Saturday, 20 June 2020

Structured diversity in tea does not mean multiple domestications


Genetic population structure of tea.
When and where were tea domesticated? And how many times? This is a question I get asked sometimes, and a paper published a few years ago was just brought to my attention that claims to have genetic evidence for three separate domestications, by Meegahakumbura et al. (2016) in PLOSone. I disagree. The paper convinced me of quite the opposite. It remains entirely likely that tea was domesticated only once in ancient times, even if there were widespread use of wild tea leaves across their wild range by people who encountered them-- but a key problem remains determining what that wild range was. The Meegahakumbara paper is an interesting one in terms of raising the question as to how much underlying geographical structure there is in the genetic diversity of tea, and how this relates to different cultural traditions of use, and which (if any) can be regarded to evolving into domesticated forms. But in the end there are severe limitations to their study. Despite some sampling from "wild" populations in India and China, as well as cultivars, there is a lack of samples from in between (northern Vietnam, Myanmar and presumably Laos, which are all presumably within the range of wild teas. What is really at issue is whether or not the large tree forms of tea are primarily wild (and recently managed or cultivated) or represents a distinct domesticated form, and this in turn raises the issues of what is "domestication" in tea. Domestication implies morphological adaptations, underpinned by genetics, which have been favoured through the propagation over generations by people. We know full well what this entails in cereals and seed crops (see, e.g. this Annals of Botany article); we can propose what is involved in tree fruits (which are harvested for fruits and presumably early on cultivated from seed: see "long and attenuated"); and we can also propose what is common about domestication in tubers and other vegecultural crops (see this recent Annals of botany article). But what about tea? Since tea can be propagated by seed or by cuttings it might share some similarities with either tree fruit or vegecultural domestications, but its use for leaf harvesting makes it quite different. Characteristics that might fit in the vegecultural domestication syndrome, include increased proportion of edible (leaf) to inedible (trunk) parts, and more asynchronous production of those. In developmental terms there appear to be selection for dwarfism-- smaller leaves, smaller stature, and more compact growth, which together with human management give teas their very trimmed hedge look (below).
Tea (C. sinensis sinensis) cultivation in Zhejiang
(Photo: DQ Fuller, 2004)

So what did Meegahakumbara et al. find? They demonstrated 3 distinct populations, and a 4th (lasiocalyx) that looks admixed between the Indian and Chinese tree teas (Figure at top). Truly domesticated tea (C. sinensis sinesis) looks to a be a strong clade. That is (in my view) the only really domesticated form. It has distinctive morphology (dwarfism).  It could already be distinct by the Han period, or at least under selection, since small, immature leaves are among the grave goods in the Yangling tomb of a Han Dynasty queen in Shaanxi, indicating trade from distance Yunnan already by 2200 years ago (see this Scientific Report). While it could be that tea was still a wild utilized plant, it might make sense that by this period experiments with cultivation had begun with an ancestral form of C. sinensis sinensis. It is also plausible that some cultivation developed in parallel with the distinct species C. taliensis, but its cultivation never spread beyond the Myanmar-Yunnan border regions (see, this paper on taliensis tea), so it is less relevant to the main domestication story of tea.

https://onlinelibrary.wiley.com/doi/abs/10.12705/666.11But is there any reason to assume that the distinct populations of large tea forms of tea, which they refer to as Assam tea (C. sinensis assamica). They divide this tall tree tea (see left) into two geographical groups (Indian assamica and Yunnan assamica, which leads to the somewhat oxymoronic terminology of "Chinese assam tea"). I do not see any reason in these data, or rationale provided, to not assume that Chinese tea is a domestication bottleneck from the Yunnan tree tea (assamica sensu  lato). The Assam and Yunnan populations of tree teas are distinct as one would expect of any geographically distant populations of a wild species. Trees of this assamica tea are also wild through large parts of northern Vietnam (see Zhao et al. 2017), and I would expect them also in Laos and parts of Myanmar.  In India these tree teas were used from the wild by a few local minority tribes prior to the British introduction of tea drinking and Chinese domesticated tea to India. But these were presumably only ever gathered wild. These Assam tree teas only came into cultivation (equivalent to pre-domestication cultivation) in the colonial era.  though they have now been cultivated by ~200 years, there seems no basis to assign them to domesticated status.

The Yunnan tree teas (What they call Chinese Assam tea) are presumably also native to the forests of parts of Yunnan, where they were also used by indigenous people. In this region their cultivation and management has continued while perhaps unmanaged populations went extinct in the wild. They could still be also the source of domesticated C. sinensis Chinese tea, or related to that source.

Tree (assamica) tea, leaves and fruits
(after a photo in the Hangzhou Tea museum)
The Cambod teas (C. lasiocalyx) look to me like part of the structured variation of a wild species that included geographical populations: i.e that the tea trees of Yunnan, India and adjacent Northern SE Asia (Vietnam, etc) borderlands should probably all be C. assamica (or  C., sinensis assamica, as taxonmically described by Zhao et al. 2017).  In their K=3 structure analysis these are grouped with Indian Assam wild tea trees, so it may be that lasiocalyx is just a northerly somewhat smaller variant of wild tea trees. It is plausible that domesticated Chinese tea came from this Cambod end of the wild range, i.e. further east, like SE Yunnan and is more distance from the Assam end of the geographical variation. It is a pity their is no geographical information on the assamica and lasiocalyx populations-- in terms of where and what habitats they come from. Although with a species like this it is not really surprising if wild populations have been completely lost. One is hard pressed to identify truly wild populations of some of the world's more popular nuts, whether chestnuts or walnuts, as these trees have been so heavily anthropogenically influenced in terms of where they grow.

So the origins of tea cultivation: southwest China, some place and time still to be determined. But the use of wild tea leaves by local peoples, widespread from Northeast India through Vietnam.

On the history of tea, see especially this magnum opus by Van Driem, reviewed here.

3 comments:

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