Ripe, juicy tomatoes are one of the hallmarks of a summer picnic – and a key ingredient in salads and the classic BLT. Like many of our modern crops, today’s tomatoes have benefited from the work of crop scientists who collected, studied, and preserved their wild relatives. Wild species native to South America carry natural disease resistance that breeders have used to create modern varieties.
Charles Rick was a plant geneticist at UC Davis. His research and collecting efforts over a 50-year career led to the establishment of the C.M. Rick Tomato Genetics Resource Center. It is a seed bank of tomato germplasm. This center at UC Davis now maintains nearly 1,200 wild tomato accessions representing 16 different taxonomic species. Many of these wild accessions were collected by Rick and his colleagues during 15 major collecting expeditions. Thanks to their efforts, today’s breeders have rich sources of genetic diversity with which to study and improve crop plants.
This is the story of how one wild tomato relative, a rare and endangered nightshade from the Atacama Desert of Chile, was (re)discovered and bred (eventually) with tomato.
In July 1956 Rick embarked on a 6-month exploration of the Andean region and the Galápagos Islands in search of wild tomatoes. Plant collecting in the Andes was (and is) an adventure. Narrow dirt roads wind up and down steep canyons. Washouts and landslides are common, guardrails nonexistent. The rusting bodies of buses and trucks that failed to negotiate turns and tumbled down the mountain sides provide sobering reminders of the hazards of driving in this region. Even getting to Lima, Peru, their starting point, had its challenges.
Over the ensuing months, Rick and his family collected nearly 130 accessions of wild and cultivated tomatoes from Peru, Ecuador, and the Galápagos Islands.
In January 1957, Rick discovered a low, tomato-like shrub. It was growing on the outskirts of the vast open pit copper mine at Chuquicamata, northern Chile. This area receives less than 2 cm of rainfall annually and the surrounding landscape was nearly devoid of vegetation.
Yet this unusual plant was clearly thriving. Its branches were decorated with bright yellow-white, aromatic flowers. They were also loaded with small green fruit that dried into hard, brown shells on the plant.
Evidently a species in the nightshade (Solanum) genus, the plant was unlike anything Rick had seen or read about. Like any good plant collector, Rick recorded a detailed description. He then pressed and dried specimens, which he sent to Donovan Correll at Texas A & M University. Correll a leading authority on the taxonomy of potato* and its wild relatives. Correll was convinced this desert plant was new to science, and named it Solanum rickii, after its discoverer, by then a renowned geneticist.
Years after Rick’s visit and collection, scientists in Chile uncovered an earlier description of the same species in an obscure Chilean journal. It was without drawings or photos, and so the plant remained unknown to scientists outside of Chile. In this original publication** the species was given the name Solanum sitiens, meaning ‘thirsty nightshade’ — appropriate for a plant living in the driest desert on earth!
Back in Davis, Rick tried unsuccessfully to cross S. sitiens plants with cultivated tomato. Despite repeated attempts, strong crossing barriers prevented the formation of any hybrids. It wasn’t until 1990 that scientists (at Campbell Soup) succeeded in hybridizing tomato with S. sitiens.
More accessions of this rare species have since been collected from other sites in Chile. Many of them are threatened by mining. My research lab found that one of these accessions can be crossed with tomato, although the hybrids were nearly sterile. Using DNA markers, we bred a set of ‘introgression lines.’ Each line contains a defined piece of the wild speciesgenome in the genetic background of a large-fruited tomato variety. Together these lines capture approx. 93% of the S. sitiens genome in a breeder-friendly form. This provides a permanent germplasm resource to study and breed drought tolerance. We can also study other novel traits from this uniquely adapted crop wild relative.
Sixty-three years after Rick collected his first seeds of S. sitiens at Chuquicamata, we now have prebred germplasm suitable for evaluation and incorporation into breeding programs. Transferring genes from this hardy plant of the Atacama Desert into tomato has been a long, hard road! Exploring its genome for new and useful traits is the next frontier.
Roger T. Chetelat, University of California, Davis
*Potato and tomato are “cousins” in the nightshade family.
**(Fun fact – the first person to publish about a specie is the one who gets to name it – thus the rediscovery of Johnston’s publication resulted in the new name).
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