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.