// Twitter Cards // Prexisting Head The Biologist Is In: tomatoes
Showing posts with label tomatoes. Show all posts
Showing posts with label tomatoes. Show all posts

Thursday, April 5, 2018

The Naming of Things

If you've been following me here for a bit, you've probably noticed I'm interested in plant breeding (especially garden veggies). My main goals are to have healthy plants that grow and produce well for me with minimal inputs in my short-season climate. The measure of, "tasty" I go by is what tastes good to me and my family, with what other people consider tasty (when I occasionally do taste-tests) held to a lesser significance.

Two large cherry sized, blocky, white tomatoes. They're sitting on a notebook with a sketched map of the garden, showing where all the plants are and which plants were grown from the same batches of seed. There is a blue pen pointing at the specific plant which produced the fruit.
From 2017, with garden notes.
I've been working with tomatoes for several years and have developed some more fine-tuned ideas about what I want the plants to become. One of my lines, seen at right, is approaching stability. That is to say, most plants from one year to the next produce very similar fruit. The fruit are blocky, large-cherry sized, white (well, paler than yellow) in color, and have a very thick outer fruit-wall (not the skin). They've tested well with people in and outside my immediate family, so I've been thinking about the possibility of distributing their seed in the future.

A few dozen of the large white cherry tomatoes sitting on a white plastic cutting board.
From 2016.
In my personal notes, I've been using the rather uncreative name of, "Abbey White" for these tomatoes. It is sufficiently descriptive to let me know what I'm talking about in my notes, but it isn't a name I expect to attach to the variety when/if I start distributing it. I could easily adjust it to, "Abbey's White", but I'm not sure I want to go with that either.

In the forground is a ceramic bowl filled with diced white tomatoes. In the background is a large wooden cutting board covered in white, yellow, and orange tomatoes (as well as a few green tomatilloes).
From 2016.
"Wait. Tomatoes are red, right!?" A white tomato might seem kinda unusual, but it's just one of a very wide spectrum of colors that tomatoes can be found in. (Check out these companies I have no affiliation with: Artisan Seeds, Baker Creek Heriloom Seeds, TomatoEden, and SeedSavers Exchange. There's so much more diversity in color and taste available if you're willing to grow tomatoes from seed.) My tomatoes tend to be any color but red. Red fruit that have turned up in my garden have tended to have a taste I didn't favor, so over a few years I stopped growing as many red tomatoes. I expect I'll need to bring in some new genetics before I can grow red tomatoes that will taste good to me.



While I was thinking about how to go about naming this variety (and others in the future), I came across twitter user @JanelleCShane. She's been playing with Recurrent Neural Networks (basically a type of AI (specifically a type of machine learning)) trained on diverse datasets, like fruit names (or knitting patterns (or Irish melodies)), so I tweeted:

(I only later noticed my garbled grammar.) I was somewhat surprised when she responded back, asking if I had a list of tomato variety names she could train her AI with. I didn't, but I was pretty sure I could pull one together pretty quickly from online resources. After some looking, I found several sources ([1], [2], [3], [4], [5], [6]) with large lists of tomato variety names. To avoid spending too much time gathering the names, I wrote web scrapers to process each source and output text files with lists of names. In total, across the six sources, I collected 11,719 distinct tomato variety name strings. Some may represent extinct varieties. Some are in other languages. Some are numerical codes. There's also capitalization and spelling variations. I threw them all into a file that Janelle could use to train her AI.

Have a look at her blog post on the tomato name trained AI at: http://aiweirdness.com/post/172622965862/tomatonames



So. What did the trained AI come up with? Well, at first the AI got overly fascinated with the numerical code names in the training dataset. It produced lots of new "names" that would be quite not useful for naming a new variety. Janelle stripped out most of the code names from the list and trained the AI again.

This time there were some really good results, some really wrong results, and all sorts of weirdness in between. I've highlighted some of my favorites from each category.

The Good,the Weird,and the Wrong.
  • Floranta
  • Sweet Lightning
  • Speckled Boy
  • Flavelle
  • Market Days
  • Fancy Bell
  • Pinkery Plum
  • Mountain Gem
  • Garden Sunrise
  • Honey Basket
  • Cold Brandy
  • Sun Heart
  • Flaminga
  • Sunberry
  • Special Baby
  • Golden Pow
  • Birdabee
  • Sandwoot
  • Bear Plum
  • The Bango
  • Grannywine
  • Sun Burger
  • Bungersine
  • First No.4
  • Smoll Pineapple
  • The Ball
  • Golden Cherry Striped Rock
  • Eggs
  • Old German Baby
  • Frankster Black
  • Bumbertime
  • Adoly Pepp Of The Wonder
  • Cherry, End Students
  • Small Of The Elf
  • Champ German Ponder
  • Pearly Pemper
  • Green Zebra Pleaser
  • Flute First
  • Speckled Garfech
  • Green Dork
  • Cluster Gall
  • Shirve’s Gigant Bullburk
  • Giant Ballsteak
  • Black Crape
  • Brandywine, True Grub
  • Caraball
  • Ranny Blue Ribber
  • Roma Wasting Star
  • Scar Giant
  • Bug Beauty
  • Banana Placente
  • Bananana
  • Stoner
  • Speckled Bake
  • Ruck
  • Green Boor
  • Wonder Bagg
  • Sun Bung
  • Bellende
  • Shart Delight
  • Solad Piss


There were also a collection that would fit perfectly among the real tomato names, though they'd be kinda strange in other contexts.
  • Matt's Sandwich
  • Indigo Tree
  • Striped Hollow Potato Leaf
  • Lelly's Yellow Stuffers
  • Terra Pink Strain
  • Greek Boar
  • Ton's Oxheart
  • Babla's German Paste
  • Mortgage Lifter, Honey Blues

I really like when the AI tried to name a tomato after a person. It didn't have enough examples for real human names, but it gave it a good solid try.
  • Matt's Sandwich
  • Lelly's Yellow Stuffers
  • Ton's Oxheart
  • Babla's German Paste
  • Shirve’s Gigant Bullburk

Amusingly, the AI came up with an existing name that wasn't in the training dataset. "Sunberry" is the name of another fruit. It's a close relative of the tomato, so I think I'll call that a positive score for the AI.



