How do you breed a cybrid?

[Shule]

My understanding is that a cybrid is a lifeform that has the mitochondria of one species or breed, and the nucleous of another. How do you actually breed these on purpose? Do you just make a bunch of hybrids and hope it happens? Do you need a team of scientists to do it in a lab?

[bower]

Shule, mitochondrial DNA is inherited exclusively from the mother plant.
So if, for example, you use a different species/ wild relative of tomato as the mother in your cross, all the offspring will have that wild relative's mtdna.
The nuclear DNA, however, will be a mixture of the two species.

I would think that a "cybrid" as you defined would be impossible without some kind of genetic engineering to do the deed.

[Frosti]

Shule, mitochondrial DNA is inherited exclusively from the mother plant.
So if, for example, you use a different species/ wild relative of tomato as the mother in your cross, all the offspring will have that wild relative's mtdna.
The nuclear DNA, however, will be a mixture of the two species.

I would think that a "cybrid" as you defined would be impossible without some kind of genetic engineering to do the deed.

I believe there is a way. The mitochondrial DNA comes from the female parent, so all one would have to do is backcross a few times to achieve a plant with the nuclear DNA of one species and the mitochondrial DNA of another.
Let's say you want the mitrochondrial DNA from plant A and the nuclear DNA from plant B.

1. Use the pollen of plant B on the stigma of plant A to create plant C with the mitochonrial DNA of plant A and the nuclear DNA of both plant A and B at a 50:50 ratio, so 50% of the nuclear DNA matches plant B.
2. Use the pollen of plant B on plant C. This is the new plant C. plant C still has the mitochonrial DNA of plant A but now possesses (statistically speaking of course)

   Code: Select all

   previous_match + ((1-previous_match)/2) = 0.5 + ((1-0.5)/2) = 0.75 = 75%

   of the nuclear DNA of plant B.
3. repeat step 2 until your threshold is reached.

At least that's my 2 cents, what do you guys think?

[bower]

Absolutely @Frosti, you can do this exactly as you said.
Liberally editing out my mistaken ramblings

Of course it's the same as getting to stability for an OP. And no selection is required. Same cross every year for seven years. Two plants, no need for a lot of space either.

[bower]

The cybrid is a great project. Assuming that important traits like stress tolerance or disease resistance are governed in the mtdna - and chloroplast DNA is also inherited from the mother. That could be a factor in adapting to certain environments.

Here's another thought: by the same math, you could also cross your cybrid F1 with a different tomato every year for seven years, and at the end of the day you will have the same proximity to 100% tomato DNA but still 100% mtdna from the wild mother.

[bower]

There's a tomato I really wanted as the female parent in a cross. Time after time I tried to make crosses with this mother plant, and she refused. I finally gave up on that and used her as the pollen donor.
Another breeder commented that he had the same experience - some plants don't make good mothers for crossing.
Interspecies crosses sometimes are easier with a specific one of the pair being the female parent and not the other.

There are still methods of doing that, embryo rescue and so on.

But maybe it would also help to choose as the father, a tomato that is a good mother? Just in case it might make it easier with the F1..
What do you think.

[Shule]

Thanks for all the responses, everyone. I hadn't considered backcrossing.

@Bower
Regarding some mothers not crossing easily, I've had a hypothesis about that for a while. My hypothesis is that if they don't cross easily in one climate, they might cross easily in another. Maybe with lower or higher humidity, cooler or warmer nights, etc.

[Frosti]

Absolutely @Frosti, you can do this exactly as you said.
Liberally editing out my mistaken ramblings

Of course it's the same as getting to stability for an OP. And no selection is required. Same cross every year for seven years. Two plants, no need for a lot of space either.

Well, I went down the rabbit hole anyway and created a tiny toy model of tomato reproduction, to check if my theoretical gut feeling isn't blatantly wrong.

The parameters are as follows:

• Number of chromosomes: 26 (2n)
• Number of base pairs overall: 900 million (at least that's the value I found for the genome of a tomato)
• Hence number of base pairs per chromosome: 34615384 (of course not true in reality)
• Crossover frequency: 1.6 per chromosome pair per meiosis (pretty hard to find a source for this one, used a value I found for another plant)
• Number of iteratons (one iteration is backcrossing until 100% match is reached): 500

Results:

mean_match.png

So yeah, at least according to this outrageously simplified model, the originally suggested match progression holds true

[bower]

Well - my excuse for not working with wild relatives is that they taste terrible and I have to grow food in my limited space. But it's not really a good excuse, if you follow this approach and only lose two spaces each year. I wonder how many generations it would take for the fruit to be edible and good. Probably much less than the threshhold for a "pure cybrid". The fruit from your Habrochaites cross looks very tempting @Frosti even in the F1.

[Frosti]

Well - my excuse for not working with wild relatives is that they taste terrible and I have to grow food in my limited space. But it's not really a good excuse, if you follow this approach and only lose two spaces each year. I wonder how many generations it would take for the fruit to be edible and good. Probably much less than the threshhold for a "pure cybrid". The fruit from your Habrochaites cross looks very tempting @Frosti even in the F1.

S. habrochaites does indeed taste rather terrible. Strong and weird. I'm hoping for a strong and weirdly good taste somewhere down the line .
S. pimpinellifolium does taste very good though, right from the get-go.
I suppose it will take at least until F2 to produce an offspring with acceptable taste. @Bower are you sure you're not confusing the parental fruit (ordinary yellow dwarf tomato) with the f1 fruit? . I'll post some closeups of the f1 fruits soon.

[bower]

@Frosti I totally did confuse the fruit with the wrong generation.

I grew a Pimpinellifolium once and the fruit was so repulsive, I never grew another. I couldn't imagine having to taste the offspring of this horrible (but pretty looking) fruit in order to weed out the nasty ones. Call me delicate. Not a supertaster, but I may be borderline allergic to some of the substances the tomato family produces. So my taste buds get alarmed.

[Frosti]

There's a tomato I really wanted as the female parent in a cross. Time after time I tried to make crosses with this mother plant, and she refused. I finally gave up on that and used her as the pollen donor.
Another breeder commented that he had the same experience - some plants don't make good mothers for crossing.
Interspecies crosses sometimes are easier with a specific one of the pair being the female parent and not the other.

There are still methods of doing that, embryo rescue and so on.

But maybe it would also help to choose as the father, a tomato that is a good mother? Just in case it might make it easier with the F1..
What do you think.

I agree, it's important to choose the right plant as female parent. As far as I understand it, compatibility is entirely governed by the female parent, is it not? That would explain why crosses from wild to domesticated tend to be more successful than the other way around. I found a table of compatibility between closely related tomato cousins some time ago, that showed exactly that. If only I could remember where I found it ...
I can confirm that crossing S. lycopersicum (male) onto S. peruvianum (female), did not work out. The table said the same thing. The other way around was only supposed to work with embryo rescue. Unfortunately I did not try the other way around. But if I did, I figure I would have seen successful fruit set with inviable seed (due to embryo abortion).