"Are hybrid seeds designed to be one use only like terminator seeds? How is the vendor lock-in structured?"
Plant breeding 101:
Every individual has two copies of each gene: one comes form the father, on from the mother. It has been observed that individuals who mostly have different maternal and paternal versions, are more vigorous. This is called heterosis, or hybrid vigor [1].
Now, just to produce children with a high number of genes that have different paternal and maternal copies, it would be enough to have a father and a mother who mostly do not have same copies. But if father has versions G1 and G2, and mother has versions G3 and G4, the child can have any of (G1,G3), (G1,G4), (G2,G3), (G2,G4). So the features of the children are not predictable.
So plant breeders have invented this method, to use inbreeding to produce highly homozygous [2] lines first. Suppose (G1,G3) is the type you want. So first breed line A of plants which always have (G1,G1) and similarly for other genes. And breed line B which consistently have (G3,G3).
Now use A as father and B as mother, and voila, you can produce offspring that consistently have (G1,G3). So the offspring exhibit hybrid vigor, but are also predictable and similar to each other. Quality and consistency!
Now sell these seeds, everyone wants to buy highly productive consistent quality seeds. Profit!
But as long and you are the only possessor of the parent lines A and B, you are the only one who can produce this particular type of hybrid offspring. The customers only get the offspring, and you keep the lines A and B safe and secret.
If the customer tries to mate those (G1,G3) individuals with each other, they will get a mix of (G1,G1), (G1,G3) and (G3,G3), so the next generation will not be nearly as productive crop as the pure hybrid seed was.
So, the "vendor lock-in" is structured by not giving anyone else access to the parent lines A and B.
This is not intentional. Its how genes work. The seed on a plant depends on the pollinator. Just like fruit trees - you plant that seed from the apple you just ate, it won't grow more of those apples. It will grow whatever that apple was pollinated with.
This is true for many flowers and garden crops, it is not true for corn, soybeans, & similar.
The viability of hybrid offspring depends on the closeness of the parents. Lots of flowers can be bread from very diverse stock and the offspring are sterile, cash crop hybrids are not made from such diversity and are generally re-plantable.
Corn is generally carefully crossed as a last step in hybridization. The next generation will grow a plant almost completely dissimilar from the one being sold.
Its an issue trying to support farmers in developing countries. Our Romanian sister church didn't accept our corn hybrids. They had their own 'land race' seed they've been creating for generations, which works well on their difficult soil. If we sent them hybrids, they'd work for one season. And then they would be dependent on us for continuous shipments of seed each spring. We left them with their self-sufficient seed.