Do any of these names fit my tomato? I'm not sure. I do rather like, "Flavelle" and, "Mountain Gem". I'll probably have to let the ideas ferment a while before I come to a decision.

I have recently seen a tomato that the name, "Speckled Garfech" would be perfect for. It came out of someone else's breeding program, so I won't share a photo. Imagine a yellow/orange striped tomato covered in green spots.
Two photos combined. The top half is a photo of a large yellow ceramic bowl filled with small cherry tomatoes. The cherry tomatoes area a mix of white and pale orange with a pink blush on one end. The bottom half is a photo of a closeup of a single larger tomato that is white with pale dark stripes. There are smaller red tomatoes and other items in the background.
From 2017.

I've got a couple more tomato lines that I'd like to stabilize (photographs at right). The upper photo shows a mix of small, very sweet cherries in pale-yellow/white with a pink blush on the bottom end of some. I'll be growing seeds from the ones with the blush. I expect the same phenotype will turn up next year, but I'm also sure there are lots of recessive alleles still hiding in them (for larger fruit, other tastes, and not having the blush).

The lower photo is of a larger, meaty white with pale stripes. This one is a bit further along already thanks to some lucky genetics, even though this phenotype only appeared in the last year. The fruit color, size, and shape are all due to recessive alleles, so those traits should already be stable. The stripes, flavor, and plant growth details probably won't be stable yet. I'll be growing several seeds from this fruit this year to find out.


References:

Tuesday, December 6, 2016

Unstable Genetics

Collecting germplasm is a key step for any plant breeding project. For the amateur plant breeder, this can seem like an arduous task. Fortunately, you can take a quick shortcut by saving seeds from hybrid plants. A hybrid plant will be heterozygous for many alleles, because it was made by crossing two (more or less) unrelated plants. The seeds produced by a hybrid will be segregating out a diverse set of different combinations of the alleles.

Growing these seeds means you may get some plants that are simply worthless, or wonderful in your eyes. Farmers (or others wanting a precise and predictable crop) won't generally accept this uncertainty. (This is probably why there is so much online dismissing the idea of saving seeds from hybrids.) However, if you're ok with each plant being unique and changing from year to year, this may be exactly the sort of thing you're looking for.

Some small plant breeders sell seeds from the unstable early stages of their breeding projects. The good ones will be entirely clear about the unstable nature of the seeds they're selling. The bad ones won't even let you know there is an issue. I have no connection to the breeders I've linked to below, but they seem to be up-front about how their seeds are not a stable end-product of a breeding program. Their seeds should give you plenty of variation to work with.

Seed sources have been periodically updated since first posting:
This company is closing soon!!!

If you know of any other vendors offering similar seeds, please let me know!


References:

Thursday, November 27, 2014

Genetic Assimilation.

1. [source]
Tomatoes sporadically produce fruit with horns, fleshy extensions adjacent to the calyx. Do a web search for, "Devil Tomato" and you will find several like the one in image #1. Generally, there is no evidence for these being the result of a genetic mutation. Rather, they represent the sort of thing that can happen when the normal development program of the fruit is disrupted in some way. Seeds taken from such a horned fruit will be no more likely to produce a plant that has similar fruit than seeds taken from any other fruit on the plant.

2. [source]
 There is a related species, Solanum mammosum, that has multiple such horns (image #2). (Though, there are example plants without horns.) The fruit of S. mammosum are rather toxic, so it wouldn't be a great idea to try and make a hybrid between the species and domesticated tomatoes.

3. [source]
Because there is the developmental potential for horns to be generated in tomatoes, there is the potential for a mutation to emphasize the trait. In the Tomato-TILING project, a few such mutations turned up (image #3). I'm not a professional plant developmental biologist, so I don't expect to get access to these interesting mutant seed lines any time soon.

I like the idea of looking for something that everyone else is trying to avoid. Every tomato breeder I've come across has been trying to breed away from a horned tomato, to produce a more "perfect" fruit shape, so I instead want a tomato that is all horns. I have the mental image of a tomato covered in fleshy projections featuring on a counter in some new science fiction movie.

As the previous examples have certain difficulties as a source for this trait, I've been looking for tomato lines which show a higher rate of these "deformations" to use as starting material in a project to breed a tomato that has the trait more consistently.

A rarely studied evolutionary model called "Genetic Assimilation" describes the process where an aberrant trait produced as the result of some stress is selected for and eventually becomes genetically fixed even without the presence of the stress. This mechanism sounds like Lamarckian evolution, except that it relies on the natural selection and the developmental plasticity of organisms…  rather than the personal experiences and intention of the organism that was favored by Lamark. It works because every trait is impacted by the genetic background, the combination of many subtle influences from other genes throughout the genome.

