Last updated February 8, 2018 at 1:13 pm
Aussie fruit are late bloomers, genomic study discovers.
The geographical origins of citrus fruit have been unclear until now – but genomics has changed all that.
Their evolution began in Asia, but it took the fruit four million years to make the relatively small jump from there to Australia, researchers have found.
The study also throws light on the domestication of the fruit from the the pure pomelo, or pummelo, that is one of the original citrus species.
The fruitful evolution of citrus
Researchers analysed the genomes from 60 different citrus fruits, using both previously published sequences and the addition of 30 new genome sequences.
Each selected to represent a diverse sampling of citrus species, including hybrids.
Among those chosen were three Australian citrus fruits – the Australian round lime, Australian finger lime, and Australian desert lime.
They revealed the whole picture that stitches together the genetic origins of citrus fruits that were previously unknown.
Using this data, they report that today’s citrus trees are derived from at least 10 natural citrus species.
In their paper, they conclude that “the centre of origin of citrus species was the southeast foothills of the Himalayas, in a region that includes the eastern area of Assam, northern Myanmar and western Yunnan.”
They propose that citrus diversified during the late Miocene epoch, six to eight million years ago, through southeast Asia.
This coincided with a weakening of Asian summer monsoons, which provided an opportunity for the migration of mammals and the rapid genetic diversification of different plants.
Genomic analysis bears migrant history for Aussie citrus
In particular, the researchers were able to rule out an Australian origin for citrus species. Instead, citrus arrived in Australia from southeast Asia, mostly likely by floating there.
It was then that Australian citrus species developed during the early Pliocene epoch, around four million years ago.
The closest relative to Australian citrus is Fortunella, a wild-growing native species in southern China.
It is likely that a precursor fruit migrated across the Wallace line, a transitional zone between Asia and Australia.
Citrus domestication and selection with pomelos
The researchers also examined their data to find patterns of pomelos, a pure citrus species, among mandarins, a result of hybridisation, to uncover the early history of citrus domestication.
The mixture with pomelo is correlated with fruit size and acidity, suggesting a role for pomelo hybridisation in citrus domestication.
They were able to show that fruit sizes had a positive correlation with the more pummelo genes it shared.
Another important factor in fruit domestication is palatability; in citrus low to moderate levels of acidity are favoured. Known palatable mandarins shared relatability with pomelos, while none of the known acidic mandarins they looked at shared any traits with pomelo.
The researchers cross-referenced some of the mandarin species such as the ancient Chinese cultivar Sun Chu Sha Kat with historical artefacts. This species was likely described in Han Yen-Chih’s AD 1178 monograph “Chü Lu” which documents citrus cultivation in ancient China.
The sharp, citrus tang that has been a part of human history has a story of its own to tell. The genus Citrus has some of the most widely cultivated fruit crops worldwide.
It has historically saved ocean explorers from scurvy, its preserved peels have been used in Chinese medicine for hundreds of centuries, it has counterbalanced rich, heavily spiced foods that needed a squeeze of freshness to liven the palate or freshen up a gin and tonic, and revolutionised children’s science experiments with its acidity and low pH.
Citrus fruits such as the Australian finger lime and desert lime have been foraged by one of the oldest surviving human civilisations, Indigenous Australians.
With such a rich history, the insights from this study have beared fruit to the origin, evolution, genealogy and domestication of citrus.
The research was published in Nature.