Stripes, stripes, and more stripes!

From Brad Gates’s Pink Berkeley Tie Dye and Boar lines to Fred Hempel’s Artisan Bumblebee series – and all the way back to one of the pioneering lines of stripes, the Green Zebra, bred by Tom Wagner – there’s just something so visually fascinating about these eye-catching striped tomatoes. There are pinstripes, wavy stripes, broken stripes, and more, but where do they come from?

My hunger for this knowledge led me to ask my friend and fellow breeder Blane Horton to help. He began a thread in the online forum Doc and Katie’s Garden Xanadu asking for information and visual examples of different kinds of striped tomatoes. While Blane collected “evidence,” I questioned Tom Wagner and two more esteemed breeders with backgrounds in genetics about why these stripes appear. I received the same response from all three sources: There is very little genetic explanation for what we visually recognize as stripes!

One of the breeders offered information on three known genes associated with some forms of striping, and I struck out on my own to find out as much as I could about them. My most valued resource in researching tomato genetics is the C.M. Rick Tomato Genetics Resource Center (tgrc.ucdavis.edu). This website offers a downloadable list of all known genes associated with tomatoes, among other valuable information.

Here’s what I found: One gene responsible for some stripes is a green stripe (gs), a recessive gene that causes uneven green striping to appear on unripe fruit. As the fruit ripens, these stripes retain chlorophyll for longer than other parts of the fruit, but they will eventually turn a paler shade of the ripe fruit color. This striping is only found within the fruit’s epidermis (skin).

Another known gene, fruit stripe (Fs), causes dark-green radial striping opposite the locules (seed cavities). These stripes may be more prominent when affected by environmental changes and tend to fade as the fruit matures. However, in some instances where this gene is found in combination with the anthocyanin fruit (Aft) gene, the anthocyanin pigmentation can express in the same manner (radial striping), but it doesn’t fade as the fruit ripens.

And lastly, there’s the uniform ripening (u) gene with the dominant allele of fruit stripe (Fs). This expression is similar to the Fs gene in its display, but it also expresses dark-green shoulders that run down into the radial stripes.

To find further explanations and visual examples of these gene expressions, I visited a blog by breeder Mark McCaslin of Frogsleap Farm, another valuable source for research and understanding. The specific blog entry is “Genetic Control of Fruit Stripes in Tomato.”

In the Facebook group World Tomato Society Galactic Project, some members—many of whom are breeders—offer superior scientific knowledge on this topic. I created a post in the group asking breeders to show their F1 fruit pictures of any crosses they’d made with either one or both parents being stripped. From scanning these photos, I hoped to see a clear pattern of either stripes or no stripes that could clue us into any dominant or recessive pathways.

I was astonished at the “muddy” outcome, and the conversation that my question sparked was remarkable. On this quest for knowledge, I learned that sometimes there are no complete answers in nature. There are many unknowns for scientists and research teams to discover, and for their work I am grateful.