I frequent the Tomatoville forums, including the "Crosstalk: Tomatoville Research and Development™" forum. I started doing so because people there have a tendency to post lovely photos of the interestingly colored and patterned tomatoes they have been growing. Recently, a user was posted images from the results of a complex cross (["Pink Furry Boar" x "Ananas Noir"] x "Bosque Green Cherry") that they were working with. One of the diverse progeny they grew (image #4) had horns on 4 of the 20 fruit. 20% is a far higher rate than I'd otherwise come across, so I asked for a few seeds.

In a few years, I'll have a better idea of where this project is going. The good thing is that I can eat all the rejects along the way.



References:
  1. Genetic Assimilation:
    1. http://jeb.biologists.org/content/209/12/2362.full
    2. http://en.wikipedia.org/wiki/Genetic_assimilation
    3. http://eebweb.arizona.edu/faculty/badyaev/ecol596e/assimilation.pdf
    4. In tiger snakes: http://blogs.discovermagazine.com/notrocketscience/2009/10/30/big-headed-tiger-snakes-support-long-neglected-theory-of-genetic-assimilation/
    5. In fruit flies: Waddington, C. H. (1942) Canalization of development and the inheritance of acquired characters. Nature 150:563-565.
  2. Horned Tomatoes:
    1. http://www.tomatoville.com/showthread.php?t=34162
    2. https://www.flickr.com/photos/farflung/6462879911/
  3. Solanum mammosum
    1. https://www.flickr.com/photos/30372914@N03/3895429577/
    2. https://www.flickr.com/photos/22012266@N02/7164709249/
  4. Tomato Tiling project
    1. http://tilling.ucdavis.edu/index.php/Tomato_Tilling
  5. Tomato Varieties:
    1. Pink Furry Boar
    2. Ananas Noir
    3. Bosque Green Cherry

Wednesday, October 15, 2014

Making a new "Blue" tomato


1. Tomato "Indigo Rose".
"Blue" is the color label applied to the new breed of anthocyanin rich tomatoes. "Indigo Rose" (image #1 at left) is the first officially available variety with the trait. The variety was bred at OSU, using two genes from wild relatives of tomatoes. The atroviolaceum ('atv') gene was introgressed from Solanum cheesemanii. The anthocyanin fruit ('Aft') gene is a transcription factor introgressed from S. chilense. The two genes combine to result in a tomato with dark purple anthocyanin pigment production when exposed to sunlight.

The high-anthocyanin traits managed to escape from the OSU breeding program before the official release, under the names "OSU Blue" or "P20". This variety was not yet stable and didn't taste very good to most people, but it did successfully introduce tomato breeders to the interesting traits a few years early. Breeders quickly took to trying to incorporate anthocyanin expression into better tasting types of tomatoes.



2. F2 tomatoes, showing pigment on fruit and calyx.
I've been growing a miniature tomato variety called "Tiny Tim" for the last several years. I saved a batch of open-pollinated seeds two years ago, as my previous batch was running out. Last year, one of the seedlings turned out to grow much faster and larger than all the others. It was the result of a cross to one of the other tomatoes growing the previous year. I grew several F2s this year, allowing me to identify the other parent as a "Roma" tomato.

Among the F2s, I noted a range of anthocyanin phenotypes in the fruit and leaves/stems. The anthocyanin pigment produced on the fruit when sun-exposed came in three levels (none, middle, and high in image #2.)

3. F2s (top two) & "Indigo Rose" (bottom).
The anthocyanin pigment produced in the calyxes also came in three levels (high, none, and middle in image #2), but independent from the fruit pigment. The pigment produced in the rest of the plant wasn't as obvious. The no-pigment plants were entirely green. the medium-pigment plants had the anthocyanin highlights on the calyxes and leaf edges. The high-pigment plants showed increased pigment over the entire plant where sunlight hit, at a level about half of that seen in "Indigo Rose" tomato plants (image #3).

4. Original; color-enhanced; postureized.
The high-pigment plants also appeared more of a red/brown color rather than the purple of "Indigo Rose" plants. Image #4 shows a section of the image #3 after using the color-enhance filter in GIMP (center) and then the posterize filter in GIMP (right). The enhanced images more clearly convey the difference in color which is visually seen on examining the plants. This either indicates a different mix of anthocyanin pigments, or is a visual artifact caused by the blending of green chlorophyll with the anthocyanin purple. I would need to do some chromatography or micro-dissection experiments to discriminate between these possibilities.



5. Anthocyanins on unripe "Tiny Tim".
The level of pigment in the F2s was a surprise as I hadn't noted any anthocyanin expression in the parent varieties. After seeing the F2s, I re-examined some "Tiny Tim" plants grown this year and found they did have anthocyanin expression. The fruit show a level of anthocyanin production comparable to the high-fruit-pigment F2s (image #5), but the small size of the fruit made it hard to notice. The F1 showed a pigment level like the medium-fruit-pigment F2s, suggesting it is a single trait with partial dominance. The color of the calyx and leaf/stem was comparable to the F2 plants with the middle level of pigment for each feature. Looking into the lineage of "Tiny Tim" suggests the middle-pigment trait was contributed from S. pimpinellifolium, used in the breeding to contribute small fruit size to "Tiny Tim". As anthocyanin pigments on the shoulder is common in many wild tomato relatives, I suspect the fruit trait also came from S. pimpinellifolium.

"Tiny Tim" is an open-pollenated variety, so it should be homozygous for any alleles impacting pigment production. The increased calyx/leaf/stem pigment intensity in the F2s over what is seen in "Tiny Tim" suggests the involvement of a second gene from the "Roma" parent that enhances the expression of the first gene. This second gene would have been hidden in "Roma" because that variety doesn't have any anthocyanin pigment production.



