Crimson Crush tomatoes

[Martin H]

Thanks for the research Mark, I've never understood the science behind hybrid vigour. The RHS article led me to the Wikipedia article on Heterosis, which tells me the scientists aren't sure of the reason either. So I don't feel bad...

[Chris11]

I think the best way of understanding hybrid vigour is to see it as the opposite of the potential negative effects of inbreeding.

For many genes, there are two or more variations (alleles) that can exist in a population, all part of natural genetic diversity. An individual will have two copies of a gene, one from each parent. This pair of genes could be the same (homozygous) or different (heterozygous).

There are genes which may be beneficial when there is only one copy, but detrimental when two copies are present. Where there are two different versions of a gene present, there is the possibility that this will be more beneficial for the individual than having two copies of either gene.

The breeding lines that are used to create F1s are highly inbred and highly homozygous, so the plant breeders know what each parent breeding line will be contributing. By contrast, the F1 hybrids will be highly heterozygous, leading to hybrid vigour. Commercial F1s are being bred for other characteristics, the hybrid vigour will be a welcome side effect I guess, and likely a stronger effect in some F1 varieties than others.

I've bred out F1s myself, both tomatoes and chillies, with predictably diverse results which usually get abandoned after a generation or two. I'm up to at least an F4 for a couple of lines, which is fascinating. So I wouldn't suggest not doing it, but I would make it a small part of your growing plans, especially if you have limited space.

[Cadalot]

The way I look at it is I have the seed from last years toms, because I wanted to see what I would get if I sowed them this year. With the amount of toms I grow, its got to be worth a try just to see

[MarkPelican]

This could be a completely different thread I suppose and I apologise if I'm hijacking the thread. However:

If inbreeding is "bad" (and I understand how it can be) how do Heritage tomatoes survive? They are obviously "in-bred" because otherwise they couldn't run true. I recognise that where there is a field of the same variety the ones in the middle are likely to be pollinated by different plants of the same variety but I suppose my question is "how diverse is the population of (say) Moneymaker? If they're diverse then why do they remain Moneymaker and if they're not diverse why don't they die out through inbreeding.

Following on, how diverse are the seeds in a single tomato? Are they all genetically identical? The reason I ask is mainly just interest but also because is keeping seed from a single tomato or plant enough genetic variation to prevent inbreeding in future generations or should I be keeping seed from multiple plants and making sure that I grow plants from different collections to maintain the diversity?

(Note I've been keeping seed for a number of years from a number of varieties and haven't noticed any reduction in crop rates).

[Chris11]

Mark, those are good questions

Many plants, like tomatoes, are largely self-pollinating. Others are largely cross-pollinating (outbreeding), and suffer inbreeding depression very quickly if there isn't a sufficient cross-pollinating population, sweetcorn being the usual textbook example.

One of the factors with inbreeding depression is increased homozygosity for detrimental (deleterious) genes. In other words there is an increased chance of having two copies of the same deleterious gene.

Selfing species will be far more inbred and homozygous, outbreeders will be far more genetically diverse and heterozygous.

Selection pressures on deleterious genes are different in selfing and outbreeding species. These selection pressures are both natural selection, and artificial selection by you as the breeder saving from your best plants and fruits. In selfing species, selection pressures keep detrimental genes at much lower frequencies.

Heritage/OP tomatoes are inbred by definition, so they don't have hybrid vigour or much genetic diversity (at least within each variety), but they have very few deleterious genes because those genes are naturally, and necessarily, removed by selection pressures. Genetic diversity for Heritage/OP tomatoes is of course maintained across all the different varieties collectively, not so much at the individual variety. Confusingly, OP means they can be open-pollinated, but in practice they're largely self-pollinating.

Outbreeding species can tolerate greater frequency of these deleterious genes, because most of the time they will only exist as one copy. And some of these genes are not actually detrimental and may be neutral in effect, or even benefit the individual plant providing they only have one copy.

In terms of our own tomato seed saving, then ideally we'd have several plants of the same variety and select the best plants and fruits to save from. In practice, as you've found, as long as the plants you've grown seem to be healthy and true to type, then they will probably be ok for viable seed saving.

I think all the seeds produced in a single tomato will be fraternal twins, and not identical.

[MarkPelican]

Chris,

That's a wonderful answer. So, if I went to the effort to cross two similar Heritage tomatoes, would the resulting F1 hybrid be more vigorous than the original varieties? Or is hybrid vigour something you have to experiment with?

[Chris11]

Thanks Mark.
If you can successfully make an F1 cross from two different varieties, then in theory at least, you are likely to see some hybrid vigour. Although it may be hard to quantify when you see it, because so many other factors are affecting our plants and we're not growing enough plants to control for other variables.

What varieties are you thinking about trying to cross?

[MarkPelican]

Of the ones I'm growing this year, I could try Jubilee and Summer Cider. They're both Orange Beefsteak so I would expect to get an Orange Beefsteak as a result. Now I'm going to have to look how to hand pollinate them!

[TattyJJ]

I fund this whole discussion quite fascinating!

It leads me to the question.... I am planning to grow a mix of tumbling tom, jersey devil and san marzano in my greenhouse, all of which are (i think) open pollinated.
So would any saved seeds likely create new or random or just dodgy tomato plants?

[Martin H]

Unless you do anything special they will most likely self fertilize, so each tomato will contain seeds that will grow just like their parents.

If you want them to cross you will have to manually open the flowers and remove the stamens to stop them self-fertilizing.

I think.

[Chris11]

Of the ones I'm growing this year, I could try Jubilee and Summer Cider. They're both Orange Beefsteak so I would expect to get an Orange Beefsteak as a result. Now I'm going to have to look how to hand pollinate them!

That looks like a good choice Mark. I've read the notes for both varieties on your website, so I'd guess you'd be hoping for an earlier and more productive Summer Cider, which was bigger than a Jubilee?

[MarkPelican]

My own experience is that in general tomatoes (except F1 Hybrids) are self fertile and will run true. There are some varieties which are more prone to cross and these tend to be the potato leafed varieties, particularly those where the stamens stick out from the end of the flower.



The above is a picture of flowers from an Orange Berry, you can see the stamen sticking out from the end of the flower.

Whereas with these (Maskotka), the stamen is within the flower.


I have had some accidental crosses, in particular a yellow cherry (Oleron Yellow) where I ended up with a mix of yellow and red cherry tomatoes. I have been growing these out for a few years, selecting the yellow varieties. At the moment, I am on the fourth generation and have two distinct yellow cherry varieties which are slightly different colours and ripen at different times. Interestingly the seeds of the yellow varieties and the red ones (which I'm not growing on) looked different so I was able to select by eye before sowing.

[Cadalot]

OK I now they are F1 and may not run true, but I saved seed from the free plants I got last year because I want to see what I get if I sow them this year.

[Thelma Sanders]

I cut all the armpits off the over-wintered plant, except the bottom one, and pruned the plant down to that level and it is slowly growing.
None of the armpits seem to be growing roots, though

[sparrow100]

My one surviving overwintered plant is flowering. But I don't think I'll be repeating the experiment - too much faffing!