Questions about crossing tomatoes

[dfollett]

I understand that seeds from any fruit off the same tomato plant are identical, so it doesn't matter whether the seed is from a large or small tomato; the potential of each is the same. Now the question:

If I cross-pollinate several tomatoes on a single plant from pollen from the same donor plant, do those fruits each carry seed that are identical or do they have different potential (assuming the cross-pollination was successful in each case)? Same question, but involving crosses with the same variety of parents as above, but using pollen and female from different plants of those same varieties?

Next, will the F1 seeds from each fruit produce identical (with the same potential) offspring or would seeds from larger/earlier fruits have different potential? I know production from the F2 seeds can go anywhere the genetics allow, but do seeds from each F1 plant carry the same potential, or is there different potential depending on the size/vigor/earliness of the F1 plant?

[joseph]

No matter how many generations of inbreeding are attempted on a tomato plant, there is always some small chance that a tiny amount of variation is preserved. So even when crossing a plant with itself there is the possibility for something new to arise. If two highly inbred varieties are crossed then most of the offspring most of the time will turn out like each other, again with a tiny chance of something new showing up.

If you make crosses using varieties that contain more biodiversity, then you get more chances for something new to show up. Some varieties are highly inbred, while other varieties contain more diversity. I don't know of a good source that rates cultivars based on genetic diversity.

If you use hybrids or the recent offspring of hybrids in the crossing, then all bets are off regarding what the offspring will be like. In very general terms, the offspring of hybrids tend to resemble their parents, and the traits tend to fall about mid-way between those of the two parents.

Originally Posted by dfollett

is there different potential depending on the size/vigor/earliness of the F1 plant?

I believe that there is different potential even among varieties that are highly inbred because if a plant is growing better and producing larger fruits it may also produce slightly larger seeds. The difference is size might be subtle, but adding 5% or 10% to the weight of a seed gives it that much more energy to germinate a little earlier and to capture a little more sunlight, and to out-compete it's clone that got a slightly slower start.

[Lee]

Originally Posted by dfollett

If I cross-pollinate several tomatoes on a single plant from pollen from the same donor plant, do those fruits each carry seed that are identical or do they have different potential (assuming the cross-pollination was successful in each case)? Same question, but involving crosses with the same variety of parents as above, but using pollen and female from different plants of those same varieties?

Simple answer, Yes. That's how hybrid seed is made.

Lee

[Fusion_power]

Joseph's answer gives a lot of detail. I would add that it all depends on how heterozygous the parents are. The more inbred, the less heterozygous, therefore the more alike the F1 hybrid plants will tend to be.

Epigenetics is when the environment a plant grows in can affect genetic expression in the seed. This can for example increase or decrease cold tolerance in a seedling depending on how the parent plant grew. Epigenetic effects tend to emphasize what is already present, it does not usually express a trait that is unexpected. One exception to this is the gs (green stripe) gene which can in some cases be increased or decreased in expression in the seed depending on how the parent plant expressed the gene as a result of growing conditions.

[carolyn137]

Originally Posted by Lee

Simple answer, Yes. That's how hybrid seed is made.

Lee

I'd have to look it up but I think there can be a difference in switching the parents due to cytoplasmic inheritance.

I was sent seeds of four varieties, early varieties, and each were crossed with Brandywine.

I was sent the reciprocal crosses for each as in:

Brandywine X Stupice
Stupice X Brandywine

... the female parent being the first one listed in a cross.

I saw no difference in the reciprocal crosses of those, but the subject of maternal influences being important did come up then.

I forgot what was said, plain and simple, but I know it's probably in someones brain somewhere, or Google, which has no brain.

Carolyn

[dfollett]

Thanks all for your answers.

Originally Posted by joseph

If two highly inbred varieties are crossed then most of the offspring most of the time will turn out like each other, again with a tiny chance of something new showing up.

Originally Posted by Fusion_power

Joseph's answer gives a lot of detail. I would add that it all depends on how heterozygous the parents are. The more inbred, the less heterozygous, therefore the more alike the F1 hybrid plants will tend to be.

Is there a simple way to determine how highly inbred or heterozygous a variety is - a place to go to look up for common varieties, or characteristics to look for that indicate how inbred a variety is?

[joseph]

Originally Posted by dfollett

Is there a simple way to determine how highly inbred or heterozygous a variety is

Plant a bunch of seeds in a flat. If every seedling comes up on the same day, and has the same shape, and is the same height, and the same color, and grows just as fast as every other then chances are that the cultivar is highly inbred.

On the other hand, if germination dates vary widely, or some cotyledons are long and skinny while others are short and fat, or if some stems are almost purple while others are green, or if the height varies widely from plant to plant, or if some stems are highly hairy and others have fewer hairs, then chances are pretty good that there is a lot of genetic diversity in that cultivar.

Later on you may notice other differences in things like leaf shapes, or stem thickness, or slight differences in fruit color/shape/size, or growth rates, etc. Some cultivars are strikingly uniform. Other cultivars have more variation. There may also be variations in soil/light/water/bugs/weeds/etc that cause differences in growth between highly inbred cultivars.

A good strategy for weeding out inadvertent crosses is to plant the seeds in bulk, and then transplant only the uniform looking plants, tossing aside the odd-balls. (In my garden I do the opposite, the oddballs that might represent a natural cross pollination event get a place of honor in the field.)

[Stvrob]

I've often wondered, for seed saving purposes, would there be any point in crossing between 2 plants of the same variety?