What are the expected genetics for this cross?

The fruit pigment appears driven by one gene. Under the model of partial dominance, the cross ...

1tt1tt x 1R1R

… produces an F1 …

1tt1R

… that shows a low level of anthocyanins in the fruit. Low amounts of anthocyanin pigment was noted in the fruit of the real F1. Selfing the F1 produces F2s …

1tt
1R
1tt 
1tt1tt
1tt1R
1R
1tt1R
1R1R

… where 1/4 have high-pigment on the fruit (1tt1tt) and another 1/2 have low pigment on the fruit (1tt1R). I only grew 10 F2s this year, so it is hard to estimate real ratios, but all three color classes were observed.


The calyx/leaf/stem pigment appears to involve two genes. If we assume both involved alleles are recessive, the cross …

2tt2tt3TT3TT x 2R2R3r3r

… produces an F1 …

2tt2R3TT3r

… that shows no anthocyanins in the calyx/leaf/stem. No anthocyanin pigments were observed in the calyx/leaf/stem of the real F1. Selfing the F1 produces F2s …

2tt3TT
2tt3r
2R3TT
2R3r
2tt3TT 
2tt2tt3TT3TT2tt2tt3TT3r2tt2R3TT3TT2tt2R3TT3r
2tt3r
2tt2tt3TT3r2tt2tt3r3r2tt2R3TT3r2tt2R3r3r
2R3TT
2tt2R3TT3TT2tt2R3TT3r2R2R3TT3TT2R2R3TT3r
2R3r
2tt2R3TT3r2tt2R3r3r2R2R3TT3r2R2R3r3r

… where 1/16 are expected to express the recessive alleles from both parents and thus show the high-pigment trait. Another 3/16 are expected to express the recessive allele from "Tiny Tim" and show the medium-pigment trait. The remaining 12/16 should only have green chlorophyll evident in the unripe fruit. This year I grew 10 F2s and only one shows the high-pigment trait. 1/10 approximates 1/16 reasonably well for the numbers I grew. Some, but not all showed the middle-pigment trait. I didn't note exactly how many F2s showed the middle-pigment trait and they've begun dying back from the cold, so I will have to screen more F2s next year at an earlier stage to better estimate the true ratios of the different color classes.



The dark pigment of "Indigo Rose" fruit is due to the interaction of two traits, the anthocyanin fruit ('Aft') trait combined with the atroviolaceum ('atv') trait. The 'Aft' trait by itself only produces a small amount of pigment on the fruit shoulder. The 'atv' trait by itself only produces dark pigment on the calyx/leaf/stem of the plant.

If the fruit pigment in the F2s is driven by a single gene, as it appears, and two genes are responsible for the calyx/leaf/stem pigment, then 1/64 of the F2s will contain both high-anthocyanin traits.



6. Derived from S. hirsutum.

There are several anthocyanin traits floating around that have been introgressed from different wild tomato relatives.
  1. S. cheesemanii
    • "atv" gene: pigment throughout plant. Seen in variety "Indigo Rose" (image #1).
  2. S. chilense
    • "Aft" gene: pigment on fruit shoulder. Seen in variety "Indigo Rose". (image #1)
  3. S. hirsutum
    • [unnamed] gene: pigment on fruit shoulder, similar to "Aft". Described at maprc.blogspot.com. (image #6)
  4. S. peruvianum

  5. 7. Derived from S. peruvianum.
    • [unnamed] gene: pigment on fruit shoulder, similar to "Aft". Seen in variety "Purple Smudge". (image #7)
  6. S. pimpinellifolium
    • gene #1: pigment on fruit shoulder, similar to "Aft". Described here.
    • gene #2: pigment throughout plant, similar to "atv". Described here.
  7. Conventional tomatoes
The four fruit pigment traits and the two plant pigment traits seem to behave similar to the others in each category. Because the species are so closely related, the traits may represent different alleles of the same genes. If so, combinations of a trait from each category (like the "atv" and "Aft" in "Indigo Rose") should result in a strong increase in the total pigment produced relative to either trait alone, especially when the modifier trait (gene #3) is also present.

I've isolated a line that appears homozygous for gene #1 and one that appears homozygous for both genes #2 and #3. Unfortunately, crossing these two lines would simply recreate the F1 (heterozygous for all three traits) rather than help me generate a triple-homozyous line.

Comparing the lightly-pigmented fruit in images #6 and #7 to my pigmented F2s suggests they are showing a different mix of anthocyanins from the other "blue" lineages. I look forward to finding one of the rare segregants which contains all three genes, so I can find out!



References:
  1. "Indigo Rose" tomato: http://extension.oregonstate.edu/gardening/purple-tomato-debuts-indigo-rose
  2. 'Aft' gene: http://www.esalq.usp.br/tomato/Aft.pdf
  3. 'atv' gene: http://www.esalq.usp.br/tomato/atv.pdf
  4. Escape of "P20" tomato : http://www.tomatoville.com/showthread.php?t=16989
  5. "Tiny Tim" tomato: http://tatianastomatobase.com/wiki/Tiny_Tim
  6. "Roma" tomato: http://tatianastomatobase.com/wiki/Roma
  7. "Orange Smudge" tomato: http://tatianastomatobase.com/wiki/Purple_Smudge

Saturday, September 20, 2014

Making Micro-Tomatoes

Tomato fruit comes in a range of sizes, from the very tiny currant tomatoes to the very large beefsteak tomatoes. What isn't so obvious is that the plants also come in a range of sizes. Most varieties grown under ideal conditions can top 6 feet during a summer, but one of my favorite varieties ("Tiny Tim", at left) is a miniature that would be lucky to top 6 inches.

