[lathyrus_long]

It's not fundamentally incompatible, but non-proprietary hybrids are extremely rare. The effort to maintain the two parents and do the cross is much greater than that of saving seed from an open-pollinated variety, so very few growers will undertake that even given the chance. Even if multiple growers do, the smaller population of plants in each of the pure lines increases the chance of significant divergence between sites, at which point the multiple production sites implicitly become multiple different varieties.

Anyone interested here might also wish to read Carol Deppe's "Breed Your Own Vegetable Varieties". She's a Harvard-trained geneticist and amateur vegetable breeder, with special interest in open-pollinated varieties derived from proprietary F1 hybrids. Her book extensively discusses the underlying biology, the practical breeding process, and the legal situation of such work.