Why does F1 Hybrid Vigour occur ?

[cryptolab]

Hello Denizens,

I find this whole topic of F1 Hybrid Vigour, particularly interesting.
I understand what F1 Hybrid Vigour is, as a description of its effect.
And I understand under what circumstances it will happen.
What i don't understand, is 'why' ?

Why does F1 Hybrid Vigour occur ?

The picture below, pretty much sums up the effect:

I'm sure many of you have spent plenty of time studying and working
on issues relating to plant breeding and hybridization... Perhaps you
can offer a perspective on this issue that sheds light and understanding
on the whole topic.

Thanks to those who offer a reply

have a nice day

cryptolab

 

[tobe]

Hello Denizens,

I find this whole topic of F1 Hybrid Vigour, particularly interesting.
I understand what F1 Hybrid Vigour is, as a description of its effect.
And I understand under what circumstances it will happen.
What i don't understand, is 'why' ?

Why does F1 Hybrid Vigour occur ?

The picture below, pretty much sums up the effect:

I'm sure many of you have spent plenty of time studying and working
on issues relating to plant breeding and hybridization... Perhaps you
can offer a perspective on this issue that sheds light and understanding
on the whole topic.

Thanks to those who offer a reply

have a nice day

cryptolab

Hi cryptolab
The f1 hybrids carry the best of both parents. In the f2 generation the genetic split, that's the generation where breeder have to select carefully. Take a look at Mendel rules that's the basic for breeding.

I hope I understand you right and this can help you.

Have a nice day
cu tobe

 

[fryge]

as far as I know, it's still not completely understood what happens, but we can observe it does.

as part of a plant breeding course I did get a few theories on what causes/could cause hybrid vigour. would have to look it up again to refresh it again, I think there were 3 prevailing theories but I only remember one of them well enough, the reality is likely a mix of those 3(for some genes it's one mechanism, for others the other, and depending on plant species/population/gene you're looking at you may encounter one mechanism more as the others).

that one theory I clearly remember is that for a certain random gene, heterozygotes are better than homozygotes. in this case, the hybrid vigour can never be fixed, since it depends on being heterozygous.

I'll look up the others and edit them in

edit: that theory where heterozygotes outperform homozygotes is called overdominance.

another is 'directional dominance'. here the theory is there are 2 genes influencing the trait, with the dominant one being the better one. since it's dominant it doesn't matter if it's heterozygous or homozygous, but it needs the dominant version of both genes to perform best(they simply add up though). in this case you would theoretically be able to fix it, if you create a line that's homozygous for both of those dominant alleles.

the third is 'complementary gene action'.
it's pretty much the same as directional dominance, except now there's an interaction between those both genes, making individuals who have a dominant allele of both better as the effects of both genes added up.


on another level, you could also look at it as the removing of negatve inbreeding effects. generally, hybrid vigour works better when the parents are further related(although that's just a general rule, if you want it more precise you should delve into general combining ability and specific combining ability, and do a lot of testcrosses).
so with an outbreeder like weed, it's likely you'll get some deleterious recessives stabilized by accident(you're not doing selection on them) while you're inbreeding.
if you then cross a distantly related plant, it's likely it'll not have the same deleterious recessives, but it has a dominant version of those(but possible other negative recessives), so you'll cancel out the inbreeding efffects of both lines by crossing them together.
that's also why it doesn't matter that much if your parentline is inbred as fuck if you're making an f1-hybrid cultivar, as long as it produces enough fertile seeds, you loose the inbreeding depression in the final generation anyway.

 

[cryptolab]

as far as I know, it's still not completely understood what happens, but we can observe it does.

as part of a plant breeding course I did get a few theories on what causes/could cause hybrid vigour. would have to look it up again to refresh it again, I think there were 3 prevailing theories but I only remember one of them well enough, the reality is likely a mix of those 3(for some genes it's one mechanism, for others the other, and depending on plant species/population/gene you're looking at you may encounter one mechanism more as the others).

that one theory I clearly remember is that for a certain random gene, heterozygotes are better than homozygotes. in this case, the hybrid vigour can never be fixed, since it depends on being heterozygous.

I'll look up the others and edit them in

edit: that theory where heterozygotes outperform homozygotes is called overdominance.

another is 'directional dominance'. here the theory is there are 2 genes influencing the trait, with the dominant one being the better one. since it's dominant it doesn't matter if it's heterozygous or homozygous, but it needs the dominant version of both genes to perform best(they simply add up though). in this case you would theoretically be able to fix it, if you create a line that's homozygous for both of those dominant alleles.

the third is 'complementary gene action'.
it's pretty much the same as directional dominance, except now there's an interaction between those both genes, making individuals who have a dominant allele of both better as the effects of both genes added up.


on another level, you could also look at it as the removing of negatve inbreeding effects. generally, hybrid vigour works better when the parents are further related(although that's just a general rule, if you want it more precise you should delve into general combining ability and specific combining ability, and do a lot of testcrosses).
so with an outbreeder like weed, it's likely you'll get some deleterious recessives stabilized by accident(you're not doing selection on them) while you're inbreeding.
if you then cross a distantly related plant, it's likely it'll not have the same deleterious recessives, but it has a dominant version of those(but possible other negative recessives), so you'll cancel out the inbreeding efffects of both lines by crossing them together.
that's also why it doesn't matter that much if your parentline is inbred as fuck if you're making an f1-hybrid cultivar, as long as it produces enough fertile seeds, you loose the inbreeding depression in the final generation anyway.

Hi Fryge,

A+ great answer. Thanks !

It seems the boffins at Iowa state University have been looking, at a molecular level, at this whole issue of F1 Hybrid vigour.
They haven't come up with a full explanation yet, but they have been comparing 1400 genes at a time, in the parents and hybrids.
Some genes are more active in the hybrids than in the parents.
And some genes what are active in the parents are relatively inactive in the hybrids.
So they are able to determine what pattern of gene activity corresponds to F1 vigour.
... what they still don't know is... why.

See here:
https://phys.org/news/2006-05-mystery-hybrid-vigor.html#jCp