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Author Topic: Theory: percentage of nucellar seeds vary depending on the pollinator  (Read 564 times)

SoCal2warm

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I have theory. I don't have any evidence for it, but let me share the reasoning for it.

Among different citrus varieties there are some which produce a higher percentage of nucellar seeds than others. Some citrus species produce entirely zygotic seed, while other varieties produce almost entirely nucellar seed, and there are several varieties with percentages in between. If the seed is nucellar, it will be a genetic clone of its parent. Not really a good thing if you're trying to hybridize new citrus varieties, but a good thing for those growing rootstock from seed, where uniformity is important.

The reason, it is believed, some citrus varieties form nucellar seed is that the sexual gamete cells inside the seed are not vigorous enough and can't compete with the nucellar cells.

The original origin citrus species are, almost entirely without exception, all zygotic. But when different citrus species are hybridized the percentages of nucellar seed shoot up. This could be because the hybridization between different species (which are not entirely compatible) results in a weaker gamete (megagametophyte), and so the nucellar lining takes over.

My theory is that if a hybrid citrus flower is pollinated with pollen from one of the original two species from which the hybrid originated from, the percentage of nucellar seed is likely to be lower. This would be because, the genetic composition of the megagametophyte would be less heterogenous, and thus presumably have more vigor.

So, for example, if Yuzu were pollinated by C. ichangensis, there might be some zygotic seeds form in the Yuzu fruit.

If this theory is true, this may confer some advantage to hybridizing new citrus varieties, since normally trying to hybridize a variety that is highly nucellar and using it as the female parent presents some pragmatic challenges.

Millet

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Re: Theory: percentage of nucellar seeds vary depending on the pollinator
« Reply #1 on: September 05, 2018, 09:05:00 PM »
Dr. Manners once wrote that the reason that zygotic gametes loose out against nucellar is that the zygotic gametes began growing later than the nucellar.  Thus the nucellar have a head start and crowd out the zygotic.
« Last Edit: September 05, 2018, 09:57:41 PM by Millet »

Ilya11

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Re: Theory: percentage of nucellar seeds vary depending on the pollinator
« Reply #2 on: September 06, 2018, 05:48:36 AM »
Usually it is quite an opposite. Due to the hybrid vigor the heterogeneous offspring grows better than genetically homogeneous parents.
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Millet

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Re: Theory: percentage of nucellar seeds vary depending on the pollinator
« Reply #3 on: September 06, 2018, 12:31:53 PM »
Ilya, I am just stating Dr. Malcolm Manner's statement on how nucellar reacts against zygotic gametes. I'm not saying your incorrect, but Dr Manners is rarely or never wrong. 

Walt

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Re: Theory: percentage of nucellar seeds vary depending on the pollinator
« Reply #4 on: September 06, 2018, 02:15:16 PM »
This is something for me to pay attention to.  I'll be making many crosses, and I will of course try to use seed parents with a high percent zygotic seedlings.  But now I'll be paying attention to pollen parent too, and see which ones give higher percent zygotic seedlings.  It may not make any difference, but it won't be extra work to keep track.

Ilya11

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Re: Theory: percentage of nucellar seeds vary depending on the pollinator
« Reply #5 on: September 07, 2018, 03:40:18 AM »
Millet, sorry for misunderstanding, my reply was addressed to initial post of Socal2warm, not yours. Completely agree with you and Dr.Manners on mutual competition between zygotic and nucellar embryos.
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                       Ilya

SoCal2warm

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Re: Theory: percentage of nucellar seeds vary depending on the pollinator
« Reply #6 on: September 07, 2018, 01:48:08 PM »
Usually it is quite an opposite. Due to the hybrid vigor the heterogeneous offspring grows better than genetically homogeneous parents.
Outbreeding depression is more common than hybrid vigor.

I think in general hybrid vigor might be more likely to occur if the two species are very closely related to each other, and each has different types of genes for vigor, whereas outbreeding depression is more likely to occur with more disparate species, that have long ago evolutionarily diverged.

I think it can be assumed that the formation of nucellar seed is either due to hybrid vigor or a form of outbreeding depression, but I am not truly sure which.

Quote
Outbreeding depression refers to cases where offspring from crosses between two different populations have a lower level of fitness than offspring from crosses between individuals from the same population.
 
