The Testimony Of The Mandarin

Mandarin citrus fruits were first domesticated in the mountainous regions of Southern China, and spread widely from there. 

Hybridization of these mainland Chinese fruits and some wild species native to Japan's Southern Ryukyu Islands accounts for most important modern varieties of them.

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."

From Science Daily.

The body text of the source paper explains that:

We find that the complexity of mandarin relationships is considerably simplified by the discovery of three ancestral lineages which, together with pummelo, gave rise to all extant mandarin diversity by hybridization and introgression. One of these groups is a previously unknown wild species currently found in the Ryukyu islands; the other two are previously unrecognized sister subspecies of mainland Asian mandarin. 

Our analysis leads to a comprehensive revision of the origin and diversification of east Asian citrus, including the elucidation of the origins of apomixis in mandarin and its spread to related citrus including oranges, grapefruits and lemons.

The paper and its abstract are:

The origin and dispersal of cultivated and wild mandarin and related citrus are poorly understood. Here, comparative genome analysis of 69 new east Asian genomes and other mainland Asian citrus reveals a previously unrecognized wild sexual species native to the Ryukyu Islands: C. ryukyuensis sp. nov. 

The taxonomic complexity of east Asian mandarins then collapses to a satisfying simplicity, accounting for tachibana, shiikuwasha, and other traditional Ryukyuan mandarin types as homoploid hybrid species formed by combining C. ryukyuensis with various mainland mandarins. These hybrid species reproduce clonally by apomictic seed, a trait shared with oranges, grapefruits, lemons and many cultivated mandarins. 

We trace the origin of apomixis alleles in citrus to mangshanyeju wild mandarins, which played a central role in citrus domestication via adaptive wild introgression. Our results provide a coherent biogeographic framework for understanding the diversity and domestication of mandarin-type citrus through speciation, admixture, and rapid diffusion of apomictic reproduction.

Guohong Albert Wu, et al., "Diversification of mandarin citrus by hybrid speciation and apomixis." 12(1) Nature Communications (July 26, 2021) (open access). 

DOI: 10.1038/s41467-021-24653-0