Tomatoes' Genetic History from Wild to Salad

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Domestic and wild tomatoes. L to R: Solanum lycopersicum, and wild relatives S. pimpinellifolium, S. habrochaites and S. pennellii. (Brad Townsley, UC Davis)

The fruit synonymous with summer, plump tomatoes, descended from tiny wild ancestors. The genetic footprints left by that journey recently were traced by an international team of plant biologists. The deciphered DNA map of the tomato and its wild relatives points to ways that the fruit could be bred to survive in harsh, desert conditions, which may become more common as climate change reshapes the world.

Domesticated tomatoes’ (Solanum lycopersicum) DNA showed signs of the artificial selection and breeding efforts that  brought about their significantly larger, redder fruit compared with wild relatives. However, the domestication of the tomato came with genetic drawbacks.

Centuries ago, the tomato appeared on European tables after Spanish conquistadors brought the plant from its Western Hemisphere homeland. Only a few plants and seeds made the journey across the Atlantic, which created what geneticists call a bottleneck, or a serious reduction in population that resulted in lower genetic diversity.

Many of tomato genes stayed in the Aztec gardens and wilderness of the Western Hemisphere. The imported plants grown in Europe ended up with a number of potentially damaging genes, which became widespread in the cultivated plants, according to the recent study published in Proceedings of the National Academy of Sciences (PNAS). These genes may have made the domesticated tomato slower-growing and frailer than their wild kin.

Plant breeders have tried for decades to improve the domesticated tomato by cross breeding it with wild relatives. The new maps of three wild tomato relatives’ DNA (Solanum pennellii, S. habrochaites and S. pimpinellifolium), may help breeders focus on specific traits more efficiently. Genetic testing of new tomato hybrids can now look for DNA markers from wild traits that could make domestic tomatoes as hardy as their wild kin.

In particular, one desert-dwelling tomato, S. pennellii, contains genes that give it a thicker waxy coating on fruits and leaves that prevent water loss, according to the PNAS study led by University of California – Davis biologists. The leaves of the desert tomato grow smaller and have fewer openings, called stomata, used for respiration. Also, the plant has hardy roots that can tolerate saltier soil.

Breeding this desert tomato with domesticated varieties could lead to plants that produce tasty, large fruits, yet can tolerate climate conditions that would wither other tomatoes. As the world’s climate continues changing, the ability to adapt tomatoes to arid deserts could keep the “T” in BLT sandwiches.