I like growing "Tiny Tim" plants on the balcony railing just outside my kitchen and I've grown several each of the last few years. Last year, one of the seedlings grew much faster than expected. It eventually grew into a shrub some 4 feet tall. This hybrid (F1) had occurred naturally in my garden the previous year. Because the cross was performed by some enterprising insect, I couldn't tell what other tomato plant contributed its pollen to the cross. Being a biologist, I decided to save the F2 seeds that the hybrid produced so I could explore the genetics captured in the cross and maybe figure out what the father was.

F2 seedlings.
There are two known recessive dwarfing traits in tomatoes. The gene label for the first is 'd' for 'dwarf'. The gene label for the second is 'sd' for 'sun-dwarf' because it results in a plant that grows very short when exposed to bright light. A dihybrid cross like this is expected to produce progeny in a 9:6:1 (normal:dwarf:micro) ratio, assuming the genetic loci are far enough apart or on separate chromosomes. I grew out 10 of the F2s this year (at right) and the resulting plant sizes were in a 5:4:1 (normal:dwarf:micro) ratio. The dwarf plants are distinguishable very early because the leaves remain splayed (yellow 'D's at right), while in normal plants they fold upwards at night (red 'N's at right). The one micro plant (green 'M' at right) sprouted and grew very slowly compared to all the others.

    F2 Micro #1
This one tiny micro seedling did eventually grow up to a full height of 8 inches before the first frost. It produced three fruit, each was more than twice as large as the average sized fruit from the "Tiny Tim" grandparent. The fruit did not taste all that good, but they did have an interesting elongated shape. I've saved most of the F3 seeds for growing out next year.

I accidentally killed two of the F2s, but the remaining eight  displayed a range of unexpected traits. Most (7/8) plants produced fruit with inflated locules (the part of the fruit with the seeds). This trait was only seen in one of the other tomatoes growing the year of the original cross, a "Roma" tomato. The F1 didn't have inflated locules, so the trait is caused by recessive alleles. A single recessive allele in the F1 should have produced a minority of F2s with the trait, though a majority of the plants I grew had inflated locules. If I grow a few dozen more F2s next year, I should see the expected 25% without inflated locules or otherwise be better able to discern what genetics are at play.

All eight plants produced fruit which was much larger than those from "Tiny Tim", like the F1, suggesting dominant alleles are responsible for the increased size. I should be able to find some F2s with the tiny fruit like those "Tiny Tim" produces.

One of the eight plants produced fruit with a high level of yellow pigment in the center of fruit. This plant also had the best tasting fruit by far. "Tiny Tim" has this same trait, but the F1 did not, indicating a recessive allele. There doesn't seem to be any genes named which would produce this trait, however. I saved seeds from this plant and intend to screen later F2s for the trait.

Most of the plants developed some light brown coloring on the sun-exposed tops of the unripe fruit, as well as differences in purple pigmentation of the calyx and leaves/stems. The brown coloring was found in two distinct levels, in addition to clear green (image below). The intermediate brown color was also noted on the F1 fruit, indicating a codominant trait. The pigmentation of the green parts of the plant also came in three distinct levels, independently of the fruit color. The indicated two genes have not been named, though their activity is similar to the genes (Aft and atv) introduced from some wild tomato relatives (Solanum chilense and S. cheesmaniae) to produce the dark purple of the variety "Indigo Rose". I'm interested in trying crosses later to examine the connection between these traits.
I'll grow many more of these F2s next year. Ideally I will be able to find more micro plants, so I can select for those that have some of these other interesting segregating traits. I'll also grow out F3 seeds saved from the one micro plant from this year and from the one plant with enhanced yellow pigment in its fruit.

Tuesday, April 1, 2014

The Color of Tomatoes


Tomatoes can be found in a wide range of colors (red, pink, orange, yellow, white, green, brown, purple, blue) at your local farmers' market or grocer. Those color differences represent differences in the types and proportions of biologically synthesized pigments. Because tomatoes are such an important crop and adding those pigments to our diet has been shown to give us positive health impacts, researchers have spent some time studying the pathway they use to build the various pigments.

Information about the pathway is spread across numerous papers encompassing decades of research. Not every paper examining the topic will contain all the details of the pathway and there are sometimes conflicting results that have to be negotiated.

The fortunate thing is that these days, more and more of the research behind any particular topic is being placed online. This gives amateur and professional researchers alike access to the same information and either can learn a great deal about many different subjects that may interest them. In general you shouldn't put too much stock in the results of any one paper, but when the results of multiple independent lines of inquiry happen to align, you can be more confident in the inferences you gain.



You might say I'm somewhat interested in the biology of the things I find around me. I decided to look into what was known about the interesting colors found in tomatoes.

The majority of pigments in the tomato are carotenoids. After a bit of research, I generated the figure (at right) to show the biosynthesis pathway which produces these pigments. It illustrates most of what I've learned about the pathway. The figure is probably not entirely comprehensive, as all biological systems are intricate, but it contains sufficient detail to help explain the range of fruit colors seen in tomatoes.

You can dig through the references at the end of this post if you're interested in where I got the information. In fact, I would highly recommend you do so, if you're interested in the biology of tomato color. If you've never read primary literature, you should be aware that you may have to read through dozens (or hundreds) of papers to get a solid grasp of a new topic in this way.

In my figure, I use double arrows to indicate higher reaction rates, as inferred from preferred branches of the reaction pathway or intermediates which are known but don't appear in large amounts. The colored components of the pathway are highlighted in something approaching their true colors in chromatography experiments (and in the fruit) to make it simpler to visualize how mutations might impact fruit color.