One of the ways that outbreeding depression can occur is by the breakdown of biochemical or physiological compatibilities between genes in the different breeding populations. Within local, isolated breeding populations, alleles are selected for their positive, overall effects in complementary combination with other specific alleles. Due to nonadditive gene action, the same genes may have rather different effects in different genetic backgrounds--hence, the potential evolution of locally coadapted gene complexes. 

Ilya11

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Re: Theory: percentage of nucellar seeds vary depending on the pollinator
« Reply #7 on: September 08, 2018, 02:25:44 AM »

Outbreeding depression is more common than hybrid vigor.

It is not a case, depression is rather rare, but hybrid vigor heterosis is very common, especially inside the same genus.
From my experience, based on thousands seedlings of 5* citrumelo, its crosses to FD and several grapefruit varieties are almost all very slowly growing plants, while hybrids to Yuzu and oranges have extraordinary rapid growth.
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                       Ilya

SoCal2warm

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Re: Theory: percentage of nucellar seeds vary depending on the pollinator
« Reply #8 on: September 08, 2018, 02:01:39 PM »
From my experience, based on thousands seedlings of 5* citrumelo, its crosses to FD and several grapefruit varieties are almost all very slowly growing plants, while hybrids to Yuzu and oranges have extraordinary rapid growth.
Not really the best example, since citrumelo is already a hybrid. And Yuzu already has very vigorous growth.

Ilya11

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Re: Theory: percentage of nucellar seeds vary depending on the pollinator
« Reply #9 on: September 09, 2018, 02:22:28 AM »
From my experience, based on thousands seedlings of 5* citrumelo, its crosses to FD and several grapefruit varieties are almost all very slowly growing plants, while hybrids to Yuzu and oranges have extraordinary rapid growth.
Not really the best example, since citrumelo is already a hybrid. And Yuzu already has very vigorous growth.
Than I do not understand what is your "hypothesis" is about and why you suggested "vigorous" Yuzu for its testiting.

My theory is that if a hybrid citrus flower is pollinated with pollen from one of the original two species from which the hybrid originated from, the percentage of nucellar seed is likely to be lower. This would be because, the genetic composition of the megagametophyte would be less heterogenous, and thus presumably have more vigor.

So, for example, if Yuzu were pollinated by C. ichangensis, there might be some zygotic seeds form in the Yuzu fruit.

Best regards,
                       Ilya

SoCal2warm

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Re: Theory: percentage of nucellar seeds vary depending on the pollinator
« Reply #10 on: September 09, 2018, 03:33:01 AM »
Than I do not understand what is your "hypothesis" is about and why you suggested "vigorous" Yuzu for its testing.
Because Yuzu is believed to have originated from interhybridization between sour mandarin and C. ichangensis.

Maybe that wasn't the best example to use.

Calamondin and kumquat. Calamondin, let us assume, contains 50% orange and 50% kumquat genes. If it is hybridized again, it will contain 75 percent kumquat.
 Then I am thinking the genes will be more homogenous, more like the kumquat ancestor, and it may be at least a little bit zygotic.

Or one might have to hybridize it with kumquat again, over another generation, before the seeds of the progeny become zygotic.

Sylvain

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Re: Theory: percentage of nucellar seeds vary depending on the pollinator
« Reply #11 on: September 10, 2018, 08:01:57 AM »
Quote
Yuzu is believed to have originated from interhybridization between sour mandarin and C. ichangensis.
Anyway, now we know it is not true.

Walt

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Re: Theory: percentage of nucellar seeds vary depending on the pollinator
« Reply #12 on: September 10, 2018, 03:37:13 PM »
Has anyone checked whether there are other things, like temperature, the change the percent of zygotic seedlings?  Temperature seems to change the percent of successes in interspecific wheat crosses, and also sunflowers.  This is a whole different thing, of course.

Ilya11

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Re: Theory: percentage of nucellar seeds vary depending on the pollinator
« Reply #13 on: September 11, 2018, 03:12:35 AM »
Environmental conditions are clearly very important, but I would never try to generalize in the case of citruses, they are so different.
I have only one example- for years I have been trying to cross 5*citrumelo to Thomasville, but failed with the exception of one fruit that was formed during extreme heat wave.
« Last Edit: September 11, 2018, 04:01:24 AM by Ilya11 »
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                       Ilya

Walt

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Re: Theory: percentage of nucellar seeds vary depending on the pollinator
« Reply #14 on: September 11, 2018, 12:35:36 PM »
I will be pollinating every time I have a flower.  I might as well keep track and report back, someday.
I'll google and try to see if someone has published such stuff.

 

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