Ryukyu mandarin facts for kids

The Ryukyu mandarin (Citrus ryukyuensis) is a wild type of citrus fruit. It was recently discovered and grows naturally on the Ryukyu Islands and nearby islands. This plant is a close relative of the regular mandarin orange (C. reticulata) found on the mainland. Like all citrus plants, it belongs to the Rutaceae family.

The Ryukyu mandarin and the mainland mandarin have been separate for over 2 million years. A cool fact about the Ryukyu mandarin is that it reproduces sexually, unlike many mainland mandarins. When the Ryukyu mandarin mixed with mainland mandarins, it created many special types of mandarin oranges found on the islands today.

Discovery of the Ryukyu Mandarin

Scientists had studied mandarin oranges from mainland Asia. But they noticed that mandarins from Japan and nearby islands had some strange genetic features. To understand this better, they did another study. They found something unexpected: a completely pure, wild type of mandarin orange!

This wild mandarin mixed with the mainland species. This mixing created many different kinds of hybrid mandarin oranges. Since these plants were found on the Ryukyu Islands, scientists named the new species C. ryukyuensis.

Scientists compared the Ryukyu mandarin with C. reticulata, the mainland mandarin. They found that these two types of citrus separated between 2.2 and 2.8 million years ago. This time period matches when sea level rise happened during the Pleistocene era. The rising sea levels would have cut off the islands from the mainland. This isolation helped the Ryukyu mandarin become its own unique species.

Because it was isolated, the Ryukyu mandarin kept reproducing sexually. But the mainland mandarin developed a genetic change. This change allowed them to reproduce without seeds, a process called apomixis.

How Ryukyu Mandarins Reproduce

The Ryukyu mandarin is special because it reproduces sexually. This means it needs pollen from another plant to make seeds. Many mainland mandarin types, however, can reproduce asexually. This means they can make seeds without needing pollen from another plant. This is like making a clone of themselves! The Ryukyu mandarin's ability to reproduce sexually is important for creating new types of citrus.

Ryukyu Mandarin Hybrids

The C. ryukyuensis still exists today. It is also very important because it helped create many unique mandarin hybrids in Japan and nearby islands.

Tachibana Oranges

Between 40,000 and 200,000 years ago, the Ryukyu mandarin came into contact with mainland mandarins again. This might have happened because sea levels fell or because seeds spread across the ocean. When they met, they interbred, creating different groups of related hybrids.

One type of mainland mandarin already had some Ryukyu mandarin DNA mixed in. This mainland orange then crossed with the Ryukyu species many times. The mainland orange passed on its ability to reproduce without seeds to its offspring. This created a group of very similar Tachibana orange plants that are all clones of each other.

Shiikuwasha Oranges

Another group of hybrids formed separately from the Tachibana. A type of mainland mandarin, similar to the Chinese Sun Chu Sha mandarin, mixed with the Ryukyu species. This created the shiikuwasha. Like the Tachibana, the shiikuwasha is a group of similar hybrids, not just one type of plant.

These shiikuwasha plants also got the ability to reproduce without seeds from their mainland parent. Interestingly, some shiikuwasha clones have the mainland mandarin as their "seed parent," while others have the Ryukyu mandarin as their "seed parent." Both Tachibana and shiikuwasha oranges grow wild. They are even mentioned in old Japanese and Ryukyu poems. This suggests that these hybrid oranges appeared naturally in the wild.

Other Interesting Hybrids

The Ryukyu mandarin also helped create other important hybrid oranges. One group is called yukunibu. These came from a cross with a kunenbo-type mandarin. Kunenbo oranges likely came from Southeast Asia or Indonesia a long time ago.

The Ryukyu mandarin is also part of more complex citrus mixes. For example, the rokugatsu is a cross between a bitter orange and the Ryukyu mandarin. The deedee is a cross between a shiikuwasha and a pomelo.