Energy flows through biosynthetic pathways. Though each enzymatic reaction is reversible, the overall progression of the pathway is driven by the enthalpy gradient across its components. When a mutation breaks part of a pathway, it blocks that flow of energy, resulting in a build-up of the chemical intermediates just before the break as well as a reduction or absence of those intermediates after the break.



The following figures are close-up versions of the main figure above, highlight the placement of a series of mutations in the pathway which result in color changes in the flesh of tomatoes. The mutations are indicated by a large negative or positive sign, highlighted in red, at the location of the change to the pathway.

Mutant : 'r'
The first major mutation (left) is responsible for the difference between red and yellow tomatoes. The wild-type dominant allele of the gene leads to the production of lycopene, so the gene is named 'red' ('R'). The recessive allele ('r') results in an overall decrease in the production of carotenoids. The decrease is not uniform; some carotenoids are suppressed more than others, resulting in an overall yellow appearance.
Mutant : 'gf'

The second major mutation (right), in combination with the 'r' mutation just described, is the most common cause of green-fleshed tomatoes. The recessive mutant allele leads to green flesh, so it is named 'green-flesh' ('gf'). The wild-type dominant allele of the gene ('Gf') allows the breakdown of chlorophyll and other photosynthetic components during the ripening process.

A minor mutation produces green ripe fruit by interfering with the ability of the fruit ripening system to respond to ethylene. Ethylene is a common plant hormone involved in maturation/ripening/senescence, so this mutation keeps some aspects of the normal fruit ripening from happening. This dominant mutant allele is called 'Green-ripe' ('Gr') and results in a green tomato with a red heart. This mutation doesn't impact the pigment pathway, but instead where different components of it are activated. It is not clear if it interacts with the 'red' ('r') locus in a similar way to the more common 'gf' mutation.

Mutant : 't'
The third major mutation (left) is responsible for the most common form of orange tomato. The recessive mutant allele is named 'tangerine' ('t') (after the orange variety "Tangerine" where the gene was found). The wild-type dominant allele of the gene ('T') allows the final synthesis of lycopene. The mutation results in the build-up of orange prolycopene, as well as zeta-carotene in smaller amounts.
Mutant : 'B'

Another minor mutation (right) results in a less common types of orange tomato. The dominant allele is named 'beta-carotene' ('Beta', 'B') because it leads to a large increase in beta-carotene and a decrease in lycopene. The position of this gene in the pathway and the mechanism of the mutation isn't entirely clear, but the data seems to suggest the gene is involved in the conversion of lycopene to gamma-carotene and the mutation results in increased activity.




The color of the epidermis also impacts the apparent color of tomatoes, but the mutations impacting this system are less well-understood. The typical red tomato has a transparent-yellow epidermis, giving the associated gene its name 'yellow' ('Y'). A common recessive mutation ('y') results in a clear epidermis. The epidermis color overlaid on the flesh color results in the perceptual differences is red/pink and brown/purple tomatoes.

Recently, breeders have been been working with genes that result in the production of dark purple anthocyanins in the skin of tomatoes. The two genes 'anthocyanin-fruit' ('Aft') and 'atroviolaceum' ('atv') were introgressed into cultivated tomatoes from Solanum chilense and Solanum cheesmaniae, respectively. A less common gene 'Abg' was introgressed into cultivated tomatoes from Solanum lycopersicoides, but is less useful/available because it has a recessive lethal character. All three genes are described in Mes et al, 2008.

Frogsleep Farms
There is evidence for lycopene production in the skin of some tomatoes, resulting in an opaque-red skin. The details of the genetics have yet to be worked out and published, but Frogsleep Farms has found some lovely examples. The fruit at right appears to have the genotype 'r' for yellow fruit flesh color, but has intense lycopene-red in the epidermis. (The first photo on this page shows another fruit, illustrating high-lycopene opaque-red skin.)

In my personal gardening, I've noticed an opaque-red epidermis in fruit produced by the micro-tomato variety 'Tiny Tim'. This variety was derived in part from the wild tomato Solanum pimpinellifolium, which also seems to have an opaque-red fruit epidermis. Potential variations might eventually be found which have opaque-yellow or opaque-orange skin, so I find the idea of exploring the traits of fruit skin color to be exciting.



wild-type
Another set of genes impact where pigment is produced in the fruit. A wild-type tomato has a dark-green shoulder when immature, which delays ripening at the top of the fruit. Common modern market tomatoes have the 'uniform ripening' ('u') trait which shows even ripening, but reduced overall color. This gene is a transcription factor which normally guides chlorophyll distribution and abundance in unripe fruit.

[dgdg] vs. [uu]
The 'dark green' ('dg') mutation produces a dark green immature fruit and increased levels of carotenoids in the ripe fruit. The 'high pigment 2' ('hp2') mutant is now considered to be a different allele of the same gene as 'dg', which is now known to be the tomato homolog of the Arabidopsis DEETIOLATED1 gene. This gene is involved in the perception of light levels and impacts morphogenesis.

The cause of bicolor, striped, and spotted tomatoes are less well understood. Striped tomatoes using the 'green stripe' ('gs') mutation are pretty common these days. Another type of striping is due to a dominant allele ('Ufs') of the 'uniform ripening' gene. Spotting is generally considered a commercial defect, but the 'gold fleck' ('Gfk') trait is an interesting look when it is highly expressed.



In the following section, I give limited descriptions of the different color categories of tomatoes. This description includes the genotypes and example varieties associated with them when they're commercially available.

