Hybrid corn and the unsettled question of heterosis.

George Shull's 1908 seminal article 'The composition of a field of maize' marked the 'exploitation of heterosis in plant breeding, surely one of genetics' greatest triumphs'. Hybrid corn became a 'symbol of American agriculture' and 'the paradigm for all developments of F1 hybrid crop varieties and more generally breeding. But there is still no consensus on the definition of heterosis while its biological basis, causal factors and genetic mechanisms remain 'unknown', or at best 'poorly understood'. It is thus logical to reverse the usual approach from the exploitation of a mysterious heterosis to the triumph of hybrid corn and focus on what breeders and geneticists do rather than on the theoretical reasons for their success. This factual approach produces surprising results: (i) hybrid corn extends the isolation technique of autogamous cereals to the allogamous maize; (ii) a 'hybrid' is an ordinary corn plantmade reproducible by the breeder and only the breeder. It is proprietary rather than 'hybrid'; (iii) for all practical purposes, heterosis is irrelevant; (iv) Shull justified his 'hybrid' breeding method by the ad hoc argument of maize 'hybrid vigour' which in 1914, he conflated under the name of heterosis with Edward East's concept of physiological stimulation due to heterozygosity; (v) hybrid corn can increase yield only once and by a small margin and (vi) the huge yield gains of the last 80 years came from mass selection, a process inconsistent with the theory of heterosis. In conclusion, the enduring success of 'hybrid' corn was achieved at the expense of farmers, common welfare and biodiversity and dovetails with the industrial agriculture requirements of crop uniformity and breeder monopoly over reproduction. This critical understanding of the paradigm of plant breeding could have important implications for breeders and geneticists.