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Old August 18, 2015   #1
EBHarvey
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Default Fruit Size Genetics?

Most of my crosses have involved small-fruited donors and large-fruited mothers since I've found it easier to collect a lot of pollen from the small varieties that make lots of flowers, and easier to emasculate and pollinate larger varieties with their more durable and larger flower parts to work with. I was aware of the rough formula for determining the subsequent F1 fruit size, but assumed that later generations would segregate out and show a range of fruit sizes. I have a large group of F4s (~200 plants) from a dozen+ initial crosses and I'm seeing a remarkable similarity among fruit sizes - virtually everything is turning up about the size you'd have expected from the F1s - golfball to large egg size, even though I seemed to have had much more variability in the F3s I selected seeds from to grow these F4s.

What's going one here? Are the genes that determine size not dominant/recessive in the traditional sense and sort of "mix" instead to create offspring whose size always falls somewhere in the middle between the sizes of the two parents?
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Old August 18, 2015   #2
Fusion_power
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I hate to be the one to break this to you....


But the genes most responsible for fruit size have a complex relationship. These genes regulate meristem growth. One gene turns on meristem growth. Another gene produces a chemical that turns it back off. Large fruited varieties either produce less of the chemical to turn off meristem growth or totally disable the gene that stops meristem growth. The gene that turns off meristem growth is dominant therefore most tomatoes are small! Druzba is a good example of heavy meristem growth producing large thick walled fruit.

There is another gene that causes fused ovules and is usually identified in literature as "fasciated". We usually refer to it as the "beefsteak" type. Fasciated is a penetrant gene meaning that it will always express to some extent if present, but does not necessarily cause large fruit to be produced. It usually manifests as more seed cavity locules in the fruit. I have a cherry tomato that carries the fasciated gene. Note that I said cherry, as in about 3/4 inch diameter fruit. Brandywine is a good example of a fasciated variety.

Then there is a gene that determines locule number in a regular fruit. Most cherry tomatoes have a gene that produces 2 locules. Larger fruited varieties have variants of this gene that increase the number of locules with 4, 5, or 6 being the most common variants. Eva Purple Ball is a good example of a variety with the 5 to 6 locule variant.

There is another gene with a huge number of variants most often referred to as the "el" gene for elongated fruit. This gene causes the long paste type fruits such as Opalka. It is another meristem growth regulator that affects fruit form. One version causes very flat pancake shaped tomatoes such as Costoluto Genovese. The oblate form gives fruit such as Omar's Lebanese.

So where did you go wrong? Well, it takes all 4 of the above genes plus a few more I did not describe to make large fruit. If you combine the disabled meristem growth regulator plus the fasciated gene plus the multiple locule gene, plus the oblate gene you wind up with large to very large fruit such as Neves Azorean Red.



I thought I might add one more item re introgressing traits. It is usually best to do so in at least 2 stages. Cross a medium sized variety X a small variety and then fix the desired traits into the offspring. Then cross that line with a large fruited variety and again fix the traits. This takes a lot more time to achieve results, but you don't compromise all of the fruit size traits and wind up with a small tomato and a lot of head scratching over genetics.

Last edited by Fusion_power; August 18, 2015 at 10:16 PM.
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Old August 19, 2015   #3
maf
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This also might be worth a read as it has some observations on segregation for size:
http://frogsleapfarm.blogspot.com/20...beefsteak.html

I would not give up hope with the golfball/large egg sized F4 selections completely. With the complex of several interacting recessive size genes there is the chance that you can segregate out some larger ones in future generations if large enough populations are planted. Either that or go back to your largest F3's or F2's and reboot from there.
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Old August 19, 2015   #4
EBHarvey
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Thank you for the detailed explanation. It looks like I somehow missed an important lesson despite reading many thousands of pages about genetics over the last few years - had no idea size was determined in such a way.

