The recent discussion of F1 and F2 brought back to mind a problem that has been nagging at me. I aim to stir the pot a little too.
In general the distinctions of F1 and F2 are useful when trying to do classical Mendelian genetics. For some crops like soybeans that are selfpollinating 99.9% of the time, you have to really work to make the F1. The F2 takes care of itself so to speak. It gives recombination of all possible combinations of the genes carried by the two parents. From these you can do selection for the best combinations after a couple of generations of selfing and selection.
For other crops like corn (maize) two inbred lines are crossed to produce a single-cross hybrid. It has greater vigor than either of the two parent lines. When a field full of “essentially identical” hybrids cross pollinate by wind blown pollen, they give the usual corn that we eat as sweet corn or feed to cattle etc. That is the equivalent of self-pollination if the field is big enough that only this hybrid is represented in it. But if you plant those seeds from the “self-pollination”, you will now have a mix of some millions of different genotypes which range all the way from one parent to the other, through the hybrid, and into combinations never gotten before, just like with the soybean sel-pollinating. Some will be good, but some will be poor combinations. So for a whole field grown from this hybrid F2 seed, the yield, which is what we care about, will be less than from the first hybrid (F1) seed. Maybe not a lot less but certainly less. That’s what keeps Pioneer in business.
With species of roses we may have something line an inbred line. Or perhaps we may not. As Carlson-Nilsson showed, R. rubiginosa is pretty uniform, but some other species are not. When you cross two diploids you will initially have equal numbers of genes from each parent. Because of other factors like imprinting, methylation of DNA or arrangement of genes on chromosomes, you might see that the offspring resemble one parent more than the other in some traits.
Now we get to the thing that is bothering me. It has to do with counting on parentage as if we were royalty talking about bloodlines. If there is no selection at all, and if all genes are freely assorting, then stirring them together in a series of crosses is like mixing barrels of barley. After ten generations, the barley would be diluted 2 to the tenth or 1024 x, so each barrel would contain about 1/1000 a barrel of the starting kind. Simple dilution. Looking at a pedigree we’d say the 2nd generation would be 1/4 species A if A was a starting parent. So wichuriana blood is diluted out in each cross if you think about it this way.
But in fact, we do some very strong selection, and the genes are not freely assorting like grains of barley. They are often rather tightly linked. So with strong selection you could maintain most of the wichuriana traits and introduce just one or two important characters, if they are easily selectable, controlled by single genes, and not linked to detrimental characters. If only!
My point is that bloodlines probably don’t mean much in roses. What we need to look at are the actual traits of the rose we plan to use as a parent and then select its offspring for the traits we want. So, don’t mess with non-hardy roses if you want hardy ones (unless for instance there is a color you need that is simply unavailable in hardy lines). Likewise, unless you believe in miracles or know something no one else knows, don’t hope for disease resistant offspring in crosses of susceptible lines.
I’m not trying to spoil anyone’s fun and I don’t follow these rules rigidly myself, but I do keep thinking about it. Practical implication- there’s plenty of genetic variability to work with in roses already sort of in commerce for a number of traits like hardiness or disease resistance. So a newcomer can start anywhere, if they know what they want to get and can identify interesting parents that have one or more of the traits wanted. The challenge is then to break linkages, and combine multiple genes to make proper pathways. And do lots of crosses.