At the start of this thread, I said I was using mandarine, orange, and P. trifoliata.
Mandarin because it is my favorite citrus, it is often zygotic, and it is moderately precocious.
Orange because I had access to OP Sanford seeds, and oranges are much like mandarins.
Trifoliate orange, obviously, because it is winter-hardy in zone 5. It has no other excuse for being in my garden.
Now, in addition, I plan to start another group using finger lime and precocious P. trifoliata. The F1 should be somewhat precocious because of the finger lime. The precocious P. trifoliata is recessive by many reports. The backcross of (fingerlime x P. trifoliata) to precocious P. trifoliata, I hope will segregate for precocity. Also it will be segregating for winter-hardiness and traits that go together to give winter hardiness, i.e. dormancy that lasts all winter, low temperature tolerance.