Fortunately, I really like tomatoes of the size I'm getting and rarely grow more than a few truly large varieties anyway - Opalka, Skykomish, maybe a Sudduth Brandy every other season), and I wasn't looking to improve upon any existing cherry varieties as I have my favorites there too. The whole reason for starting this project in the first place was to "fill the gap" so to speak between cherries and beefsteaks in my garden with a little more flavor and visual variation then the Jaunne Flamee, Eva Ball, and Stupice's I'd been growing and I really, really wanted a reliable saladette-sized black which I think I have now in a black pear x green zebra that looks and tastes like a 1/4 sized BTD Pink. All is well - I was just stumped on the lack of variability.
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Old August 19, 2015   #5
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Quote:
Originally Posted by Fusion_power View Post
I hate to be the one to break this to you.....
Thanks for a great explanation.
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Old August 20, 2015   #6
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Quote:
Originally Posted by Gerardo View Post
Thanks for a great explanation.
+1. Thanks Fusion for saving me some fruitless head scratching.
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Old August 20, 2015   #7
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Quote:
Originally Posted by maf View Post
This also might be worth a read as it has some observations on segregation for size:
http://frogsleapfarm.blogspot.com/20...beefsteak.html

I would not give up hope with the golfball/large egg sized F4 selections completely. With the complex of several interacting recessive size genes there is the chance that you can segregate out some larger ones in future generations if large enough populations are planted. Either that or go back to your largest F3's or F2's and reboot from there.
Thanks also maf, for this excellent link... not only about size but also earliness. This really helps me to understand some of my F2 results this year.
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Old August 21, 2015   #8
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I'm sure this has been posted and gotten buried somewhere in T'ville, but if you want more specific info on some of the genes FP described see:

http://www.plantcell.org/content/16/...S181.full.html

There may be some newer info since publication but this is a good primer.
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Old November 12, 2015   #9
maccherone
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Quote:
Originally Posted by Fusion_power View Post
I hate to be the one to break this to you....


But the genes most responsible for fruit size have a complex relationship. These genes regulate meristem growth. One gene turns on meristem growth. Another gene produces a chemical that turns it back off. Large fruited varieties either produce less of the chemical to turn off meristem growth or totally disable the gene that stops meristem growth. The gene that turns off meristem growth is dominant therefore most tomatoes are small! Druzba is a good example of heavy meristem growth producing large thick walled fruit.

There is another gene that causes fused ovules and is usually identified in literature as "fasciated". We usually refer to it as the "beefsteak" type. Fasciated is a penetrant gene meaning that it will always express to some extent if present, but does not necessarily cause large fruit to be produced. It usually manifests as more seed cavity locules in the fruit. I have a cherry tomato that carries the fasciated gene. Note that I said cherry, as in about 3/4 inch diameter fruit. Brandywine is a good example of a fasciated variety.

Then there is a gene that determines locule number in a regular fruit. Most cherry tomatoes have a gene that produces 2 locules. Larger fruited varieties have variants of this gene that increase the number of locules with 4, 5, or 6 being the most common variants. Eva Purple Ball is a good example of a variety with the 5 to 6 locule variant.

There is another gene with a huge number of variants most often referred to as the "el" gene for elongated fruit. This gene causes the long paste type fruits such as Opalka. It is another meristem growth regulator that affects fruit form. One version causes very flat pancake shaped tomatoes such as Costoluto Genovese. The oblate form gives fruit such as Omar's Lebanese.

So where did you go wrong? Well, it takes all 4 of the above genes plus a few more I did not describe to make large fruit. If you combine the disabled meristem growth regulator plus the fasciated gene plus the multiple locule gene, plus the oblate gene you wind up with large to very large fruit such as Neves Azorean Red.



I thought I might add one more item re introgressing traits. It is usually best to do so in at least 2 stages. Cross a medium sized variety X a small variety and then fix the desired traits into the offspring. Then cross that line with a large fruited variety and again fix the traits. This takes a lot more time to achieve results, but you don't compromise all of the fruit size traits and wind up with a small tomato and a lot of head scratching over genetics.

