to type seedlings caused by nucellar embryony with (more or less) true to type seedlings caused by the highly homozyguous nature of a species (pummelos for instance). Nucellar embryos are clones but the latter just happen to be similar because the parents are not much unlike each others. Now, my idea of a plan B is certainly speculative and questionable on a genetic and empiric basis. Still, I would like to discuss it as a subordinate principle of the phenomenon of nucellar embryony because it is not only the genetic outlook of the mother plants that determines the percentage of nuecllar seedlings but also the type of pollen applied. So it seems that the genetic outlook or just fitness of hybrid embryos influences the willingness - so to say - of plants to produce nucellar embryos.
melenduwir, nucellar embryony is by far not limited to citrus. It is very common in Sorbus and Aronia (another hobby of mine). Here, it is linked to the tetraploid state of cultivars or apomictic species. I have also read that crosses between Pyrus and Chaenomeles resulted in some apomictic seeds (and no hybrids) so that Pyrus seems to possess the same potential of apomixis although it is only expressed under extremly rare and artificial situations. There are many other plants that can procreate apomictically. This underlines the potential of non-sexual propagation via seeds. It enables, for example, a highly fit but improbable cross like Sorbus intermedia (S. aucuparia x S. torminalis x S. aria) to spread thoughout northern Europe for which it is perfectly adapted and survive for thousands (or millions?) of years that it can wait for its chance to outbreed into other species (via pollen) or undergo the right mutation that makes it even more fit.
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Post by melenduwir on May 23, 2022 14:09:08 GMT -5
The Common Dandelion in North America is a great example of apomixis - it's a triploid that is more capable of handling colder northern regions than the warm-weather-loving diploid native to the Mediterranean region, but it's necessarily incapable of being fertilized. It produces seeds that are duplicates of the mother plant, so aside from occasional mutations, all dandelions are clones of each other - which is also a huge risk ecologically, since a disease that adapts to one plant is also adapted to every other.
If it's merely an expression of a plant not being well-suited to normal reproduction, it suggests simply breeding enough zygotic generations would resolve the problem.
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till
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Post by till on Sept 29, 2022 15:00:13 GMT -5
Melenduwir, Dandelion seems to be an interesting example. According to German wikipedia, it exists in different ploidity levels, accumulates mutations in apomictic ploidity states and then switches back to diploid sexual ploidity levels. So apomictic races of Dandelion are not totally separated from diploid races. Apomictic races can convert into diploid sexual ones. According to wikipedia, it is not clear which race is more fit. The example of Dandelion, by the way, shows that apomixis in citrus and in other species may not be fully comparable.
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Post by pagnr on Oct 1, 2022 2:05:05 GMT -5
The example of Dandelion, by the way, shows that apomixis in citrus and in other species may not be fully comparable.
Also the lifespan of the plant and generation times are vastly different. Dandelions are possibly annual or biennial in their generation times, new offspring produced and also flowering every season. Citrus generations more like 5 to 10 years, depending on species
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