1. Who Are the Parents of the Seedless Mandarin (Unshu Mikan)?
The unshu mikan, known abroad as the common mandarin or satsuma, is easy to peel and usually has no seeds. It accounts for about 70% of domestic citrus production in Japan (as reported in 2018) [1]. But who are its parents? In February 2018, the National Agriculture and Food Research Organization (NARO) and the National Institute of Genetics announced they had decoded the entire genome of the unshu mikan. The results showed that its parents were the Kishu mikan and a fruit called kunenbo [1]. Furthermore, researchers confirmed that kunenbo is also a child of the Kishu mikan. The other parent of the kunenbo remains unknown [1].
If we draw this as a family tree, the Kishu mikan is both a parent and a grandparent to the unshu mikan. If we assume that half of an offspring's genes come from each parent, at least three-quarters of the unshu mikan’s genes come from the Kishu mikan line: one-half directly from the parent and one-quarter through the kunenbo [1]. This is a rough estimate calculated here, not an official figure. If the unknown parent of the kunenbo is closely related to the Kishu mikan, this percentage would be even higher.
Scientists explain that the lack of seeds occurs because the plant has weak abilities to produce both pollen and eggs, combined with a strong tendency for fruit to develop without pollination [1]. To understand this lineage, we must look at what kind of fruit the Kishu mikan is.
2. The Kishu Mikan: The Parent Fruit and Where the Mandarin Was Born
The Kishu mikan is a small orange, about 5 cm (2 inches) in diameter and weighing 30–50 g (1–1.8 oz), with seeds in its segments [2]. Documents from Wakayama Prefecture describe it as the root of the famous Arita mandarin. Its origin is China, and before the unshu mikan became widespread in the mid-Meiji period (late 19th century), the Kishu mikan was the representative variety in the Arita region [2].
Where did the seedless unshu mikan come from? NARO states that it is believed to have appeared in the Kyushu region about 400–500 years ago [1]. A publication by the Ministry of Agriculture, Forestry and Fisheries specifies Nagashima Island in Kagoshima Prefecture as the place of origin. It suggests that the fruit grew accidentally from citrus trees brought from China [3]. These locations are not contradictory; one is a broader region (Kyushu) and the other is a specific island within it.
As the unshu mikan was propagated by grafting in various places, people selected branches with variations called "branch sports" [1]. Today, the unshu mikan has become a parent to more than 70 varieties and strains, including Kiyomi and Dekopon (Shiranui) [1]. The Chinese-born Kishu mikan created a new fruit in Japan, which then produced many descendants. But if we trace the Kishu mikan back further, where does the entire citrus family tree lead?
3. Do Oranges and Lemons Share Three Basic Ancestor Species?
A review article in the Journal of Breeding Science explains that the genomes of hybrid citrus fruits inherit large DNA fragments from basic species like a mosaic [4]. The three basic species are the mandarin (the orange family), the buntan (pomelo), and the citron.
Encyclopedia entries note that oranges, lemons, grapefruits, and limes are hybrids created from these three species, their descendants, or crosses with other wild species [5]. However, it is not just these three. Some types also arose from crosses with wild species. There are two ways to write the scientific name for buntan. The exact parent combinations for individual varieties must be investigated case by case and cannot be listed here.
The Kishu mikan, the parent of the unshu mikan, is a relative close to the mandarin side of these three ancestors. If these three ancestors connect further back, where was their oldest common ancestor located?
4. Where Did the Ancestors of Citrus First Appear?
In 2018, a study by Wu et al. published in the scientific journal Nature analyzed the genomes of 60 lineages. It suggested that the citrus genus originated at the southeastern foothills of the Himalayas (eastern Assam in India, northern Myanmar, and western Yunnan in China) [6]. Separation from a common ancestor is said to have begun about 15 million years ago [5].
This study organized natural citrus into 10 species. It notes that the rapid increase in citrus types in Southeast Asia occurred during the Late Miocene (about 11.6–5.3 million years ago), coinciding with a weakening of monsoons [6].
However, the place of origin is not definitively settled. A Japanese research project funded by Grants-in-Aid for Scientific Research (Kitajima et al.) analyzed DNA from over 900 citrus specimens in East and Southeast Asia. They suggested that the basic species may have originated in different regions for each type, rather than one single place [7]. The southeastern Himalayan foothills are one leading theory.
At the easternmost edge of this wide distribution lies Japan. What citrus fruits grew naturally here?
5. How Many Citrus Species Are Native to Japan?
Japan marks the eastern limit of the natural distribution of citrus. The review in the Journal of Breeding Science states that there are only two native species in Japan: tachibana and shikuwasa (shikwasa) [4].
Shikuwasa has more differences in its genetic combinations than tachibana. Reports suggest it is likely a hybrid between tachibana and an unknown species, though this is not confirmed as a definite fact [8].
All other types are thought to have come from outside Japan. Encyclopedia entries mention legends that Tokijiku-no-kakunoko (said to be daidai) was introduced during the reign of Emperor Suinin and later brought to Kishu (Wakayama Prefecture), with one account citing the year 1574 [9]. These dates and stories include legends and theories, so they cannot be considered certain historical records.
Meanwhile, the Grants-in-Aid research estimates how tachibana, yuzu, Kishu mikan, daidai, karatachi (bitter orange), and kumquat arrived in Japan during the Nara period by analyzing DNA [7]. Efforts to re-read old stories using DNA are continuing today.
6. How to Start Looking for Citrus Parents with Fruit at Home
You can begin exploring citrus family trees in your kitchen. Place 2–3 types of citrus you have at home, such as mandarins, oranges, or lemons, side by side. Note the number of segments, seeds, ease of peeling, and smell in a notebook. Draw lines connecting those that seem similar. If you can find buntan (pomelo) or yuzu, it becomes even more interesting.
You can also investigate by name. Write down the variety names from labels. Check fruit tree books at your library or official agricultural websites for your prefecture to see if parent varieties are listed. If they are, compare them with your predictions.
For further reading, the press release from NARO and the National Institute of Genetics is available in Japanese and includes a family tree and glossary for the unshu mikan [1]. The Grants-in-Aid report also contains research on how citrus arrived in Japan [7].