Dear all,
someone know the name of gene for expression meristem growth and meristem off?
Thank you in advance.

Maccherone
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Old November 12, 2015   #10
travis
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I dont know much about the individual genes that control fruit size in the segregating populations, so I can only offer anecdotal information.

My cross of Green Giant x Yellow Submarine (I was originally shooting for a green grape and a green pear, both with potato leaf foliage) gave the expected golf ball size and shape in the F1. Most of my F2s were the same size, but a few expressed "fasciated" to the extent that they had multiple locules or had a boat shape (or "Popeye jaw" shape) fruit.

In the F4s, especially, some of the vines began yielding 4-ounce to baseball size tomatoes with multiple locules and evenly round or flat-round shapes. These are the recombinations that have become known as "Gail," "Melon Ball," and "Green Gail." But most of the recombinations continue to be round ping pong/golf ball or saladete sized piriforms.

Now, I obtained seeds for Suncoast tomato from Dr. Scott at the Univ. of Florida, Bradenton. I wanted the seed because Suncoast is the base input for many of IFAS's high crimson breeding lines, and I was interested in combining it with some of my gf tomato lines.

Anyway, with the seeds came a pamphlet explaining the development of Suncoast, and a handwritten note from Dr. Scott saying that Suncoast had been one of the cultivars he had found can up-size outcrosses to cherry tomatoes very quickly (thru backcrossing).

He also explained that the outcrosses had been made to wild type tomato relatives to obtain disease resistances, then using Suncoast to up-size the selections quickly.
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Old November 12, 2015   #11
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The gene names you are looking for are "wuschel" and "Clavata". Keep in mind that these gene variations are only part of the genetic changes involved in large fruited tomatoes.

There are better articles with more detail, but this one is a good starting point.
http://www.cshl.edu/news-and-feature...-tomatoes.html
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Old November 13, 2015   #12
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Another good paper: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1461635/

They find that 67% of the size difference from two test strains (tiny and huge) can be attributed to 6 genetic loci. They don't talk about the 33% of the size difference that they couldn't attribute to specific loci, which is probably caused by lots of genetic difference that have very small individual effects.

It is informative to learn about the big players (like wuschel and Clavata), but don't lose sight of all the little players that don't have names.
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Old November 13, 2015   #13
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Is it possible for an F1 cross to have fruit larger than either of its parents or will the fruits size always be smaller than the larger of the two parents i.e., when crossing two small fruited types?

Last edited by dfollett; November 13, 2015 at 10:08 AM.
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Old November 13, 2015   #14
Fusion_power
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It is possible for two small fruited lines to make a larger fruited F1, particularly when the Sun and/or El genes are involved in one line and not in the other. I have at least one cross with Rio Grande that demonstrates this effect.

But on average, crossing two different size varieties will lead to fruit smaller than the larger variety in the F1. The reason is that most of the time small fruit genes are dominant and large are recessive.
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Old November 13, 2015   #15
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If Meristem growth is recessive, should we be able to get about 25 percent MG plants in our f2, and if we select those wouldn't it stick?
I asked because my Brandywine x Bloody Butcher cross f1 was smaller tomatoes (4 Oz). I chose a beefsteak looking f2 that was larger (8 Oz), and only had two out of ten. The F3's are all 8 Oz and larger, up to a pound (18 plants grown)
Thanks just trying to figure it out too.

Fusion, Chris, Travis, others, thanks for taking the time to write posts like that.

Quick edit here to ask another question, I also selected an f2 that was a small pink tomato. So if I absorbed your lessons I know the pink is the recessive color coming back out, it will stick now, and I should still see some large tomatoes in the future lines because the Meristem growth is inactivated or disabled( dominate), but the recessive still lurks in the line? Something like that ? lol hahaha

Last edited by AKmark; November 13, 2015 at 03:13 PM.
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