Juicy past of favorite Okinawan fruit revealed

Date:
July 26, 2021
Source:
Okinawa Institute of Science and Technology (OIST) Graduate
University
Summary:
A genetic analysis of fruit in the mandarin family has unraveled
a complex journey from the mountainous region of southern China
to the markets of Okinawa.



FULL STORY ========================================================================== Citrus fruits from the mandarin family are popular throughout the world
for their tasty and healthy characteristics. Within Japan, the tiny
shiikuwasha and the ornamental tachibana are of special cultural and
historical importance.

However, the origin of these two varieties, and other East Asian citrus,
was always something of a mystery, until now.


==========================================================================
In a new study, published in Nature Communications, scientists from the
Okinawa Institute of Science and Technology Graduate University (OIST),
and collaborators from other institutes, analyzed 69 genomes from the
East Asian mandarin family, alongside their mainland Asian relatives,
to reveal a far- ranging story of isolation, long-distance travel,
and hybridization.

The story starts in the Hunan Province of southern China, which is
the center of wild mandarin diversity and the genetic source of most
well-known mandarins.

When the scientists re-analyzed previously published genomic data, they unexpectedly found that wild mandarins of this mountainous region are
split into two subspecies.

"We found that one of these mandarin subspecies can produce offspring that
are genetically identical to the mother," said Dr. Guohong Albert Wu,
a research collaborator at the Lawrence Berkeley National Laboratory in California. "Like many other plants, wild citrus typically reproduces when
the pollen of the father combines with the egg of the mother, mixing the
genes from both parents in the seed. But we found a subspecies of wild mandarins from Mangshan, in southern China, where the seed contains an identical copy of the mother's DNA without any input from a father. So,
the seed grows to be a clone of the mother tree." Back in Okinawa, the researchers looked more carefully at a strange shiikuwasha-like citrus
that produces small, acidic fruit and had been ignored by local farmers
since it has little commercial value. To their surprise, they found that
this strange citrus represented a previously undescribed species, which
they named the Ryukyu mandarin or, more formally, Citrus ryukyuensis. And
in contrast to the well-known shiikuwasha, which reproduces clonally (like
the subspecies in Mangshan), the new species always reproduces sexually.

Remarkably, the researchers found that all shiikuwasha are hybrids of a
very specific type -- one parent is from the local Ryukyuan species and
the other, from mainland Asia. Surprisingly, all shiikuwasha have the same mainland mandarin parent, meaning that all shiikuwasha are half-siblings.



==========================================================================
They concluded that tens of thousands of years ago a mainland Asian
mandarin was transported, either by people or by natural methods,
to the land that would become the Ryukyu Islands. There it mated with
the native Ryukyu citrus. The researchers traced the ancestry of this
mainland Asian mandarin back to Mangshan, where it acquired its ability
to reproduce asexually. This ability was passed on to its children.

Thus, all the shiikuwasha varieties found in Okinawa's markets today are descended from this mating, and reproduce asexually, allowing stable
varieties like golden shiikuwasha to be propagated from generation
to generation.

And what of tachibana and the other East Asian mandarin variations?
"They're all hybrids!" explained Dr. Chikatoshi Sugimoto, Postdoctoral
Scholar in OIST's Molecular Genetics Unit. "The tachibana lineage also
seems to have descended from the newly described Ryukyu species and
another mandarin from China, but its birthplace was probably what is
now mainland Japan." Once they saw the genetic pattern in shiikuwasha
and tachibana, the researchers also recognized another half-sibling
family comprising various traditional Ryukyuan types -- oto, kabuchii, tarogayo, and other unnamed citrus. This family, which the researchers
called 'yukunibu' (sour citrus in the native Okinawan language), is
much younger than shiikuwasha and tachibana. It arose when the famous
kunenbo -- also the father of satsuma mandarins -- hybridized with the
native Ryukyu mandarin. Kunenbo was brought to Okinawa from Indochina
around 4-500 years ago by maritime trade. Like the mainland parents of shiikuwasha and tachibana, it was also able to clone itself by seeds,
due to its distant Mangshan ancestry, and it passed this trait on to
its children.

"It's fascinating to puzzle out the story of mandarin diversification and
its relationship to the biogeography of the region," concluded Prof. Dan Rokhsar, Principal Investigator of OIST's Molecular Genetics Unit. "But
it also could have commercial value. What other possibly hybrid types are there? Could we create new hybrids that are more resilient to disease
or drought, or have other desirable characteristics? By looking into
the past, we can create all sorts of possibilities for the future."
To unravel this diversity, the researchers worked closely with industry
and individuals in Okinawa, including Okinawa Prefectural Agricultural
Research Center, Nago Branch, Katsuyama Shiikuwasha, and local farmer,
Hiroshi Kobashigawa.

========================================================================== Story Source: Materials provided by Okinawa_Institute_of_Science_and_Technology_(OIST)
Graduate_University. Original written by Lucy Dickie. Note: Content may
be edited for style and length.


========================================================================== Journal Reference:
1. Guohong Albert Wu, Chikatoshi Sugimoto, Hideyasu Kinjo, Chika Azama,
Fumimasa Mitsube, Manuel Talon, Frederick G. Gmitter, Daniel
S. Rokhsar.

Diversification of mandarin citrus by hybrid speciation and
apomixis.

Nature Communications, 2021; 12 (1) DOI: 10.1038/s41467-021-24653-0 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210726102135.htm

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