The Genetics of Heirloom Tomatoes

Come summertime, I enjoy the marvelous diversity, and exquisite if fleeting taste and texture, of heirloom tomatoes. They are nothing at all like the uniformly hard, shiny orange round and ovoid things that stand in for tomatoes mid-winter, more like a Spaulding pinky ball than a juicy fruit.

Heirloom tomatoes come in a variety of hues, from pale yellow to dark purple to everything in between. The skins are thin, the flesh sweet, and softening rapid. The fruits range from small and round to huge and multi-lobed. Their delectability is fleeting – they’re best eaten within two days of picking, or right off the vine. If nurtured and spaced correctly, the plants become towers of deliciousness.

I do not have a particularly green thumb, but I try, and every summer grow the components of salads for months. Here’s a useful video for nurturing heirloom tomatoes.

This season, I’m attempting to grow my own, with heirloom seeds sprouting in tiny dissolvable pots in a closet repurposed with a red glowing growing lamp. They are Cherokee purples. Other colorful names are Hungarian heart, pineapple, rosella, apricot zebra, tulip, beefsteak, globe, oxheart, plum, cherry, lucky tiger, and Brad’s Atomic Grape, “a visually striking, elongated cherry tomato developed by Brad Gates of Wild Boar Farms in California,” a sweet fruit with stripy purply coloring and a hard texture. I love them.

Breeding True
Heirloom tomatoes originated in South America as the Aztecs selected and cultivated them for their palate-pleasing traits. Spanish explorers brought the fruits to Europe in the 16th century, and by the 1800s, many varieties were being grown, the seeds typically saved and passed down in human families. They are, technically, ancient cultivars.

By definition, heirloom seeds must have been handed down for at least 50 years. They lack the gene variants bred into cultured plants that provide the expected orange color and thick skins of the bounceable supermarket versions, and are homozygous for the recessive traits that endow the fruits with their spectacular colors and offbeat shapes. In people, the homozygous recessive state lies behind many single-gene conditions, including cystic fibrosis, sickle cell and Tay-Sachs diseases, and many “inborn errors of metabolism.”

Insects and breezes spread the seeds of heirlooms. Because the two copies of the genes that provide the fruit traits are homozygous – that is, two copies of the same variant, rather than hybrids – the pleasing qualities are consistent from generation to generation. A more familiar term for homozygote is “breeding true.”

Tackling the Biochemistry
A team from the Council for Agricultural Research and Economics Research Centre for Vegetable and Ornamental Crops in Pontecagnano, Italy analyzed the biochemistry of 60 accessions of heirlooms (technically Solanum lycopersicum). In agriculture, an accession is seeds, clones, or other tissue sampled from a specific site, one time, and cultured at a gene bank, for use by researchers. An accession may be a cultivar, landrace, or wild relative.

The researchers identified and tracked ascorbic acid, the colorful carotenoid pigments (such as lycopene and β-carotene), and other compounds to specifically define the characteristics of the many variations on the heirloom theme. They tracked thousands of genetic markers called SNPs, which vary at single bases in the DNA sequence, deducing how a plum hue differs from cherry and yellow, and fruits with globe shapes differ genetically from tiny orbs and the enormous flattened, purply kinds.

The researchers identified two genes that control fruit weight and a single gene that controls a precursor molecule to the carotenoid pigments. Identifying the basis of these traits and how they vary could suggest even more variations on the heirloom tomato theme.

CODA

Next week: how the eggplant got its purple color. Against the profoundly anti-science stance of the current US administration, I’m going to stick to writing about vegetables for awhile.