Backcrossing and F2s

[Patrina_Pepperina]

This past weekend, several Tomato folk got together in Marysville Victoria for a tomato tasting, and Ray (Raymondo), Craig (Kionga) and I discussed backcrossing as a way of hastening the process of finding what you want out of a cross. This was in particular reference to the Dwarf project between north and south hemispherers. Craig brought up the idea with me as we travelled to Marysville. Ray suggested that it might be a good idea to back cross at the F2 stage.

I'm thinking that it might be a good idea too, especially with the cross that turned out to be 3-way, ie. Bashful (GDC x [Orange Strawberry x unkown] which had a smallish semi heart shape red F1). I suspect that it's going to be difficult to find large fruited segregates out of this one.

The other issue (besides backcrossing) that I'd like to know more about is how likely it is to get a large fruited F3 from a smaller fruited F2? This year I had larger globes on an F3 than on the F2 last year for a cross between Earl of Edgecombe and Maritza Rose, but maybe that's just environmental.

I'm also wondering whether it would be a good idea for a great tasting F2 (that might not have the size we want) to be backcrossed with the larger original parent.

Are the gains greater than the losses when backcrossing? I imagine it might broaden the project but maybe it could also shorten it somewhat?!

Patrina

[Raymondo]

As discussed with Patrina and Craig, I think a backcross with the F2 should provide some gains. As Keith said, you're 'loading' up more genes from the large fruited parent.
For those involved in this project, the idea would be to identify the dwarves in the F2 early enough so that you could then cross these, or some of them at least, back to the non-dwarf parent. The way this project is working, it would be the SHers (both actual and honorary) growing the F2s so they would do this cross. This clearly means growing at least that parent as well.
The NHers, who would be growing out the results of this backcross, would then be looking for dwarves in the mix they will get.
I'm really looking forward to following this project as it develops.

[Patrina_Pepperina]

Ray, I'll try to back cross any F2s I grow next season for sure. It would be good if others try it also, just in case the backcross doesn't take, or the large parent plant turns out to be not true-to-type, as in crossed.

It's not difficult to see how involved it can get when looking for something specific plus recessive, and introducing backcrossing as well, so it's great to be able to share the work as we are doing in the dwarf project.

Patrina

[Raymondo]

It would be great if dwarfism turned out to be a single gene. If so, and provided there's no interference, the probability of getting a dwarf with which to do the backcross is 1/4. Statistically, this means that by growing 11 F2s you would be 95% certain of getting at least one dwarf. This would also apply to the F2 of this cross. Obviously, the more you grow out, the better the chances of finding a dwarf.
The backcross F1, being grown out by the NHers, would all be non-dwarves, though they would be very unlikely to be uniform, unlike the original F1, which in theory at least should be relatively uniform non-dwarves.
The backcross F2, and again, it will be the SHers growing this, is where the dwarves will start to pop up again. You could if you so wished back cross again with any dwarves that do pop up but it probably wouldn't be worth the effort.

[JabulaZa]

I am willing to Backcross the F2 but would then also need some parent seeds (preferable of both parents to recognise the dwarfs very early).
Our temps and season length might just allow a second growout and be able send seeds back NH given I can receive the seeds early enough from the NH.

[Patrina_Pepperina]

Originally Posted by Raymondo

The backcross F1, being grown out by the NHers, would all be non-dwarves, though they would be very unlikely to be uniform, unlike the original F1, which in theory at least should be relatively uniform non-dwarves.

I was pondering this point too. Since the F2s are segregating, I think it would be quite interesting to see what differences turn up in the F1s of the dwarf backcrosses.

If the original dwarf F2 has good taste but is too small, backcrossing to the large fruited parent would be the go, but maybe even for a large fruited, poorer tasting dwarf F2, backcrossing could also be advisable to improve flavour?

Patrina

[Raymondo]

Keith,
Good news that the dwarf genes appear to be single. That'll make life a lot simpler.
Patrina,
Flavour, as you know, has many contributing factors: probably many genes and of course many environmental factors. A backcross will move the plant closer to the non-dwarf parent genetically, but whether or not the 'flavour genes' will be among them is anybody's guess!

[celticman]

. I have been quietly fallowing the dwarf project for a while. Seems like you folks have a very interesting project going. I am about as dense as a bar of iron when it comes to genetics however so I would like to know if I have the F2 backgrossing consept sorted out correctly.
d=dwarf
D=nonDwarf
G a bunch of genes
so you had to tomatoes T1 with d(and G1) and T2 with D (and G2)
YOU crossed T1 and T2 and got dD(andG1xG2) F1
You growout and select dd(G1xG2) F2 and cross with T2
to get dD(G1xG2XG2) and BackCross BC1
you grow out and select dd(G1xG2XG2) and begin selecting for other traits you would like potatoe leaf taste five pound fruit ectra....
If at some point you need help with the growing out one of the tomatoes let me know.

[Patrina_Pepperina]

That's pretty much how I understand it Celticman, but I'm not really up on all the genetics. I followed your train of thought just fine. Maybe someone will be able to jump in if there's something wrong in the sums.

You are welcome to join in at the next step - please remind us when the planning time comes for your next growing season

Patrina

[celticman]

I'll send you and e'mail in January to work out the details of helping with the next step. I would hate to workout the statics on a specific gene in the Backcross but I read threw a biology text more less understand the concept. It is a shame that taste is not controlled by a single gene!
Celticman

[Mantis]

As luck has it I am growing a Black from Tula so I should be able to do a cross if needed. Also have two PL Black Krims on the go. HMMMMMM