Samplings: October 2021

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Flowers of Shiikuwasha citrus plant.

Chikatoshi Sugimoto

Mandarins are fundamental to East Asian cultures and are the building blocks of most other citrus fruits. When they were crossbred with pomelos, the result was oranges; further crossbreeding created grapefruit and lemons. Varieties such as shiikuwasha and tachibana are cherished as New Year’s gifts and temple decorations, and the tradition of giving mandarins extends to Christmas stockings.

Nevertheless, mystery and misunderstanding have long shrouded the ancestry of today’s cultivated mandarins and their wild ancestors. Earlier researchers lacked the sequencing technology to compare the genomes of what was believed to be the original wild mandarin, from the Nanling Mountains in southern China, and various cultivars. And they lacked genomic data on the wide diversity of mandarins found in Japan and the Ryukyu Islands to the south.

Now, using powerful genetic tools, researchers on Okinawa, as well as Spain and the United States, sequenced the genomes of sixty-nine traditional and wild mandarins from the Ryukyus and southern Japan and compared them with previously sequenced mainland mandarins and other citrus fruits. Amidst the multiplicity of named varieties, they identified three original wild mandarins that gave rise to all existing mandarins and, together with pomelos, many other citrus fruits.

Two of these originals are mainland sister subspecies, the common mandarin and the mangshanyejan, that diverged from each other about 1.4 million years ago (Mya). The third is a newly identified wild species, Citrus ryukyuensis, discovered in the orchard of an Okinawan farmer who, as researcher Guohong Albert Wu said, “likes to grow different kinds of fruit trees.” It diverged from the ancestor of the two mainland types 2.2–2.8 Mya after the periodic land bridge between mainland Asia and the islands was inundated by rising seas.

The common mandarin and mangshanyejan can clone themselves through apomixis, or asexual seeds, which enables them to breed “true,” an advantage for cultivated and wild citrus. C. ryukyuensis reproduces sexually. But it hybridized with admixtures of mangshanyeju and the common mandarin, which migrated from China in prehistoric times, and with a third mainland variety that arrived about 500 years ago. This hybridization produced varieties, such as shiikuwasha and tachibana, that reproduce clonally, giving them reliable propagation traits plus C. ryukyuensis’s adaptations to the island environment.

The researchers suggest that their findings can help today’s growers develop mandarins, oranges, and other citrus that resist disease and are resilient in warming temperatures. “The Ryukyu Islands are subtropical, with a very warm climate,” Wu explained “Wild species there are a great resource for studying adaptation to climate change.” (Nature Communications)