Genetic uniformity of the single cross hybrids of inbred parent types of complementary genetics is actually potentially an issue as it greatly increases the vulnerability to pathogen and pests of successfully adapting to the genetic combination.

The biggest advantage of the hybrid is its increased yield and quality not its uniformity in field crops but for a crop with a highly specific crop usage that would be different.

The problem of genetic uniformity was brought up in southern corn leaf blight epidemic which was traced to uniform application Texas male cytoplasmic sterility which was used to avoid need to detassel. The quick conversion back to normal cytoplasm and cutting and pulling tassels resolved that issue.

For developing countries where stability of yield is extremely important for subsistent farming the use of blends of hybrids and 3 and 4 way hybrids and synthetic hybrid populations are attractive for combining a high productivity with a genetic diversity which will reduce poor performance in a non optimized environment.

Thanks for an expanded overview Paul Reed Hepperly . Sir, I have been exposed to cotton and in India, it is only hybrids that are being grown. High yield apart, most hybrids these days are being target marketed by private companies. Good move. Avoids the earlier problem of ill fitting hybrids to a particular environment just being pushed because of excess seed availability.

But, what I meant to ask was, is there an upper limit to heterosis? Especially when we know there is Outbreeding Depression to deal with. A hybrid can be GOOD but not the BEST ALWAYS. And most of us are targeting incremental increases in a new hybrid, over an existing check.

Disease and pest tolerance, especially in cotton hybrids today have been taken care of what with new private company players in the country with refined germplasm at their disposal. Value addition to an existing hybrid is being practiced today.

This paper is very valuable. Huang and colleagues examined the genetic state of hybrid varieties of rice with regard to heterosis. They sequenced 1,495 hybrids, performed phenotypic analysis and attempted to determine the nature of alleles associated with superior performance. The results provide both a tremendously valuable resource and new insights into the genetic basis of heterosis.

Huang, X., Yang, S., Gong, J. et al. Genomic analysis of hybrid rice varieties reveals numerous superior alleles that contribute to heterosis. Nat Commun 6, 6258 (2015) doi:10.1038/ncomms7258

Thanks @ Alireza H Hasanalideh for pointing to the paper. Somehow, I believe that even Heterosis follows the Diminishing Marginal Utility curve so well explained in Economics!!! Somewhere we hit an upper limit to heterosis and things start going downhill from there. Again, thinking of predictive models what with Artificial Intelligence tools now becoming widely available.

Studies comparing older and newer major crop varieties would indicate that about one half of the major increases in yield are due to improved genetics and the other one half from improved management strategies. The ability to make big genetics can show some diminishing returns as would input strategies but there is no indication that either strategy is tapped out and especially under low input systems the yield on a farm level is a minority fraction of the yield potential.

True Sir Paul Reed Hepperly . Wish we do not hit that predicted plateau of heterosis in the near future. There are some varieties in crops like safflower, Annigeri-1, which seems to have had the best combination of genes God could have thought of. It was released in 1969 and has no true competitor till today. Even hybrids yield less on a sustainable basis. There are other crop examples like sorghum, Bengal gram and paddy. In this backdrop, we need to have a re look in to hybrids, notwithstanding the tremendous yield potential realized with heterosis.

In hybrids, the redistribution of plastics substance is aimed at maximizing the adaptive effect of the first generation at the expense of greater potential for variability in the second and subsequent ones, which makes it possible to give more seeds and bypass other genotypes quantitatively in the population.

Thanks Yevgeniy V. Zaika. True that the hybrids are USUALLY more productive than their parents. There would be an upper limit to the maximization of the adaptive limit of the F1 as the parents would already have some adaptivity built in them. Rarely we come across 2 very poorly adapted parents giving a super hybrid. The question is how far a hybrid can maximize its productivity? Or, is there a BEST hybrid? Environment aside.

It is obvious that genetic systems are being activated which increase the efficiency of synthesis of the necessary enzymes and metabolites for a more powerful development of vegetative and generative organs of plants. In my opinion, neither the domination nor the non-allelic interaction fully explains heterosis. There is also an epigenetic explanation. The only thing I'm sure is that the maximum level of heterozygosity of the parent components is required. Your example of poorly adapted hybrids is a case where the components with high combinability have met. Such crossings are not always successful. Hybrids may also have depression of some signs.

Yes Yevgeniy V. Zaika . We have discussed Outbreeding Depression. My question is, how far a hybrid can reach in terms of yield given the best adaptability, the best combining ability and the best mix of epigenetic factors etc that come together in it. Surely, we are in a constant look out for the best mix of all the three above mentioned factors so that we end up with a very good hybrid. Thank god, nobody has got a product with the best mix and hence we keep generating new and productive hybrids every year. In sum, we keep hitting a new normal.

But, the question remains. Can we define a best hybrid AND GET IT :)