Apples are popular fruits that are loved by people. It is rich in pectin and minerals such as zinc and calcium, which help to enhance memory and concentration, maintain bone and teeth health, and make the skin smooth and supple. At the same time, malic acid can improve human immunity and has an auxiliary effect on the treatment of uremia, hypertension and other diseases. Therefore, apple enjoys the reputation of "all-round healthy fruit".
In fact, wild apples are small and sour, and the flavor is not good. How did it go down the "sweet wind" route and become a big and sweet cultivated apple? Botanical scientists have traced the evolutionary history of apple flavor, starting with genes.
Comparison of fruit sizes of wild apples ((1)(2)) and cultivated apples ((3)(4)) (courtesy of the author)
The source of the world's cultivated apples is in the Tianshan Mountains
Apple is a genus of apples in the family Rosaceae, which is mainly distributed in the northern temperate zone, including Asia, Europe and North America, with about 38 species, about 30 species in China, of which 16 species are endemic to China. Most of the species of apple are concentrated in the three provinces of Southwest Sichuan, Yunnan and Guizhou in Southwest China, so this area is considered to be the center of the origin and evolution of apple plants.
In addition to cultivated apples, other apple species are often referred to as begonias or wild apples. Wild apples are usually small, 1 to 4 cm in diameter, but the wild apple of the Tian Shan Mountains, the Severus apple, is larger in shape, up to 6 cm in diameter.
China has more than 2,000 years of apple cultivation history, "柰" is the oldest written variety in China, that is, the Severus apple that originated in Xinjiang. Apple cultivation began early in Europe, and documented apple varieties date back to the third hundred years BC.
Interestingly, there has been no genetic material exchange between the ancient European brewing apple and the ancient sweet apples of the East such as "Kao". This suggests that these two ancient varieties may have originated from independent domestication events, different from the origin characteristics of cultivated apples from the hybrid offspring of wild apples from the East and west. The results support the European view that Cider had been brewed and domesticated local wild apples from the Neolithic to the Bronze Age.
In the 1920s, the famous Soviet botanist Vavinov found that the morphological characteristics of the Severus apple and the cultivated apple were very similar, pointing out that the Tianshan Wild Fruit Forest was the origin of the cultivated apple. In recent years, geneticists have confirmed that the Severus apple is the ancestor of the world's cultivated apples through apple genome mapping and resequencing studies: after the ancestor species of Severus spread along the Silk Road to Europe, they crossed with the local wild apple forest apples, improved and bred, cultivated apples, and then introduced to the Americas, Asia, Oceania, Africa - in the past hundred years, the world's five continents have had apple cultivation.
"Sweet" evolution is simpler than "big"
The taste of the fruit depends on the composition and content of soluble sugars and organic acids, which together with the aroma recognized by the olfactory system determine the flavor of the fruit. In the apple genome, the domestication of flavor traits such as fruit acidity, organic acid components, and sorbitol content is mostly related to the selection of a single dominant gene locus, so its "sweetness" shows relatively simple evolutionary characteristics.
Recently, the Wuhan Botanical Garden of the Chinese Academy of Sciences, together with Huazhong Agricultural University and Shandong Agricultural University, published research results in Molecular Plants, revealed the evolutionary route of apple flavor traits.
The soluble sugar components in apple fruit are mainly glucose, fructose and sucrose. The average difference in total sugar content between cultivated apples and wild apples is not obvious, but wild apples also accumulate more sorbitol, and cultivated apples almost generally do not contain sorbitol.
The organic acid component in the cultivated apple fruit is mainly malic acid, and its content accounts for more than 80% of the total acid, while wild apples accumulate malic acid and citric acid at the same time, and the content of these two acids is about 3.8 times and 60 times higher than that of cultivated apples, respectively. This is why wild apples are much more acidic than cultivated apples.
Malic acid accumulation is mainly controlled by the Ma1 gene, whose loss-of-function mutations are the main cause of the formation of hypoacid traits. Interestingly, in addition to controlling the accumulation of malic acid in apple fruit, the Ma1 gene also has a pleiotropic effect of controlling the glyco-acid ratio, suggesting that the Ma1 gene plays an important role in determining the flavor of apple fruit. MdTDT is an important gene that controls the content of citric acid, and the mutation of this gene is related to the extremely low content of citric acid in the mature fruit of cultivated apples.
Genetic studies have shown that MdSOT2 is an important gene that controls sorbitol content, and its high expression in wild apples promotes the accumulation of sorbitol, but its expression level in cultivated apples is generally low. However, in the fruits of "Fuji" and its derivatives, sorbitol is accumulated at a high level. Studies have shown that sorbitol is not only a preservative that prolongs the shelf life of fruits, but also its content is significantly positively correlated with the sweetness of apples. Therefore, the high level of sorbitol accumulation in the fruit may be part of the reason for the long shelf life and sweet and juicy fruit of the "Fuji" series of apples.
Fruit size is an important trait chosen during fruit domestication. However, the change in the size of the apple fruit does not have a negative impact on the flavor quality of the apple, and both large apples and small apples can be delicious. Apple fruit size is controlled by many genes, and they undergo multi-step aggregation to achieve the characteristics of cultivating apples.
Author: Han Yuepeng
Editor: Xu Qimin