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How hybridizing crops with their wild 'cousins' can add characteristics to new varieties

Wild species native to South America have a natural resistance to diseases that breeders have used to create modern tomato varieties.

9/22/2020

Solanum sitiens, growing near Chuquicamata, Chile. / Credit: C. Rick

Ripe, juicy tomatoes are one of the hallmarks of a summer picnic and a key ingredient in salads and the classic BLT sandwich. Like many of our modern crops, tomatoes today have benefited from the work of agricultural scientists who collected, studied, and preserved their wild relatives. Wild species native to South America have a natural resistance to diseases that breeders have used to create modern varieties.

Charles Rick was a plant geneticist at UC Davis. His research and compilation efforts over a 50-year career led to the establishment of the Tomate C.M. Rick. It is a tomato germplasm bank. This center at UC Davis now maintains about 1,200 accessions of wild tomatoes 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 a rich source of genetic diversity with which to study and improve crop plants.

This is the story of how a relative of the wild tomato, a rare and endangered nightshade from the Atacama Desert of Chile, was (re) discovered and (eventually) raised with tomato.

In July 1956 Rick embarked on a 6-month exploration of the Andean region and the Galapagos Islands in search of wild tomatoes. Collecting plants in the Andes was (and is) an adventure. Narrow dirt roads go up and down steep canyons. Landslides and landslides are common, guardrails non-existent. The rusting bodies of buses and trucks that failed to round the curves and fell down the mountainsides are sobering reminders of the dangers of driving in this region. Even getting to Lima, Peru, his starting point, had its challenges.

During the following months, Rick and his family collected nearly 130 accessions of wild and cultivated tomatoes from Peru, Ecuador, and the Galapagos Islands.

In January 1957, Rick discovered a low, tomato-like bush. It was growing on the outskirts of the vast open-pit copper mine at Chuquicamata in northern Chile. This area receives less than 2 cm of rain per year and the surrounding landscape was almost devoid of vegetation.

However, this unusual plant was clearly thriving. Its branches were decorated with aromatic flowers of a bright yellow white. They were also loaded with small green fruits that dried into hard, brown skins on the plant.

Evidently a species of the nightshade genus (Solanum), the plant was unlike anything Rick had seen or read. Like any good plant collector, Rick recorded a detailed description. He then pressed and dried the samples, which he sent to Donovan Correll at Texas A & M. Correll University, a leading authority on taxonomy of the potato and its wild relatives. Correll was convinced that this desert plant was new to science and named it Solanum rickii, after its discoverer, a renowned geneticist.

Years after Rick's visit and collection, scientists from Chile discovered an earlier description of the same species in an obscure Chilean magazine. It had no drawings or photos, so the plant remained unknown to scientists outside of Chile. In this original publication the species was named Solanum sitiens, which means "thirsty nightshade" - appropriate for a plant that lives in the driest desert on earth!

Back at Davis, Rick tried unsuccessfully to cross S. sitiens plants with cultivated tomato. Despite repeated attempts, strong crossing barriers prevented the formation of hybrids. It wasn't until 1990 that scientists (at Campbell Soup) succeeded in hybridizing tomato with S. sitiens.

Since then, more accessions of this rare species have been collected at other sites in Chile. Many of them are threatened by mining. The research lab found that one of these accessions can be crossed with tomato, although the hybrids were nearly sterile. Using DNA markers, we created a set of "introgression lines". Each line contains a defined part of the genome of the wild species in the genetic background of a variety of large-fruited tomato. 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 improve drought tolerance. We can also study other novel traits of this specially adapted wild crop relative.

Sixty-three years after Rick collected his first S. sitiens seeds in Chuquicamata, we now have pre-improved germplasm suitable for evaluation and incorporation into breeding programs. Transferring genes from this hardy Atacama desert plant to tomato has been a long and difficult road! Exploring your genome for new and useful traits is the next frontier.

Source: ASA/CSSA/SSSA

Charles Rick, collecting S. sitiens near Chuquicamata, Chile, in 1987./ Photo:R. Chetelat

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