 'RR' (Kachanovsky et al, 2012)
Red Tomatoes

The typical red tomato is pigmented by a large amount of lycopene and lesser amounts of beta-carotene, driven by the 'red' ('R') gene. The skin of these tomatoes also has a yellow pigment driven by the 'yellow' ('Y') gene.
genotype = [RR; YY]
example = "Red Barn".

Pink Tomatoes

These have the lycopene (red) and beta-carotene (orange) of typical tomatoes, but they have clear skin from a recessive allele 'y'. This results in the tomatoes appearing pink when compared to the typical red tomato.
genotype = [RR; yy]
example = "Dwarf Champion Improved".

Orange Tomatoes
 'RR; tt' & 'rr; tt' (Kachanovsky et al, 2012)


The most common type of orange tomato is caused by the 'tangerine' ('t') mutation. The skin can be clear or yellow.
genotype = [RR; YY/yy; tt] or [rr; YY/yy; tt]
example = "Earl of Edgecombe", "Elbe""Tangerine".

A less common type of orange tomato is caused by the 'Beta-carotene' ('Beta','B') mutation. The skin can be clear or yellow.
genotype = [RR; YY/yy; BB]
example = "Caro-Rich","Jaune Flammée".

 'rr' (Kachanovsky et al, 2012)
Yellow/White Tomatoes

Yellow tomatoes are caused by a recessive allele ('r') of the 'red' gene. The skin can be clear or yellow.
genotype = [rr; YY/yy; TT]
examples = "Yellow Pear".

White tomatoes appear to be caused by a stronger recessive allele ('r-') of the 'red' gene.
genotype = [r-r-; YY/yy; TT]
example = "Dr Carolyn", "White Queen".

Green Tomatoes

"Coeur de Surpriz" showing 'Gr'.
Grown by Mary Hope.
The most common type of green tomato is caused by the recessive 'green-flesh' ('gf') mutation in combination with the 'r' mutation.
genotype = [rr; YY/yy; TT; gfgf]
example = "Green Zebra".

A less common type of green tomato is caused by the dominant 'Green-ripe' ('Gr') mutation. This mutation leaves the center of the fruit to ripen normally, resulting in green/'purple' outer regions and a yellow/red center. There are heirloom varieties around with this trait, but I haven't been able to find many specific names.
genotype = [GrGr]
example = "Coeur de Surpriz".

Brown Tomatoes

These are pigmented by a large amount of lycopene and lesser amounts of beta-carotene, driven by the 'red' ('R') gene, as well as by chlorophyll from the 'green-flesh' ('gf') gene. The skin of these tomatoes has a yellow pigment driven by the 'yellow' ('Y') gene.
genotype = [RR; YY; TT; gfgf]
example = "Black Russian", "Brazilian Beauty".

Purple Tomatoes

These are pigmented by a large amount of lycopene and lesser amounts of beta-carotene, driven by the 'red' ('R') gene, as well as by chlorophyll from the 'green-flesh' ('gf') gene. The skin of these tomatoes is clear driven by the mutant allele ('y') of the 'yellow' gene.
genotype = [RR; yy; TT; gfgf]
example = "Black Cherry""Black Krim".

"Indigo Rose" showing 'Aft' and 'atv'.
Blue Tomatoes 

These have anthocyanin expression in the skin, driven by the combination of 'anthocyanin fruit' ('Aft') and 'atroviolaceum' ('atv') genes.
genotype = [AftAft; atvatv]
example = "Indigo Rose".

Black Tomatoes

There is no specific genetics to describe for this category. 'Black' is often used to describe those that I would call 'brown' or 'purple'. (The example varieties I list for the 'brown' and 'purple' groups have names starting with 'black'.) I wouldn't be surprised if the 'blue' tomatoes end up being described this way, since they're actually the closest to black we're likely to get.



Bicolor Tomatoes

The 'bicolor' trait is caused by an allele ('ry') of the 'R' gene which activates of the carotenoid pathway in some parts of the fruit and not others. This results in streaks of red and yellow throughout the fruit and skin.
genotype = [ryry]

Striped Tomatoes

"Green Zebra" showing 'gs'.
Dark green stripes on immature fruit, driven by the recessive 'green-stripe' ('gs') gene. This trait has become very popular lately and is responsible for the stripes seen on many heirloom-type varieties available in markets.
genotype = [gsgs]
examples = "Green Zebra", "Striped Roman".
tomato showing 'UFs'

Dark green radial stripes, opposite of each locule, driven by a dominant allele ('UFs') of the 'uniform-ripening' gene. Frogsleep Farm has some nice photos of fruit with the pattern mixed with anthocyanin production, as well as a nice bit of discussion of the trait, but there seem to be few commercially available varieties with this form of striping.
genotype = [UFsUFs]
examples = "Siberian Tiger", "Arbuznyi".

Spotted Tomatoes

Yellow spots on ripe fruit, driven by the dominant 'gold-fleck' ('Gdf') gene.   Frogsleep Farms has some interesting images showing this trait.
genotype = [GdfGdf]
examples = "Scabitha".



