Hybrid vigor and Landrace Questions

[reed]

I'v learned here on the forum about hybrid vigor where, a new cross tends to grow better than either of the parents and that it is important in the commercial industry to make better performing varieties even though you have to buy them new each year. I always thought commercial hybrids were for specific traits but now think it might be as much about this effect. I don't really understand exactly why or how it happens but don't think that is really important for my purposes. 

I am wondering though how long you might make it last. For example if you grow an F1 hybrid and save it's seeds (even though they may separate back out). Won't they in future years just keep re-hybridizing  and re-separating over and over thereby extending the hybrid vigor effect  indefinitely? Especially if you throw in a few other OP and or F1s just to keep it stirred up good?

When it comes to making a landrace, especially one with easily out-crossing things like AD corn where hundreds of different kinds are mixed with maybe millions of possible genetic combinations, might you not end up with all hybrid vigor, all the time? Might that not remain true, at least to a degree even though some genetics could be lost over the years based on selection specific to your climate, conditions and preferences?

It occurs to me that the hybrid vigor might only be lost if you end up making a new variety with stable and predictable traits and then grow it "pure" for some period of years.

[philagardener]

Great questions!

In a cross between two inbred lines, hybrid vigor, or heterosis, will be strongest in the F1. In this case, the F1 individuals are maximally heterozygous at different genetic loci due to differences between the inbred parental lines (thought to be the basis for the effect) but phenotypically uniform because individuals are genotypically similar. In subsequent generations, reassortment and recombination will reduce the degree of individual genetic diversity with a resulting increase in phenotypic diversity in the population. This is segregation of traits commonly observed in the F2 and subsequent generations.  So the heterosis effect slowly fades in the population as a whole (even if the gene pool is maintained overall).

To take advantage of heterosis in a landrace, one would need to keep high the level of allelic diversity in individuals.  Maintaining a diverse gene pool is helpful in this regard, as is continuing to input genetic diversity by adding new varieties, but the degree to which hybrid vigor will be seen will never equal the effect seen in the F1 between two inbred lines since that maximized the heterozygosity at the individual plant level.

[DarJones]

What you are describing is a "composite". It is when several genetically distinct lines are brought together into a common pool. Composite breeding is an adjunct to hybrid breeding because it produces measurable levels of heterosis even after several generations of reproductive mixing. There are some problems with composite breeding, specifically that overall production drops to a statistically measurable level after a few generations, entirely depending on how many unique lines the composite was started with. There will be a few outstanding individuals counterbalanced by a few very poor performers. Overall production potential will be significantly lower than a selected F1 hybrid.

Landraces are in many ways just a composite breed that has been carried forward so many generations that the original unique lines are no longer distinguishable. They perform at an acceptable level as a result of the original variable genetics. As deleterious genes combine and recombine in the population, the original level of vigor declines and stabilizes when considered across the population.

A good example of this type breeding is Reid's Yellow Dent corn which was a result of deliberately mixing two varieties of corn such that the resulting composite was much more productive than the parents. Over time, RYD declined in production until is was not much better than the better of the parents.

[Joseph Lofthouse]

Sometimes I wonder if "hybrid vigor" doesn't really exist... Perhaps it's only an artifact of industrialized agriculture... Or a creation of the marketing department at the mega-chemical companies. Hmm. How to unwrap the myth. Perhaps like this: The marketing hype typically shows photos of parents that are suffering from inbreeding depression. And not just a little: Severely suffering to the point that it's startling that they can grow at all. Then a cross is made between those lines, and all of a sudden the offspring perform better than either of the very misfit parents. So what is really happening may not be "hybrid vigor" per se, but rather a result of somewhat undoing the intense deleterious effects of the inbreeding. Perhaps instead of calling it "hybrid vigor", it might be more accurate to call it "inbreeding amelioration". I say somewhat undoing because what the seed companies discovered when they first started making crosses between inbred lines is that a double cross hybrid performed better than a single cross hybrid. Double cross hybrids are made by starting with 4 inbreds, and making a cross between pairs and then crossing the resulting hybrids.

With a properly maintained landrace the intense inbreeding is avoided so the crops are constantly in a state of "hybrid vigor", or to use perhaps more accurate terminology, "Inbreeding depression is ameliorated".