References:
  1. Apel W & Bock R. (2009) Enhancement of Carotenoid Biosynthesis in Transplastomic Tromatoes by Induced Lycopene-to-Provitamin A Conversion. Plant Physiology 151:59-66. 
  2. Barry CS, McQuinn RP, Thompson AJ, Seymour GB, Grierson D, and Giovannoni JJ. (2005) Ethylene Insensitivity Conferred by the Green-ripe and Never-ripe 2 Ripening Mutants of Tomato. Plant Physiology 138:267-275.
  3. Barry CS, McQuinn RP, Chung M, Besuden A, & Giovannoni JJ. (2008) Amino Acid Substitutions in Homologs of the STAY-GREEN Protein are Responsible for the green-flesh and chlorophyll-retainer Mutations of Tomato and Pepper. Plant Physiology 147:179-187.
  4. Fantini E, Falcone G, Frusciante S, Giliberto L, & Giuliano G. (2013) Dissection of Tomato Lycopene Biosynthesis Through Virus-Induced Gene Silencing. Plant Physiology 163:986-998.
  5. Gonzali S, Mazzucato A, & Perata P. (2009) Anthocyanin pathway in tomatoes. Trends in Plant Science 14:237-241.
  6. Issacson T, Ronen G, Zamir D, and Hirschberg J. (2002) Cloning of tangerine from Tomato Reveals a Carotenoid Isomerase Essential for the Production of Beta-Carotene and Xanthophylls in Plants. Plant Cell 14:333-342.
  7. Jenkins JA & Mackinny G. (1951) Color in Tomatoes. California Agriculture  Feb 13-14.
  8. Jenkins JA & Mackinny G. (1953) Inheritance of Carotenoid Differences in the Tomato Hybrid Yellow x Tangerine. Genetics 38:107-116.
  9. Jenkins JA & Mackinny G. (1955) Carotenoids of the Apricot Tomato and its Hybrids with Yellow and Tangerine. Genetics 40:715-720.
    1. http://www.genetics.org/content/40/5/715.full.pdf
    2. Carotenoid characterization of 'apricot' ('at') mutation.
    3. Data suggests alternate pathway to generate beta-carotene.
  10. Kachanovsky DE, Filler S, Isaacson T, & Hirschberg J. (2012) Epistasis in tomato color mutations involves regulation of phytoene synthase 1 expression by cis-carotenoids. Proceedings of the National Academy of Science USA 109:19021-19026.
  11. Levin I, Frankel P, Gilboa N, Tanny S, and Lalazar A. (2003) The tomato dark green mutation is a novel allele of the tomato homolog of the DEETIOLATED1 gene. Theoretical Applied Genetics 106:454-460.
  12. Lesley JW and Lesley MM. () Linkage of sh (sherry).
  13. Ma Q, Du W, Brandizzi F, Giovannoni JJ, & Barry CS. (2012) Differential Control of Ethylene Responses by GREEN-RIPE and GREEN-RIPE LIKE1 Provides Evidence for Distinct Ethylene Signaling Modules in Tomato. Plant Physiology 160:1968-1984.
  14. Mes PJ, Boches P, & Myers JR. (2008) Characterization of Tomatoes Expressing Anthocyanin in the Fruit. Journal of American Society of Horticultural Science 133:262-269.
  15. Paran I, van der Knaap E. (2007) Genetic and molecular regulation of fruit and plant domestication traits in tomato and pepper. Journal of Experimental Botany 58:3841-3852.
    • http://jxb.oxfordjournals.org/content/58/14/3841.full
    • 'Del' has increased expression of lycopene-delta-cyclase, resulting in conversion of lycopene to delta-carotene.
    • 'Del' was introgressed from Solanum pennelii.
    • 'Beta' was introgressed from Solanum cheesmaniae.
    • 'old gold' is allelic to 'Beta', but eliminates Beta-carotene production.
  16. Powell ALT, Nguyen CV, Hill T, Cheng KL, Figueroa-Balderas R, Aktas H, Ashrafi H, Pons C, Fern Ă¡ndez-Muñoz R, Vicente A, Lopez-Baltazar J, Barry CS, Liu Y, Chetalat R, Granell A, Deynze AV, Giovannoni JJ, and Bennett AB. (2012) Uniform ripening Encodes a Golden 2-like Transcription Factor Regulating Tomato Fruit Chloroplast Development. Science 336:1711-1715.
  17. Tomes ML, Quackenbush FW, Nelson OE Jr, & North B. (1953) The Inheritance of Carotenoid Pigment Systems in the Tomato. Genetics 38:117-127.
    • http://www.genetics.org/content/38/2/117.full.pdf
    • Identification of pigments found in red/yellow/B-orange/tangerine fruit.
    • Yellow skin is a non-carotenoid pigment.
    • Yellow differs from red mostly in across-the-board reduction in pigment.
    • Tangerine fruit contains mostly zeta-carotene and prolycopene.
    • Red fruit contains mostly lycopene and less beta-carotene.
    • 'Beta'-orange fruit contains mostly beta-carotene and less lycopene.
    • [r locus < t locus < Beta locus] in pathway.
  18. Tomes ML. (1966) The Competitive Effect of the Beta- and Delta-Carotene Genes on Alpha- and Beta-Ionone Ring Formation in the Tomato. Genetics 56:227-232.
  19. Wann EV. Reduced Plant Growth in Tomato Mutants high pigment and dark green Partially Overcome by Gibberelin. (1995) HortScience 30:379.
  20. http://forums.gardenweb.com/forums/load/tomato/msg1118251224332.html
    • Mention of 'B' mutant variety.
  21. http://treecropsresearch.org/heirloom-tomatoes/
    • List of several varieties with high prolycopene => 'tangerine' mutant varieties.
    • Data suggests secondary pathway to beta-carotene.
  22. http://tgc.ifas.ufl.edu/vol7/v7p14.html
    • Bicolor trait.
  23. http://tgc.ifas.ufl.edu/vol6/v6p33a.html
    • Bicolor trait.