[jondear]

So basically trading seeds is good for maintaining a healthy dose of genes.

I'd be willing to bet if we all started growing seed from a mass cross, we'd all come up with strains that suited our micro climates. If after a period of time we traded the new crosses would pick up some stuff we lost, good or bad, but lost just the same.

[reed]

Dec 24, 2014 12:11:35 GMT -5 DarJones said:

 Over time, RYD declined in production until is was not much better than the better of the parents.

So, when Reid's Yellow Dent came to be as it's own variety it was notably better that either parent. After being grown "pure" for a while it declined to just "not much better". Not much is still more, VERY interesting. 

[reed]

Dec 24, 2014 12:42:06 GMT -5 Joseph Lofthouse said:

Perhaps instead of calling it "hybrid vigor", it might be more accurate to call it "inbreeding amelioration". 

Different critters for sure, but say a Basset Hound with back problems and a Pug with a severe overbite get romantic. You could/might end up with cute puppies, more "normal" and healthy that either parent. Nothing to do with "hybrid vigor" just a step toward reversing the damage caused by inbreeding. Throw a Beagle in the next generation and now you got genetic chaos, the horror!

As far as preservation and keeping things pure goes, if this happened to all the Bassets and all the Pugs has anything really been lost except the health problems? 

As I mentioned before if my KY Wonder Beans and my Rattlesnake beans turn into KY Rattlesnake beans with no hope of retrieving either individual parent as it was before then O'well. If a dozen others are also mixed and lost in there too, all the better. 

[reed]

It just occurred to me there could be pitfalls to the hybrid vigor effect. I'm not worried about it though because of what I'v learned here on the forum.

As an example I'm going to try to make landraces of several of my favorite things. Pole beans is one of them. Next year I am going to mix planting, not in separate rows but such that different ones will be sharing the same support. That way a bumblebee will only have to move a few inches between different ones, hopefully increasing the chance of a cross. Some of the varieties I'm using will be, Tan KY Wonder, Black KY Wonder, Rattlesnake, White Blue Lake, Black Blue Lake, Ideal Market, Fortex, Cherokee Trail of Tears and some others I can't remember right now. 

I won't know in the first year if any crosses happen. In the second year I might know if I end up with seeds that look different than any I planted. In the third year especially if a non visually noticeable cross occurred I could end up with some more vigorous individual plants without knowing why.

I want that to happen but until I understood this hybrid vigor thing I might have mistaken that for climatic and conditional adaptation and made the mistake of overly representing those seeds in the next year.  I would certainly want them and want to track them but not to the exclusion of the others. 

 

[DarJones]

If a hybrid just ameliorates the effects of inbreeding, then it should be possible to select a homozygous line that is just as productive as the hybrid. This has been attempted, but has not yet been achieved. I say "not yet" because there is strong potential for genetic tools to enable this type production. But for now, and this includes using all the tools of past generations of breeding, it is impossible to select a homozygous line that is as productive as a carefully selected hybrid.

I can illustrate why this is so by using the Single Flower Trait (sft) in tomato. It has been proven that a single copy of sft in a tomato plant can dramatically increase production of that plant. It can easily be shown that a plant homozygous for either parent, whether homozygous sft or homozygous -sft, results in reduced production for the sft plant and normal production for the -sft plant. But when you cross sft with -sft, the result is a dramatic increase in production. The reason this works traces back to regulatory machinery in the plant that tells it how many flowers to produce. If it produces too many flowers, most of them abort and the plant produces a normal crop as -sft. An sft plant on the other hand produces reduced numbers of flowers, but nearly all of them are large healthy flowers that set fruit. Because there are less flowers to set, the result is reduced production for the plant. But when sft is crossed with -sft, the result is more large healthy flowers that set fruit and a plant that produces typically 30 to 50 percent more total fruit than either parent.

Inbred lines of maize have been selected by seed companies that are capable of 140 bushels per acre production. This is dramatically higher than most composite, open pollinated, or landrace lines. It sounds really good to be able to plant an inbred that is so productive. But it does not sound nearly as good when you consider that making a hybrid with that inbred line can raise production to 210 to 250 bushels per acre.