4.2 The possible origin and domestication of D. oppositaand D. alata were speculated by genetic difference and population
structure analysis of China yam resources
Yam had gradually formed the cultivation and domestication centers of
Asian group, African
group
and American group in the long-term evolution process. Africa group,
including species of D. rotundata , D. alata , D.
cayenensis and D. dumetorum, has become the main regional belt
of yam production in the world, mainly distributed in Ghana, Togo, Benin
and other West African regions, Central Africa and Western Congo
(Coursey, 1976; Martin and Sadik, 1977). D. trofida was the
earliest domesticated variety in South America and was widely
cultivated, mainly distributed in Brazil, Venezuela, Paraguay and other
regions (Coursey, 1976). Asia was also the main distribution center of
yam. The main cultivated and domesticated species are D. opposita,
D. alata, D. esculenta, D. japonica, D. bulbifera, D. hispida andD. quinquelaba (Gong et al., 2004). China is one of the important
domestication centers of yams, with records about yams from 4000 years
ago. Thus far, the species of yams widely cultivated in China areD. opposita , D. alata , D. persimilis , D.
fordii , and D. esculenta (Li et al., 2009).
At present, the origin of yam could be traced back to the late
Cretaceous (Maurin et al. , 2016). Dioscorea is considered
to be a monophyletic group originating from a common ancestor (Wu,
2012), which represents an early-diverging lineage of monocots just
internal to Acorus (Hansen, 2007).
However,
there are still many different arguments about its origin, evolution
process and domestication process. In this study, a total of 112
cultivars, landraces and wild varieties of yam were collected in 21
provinces (cities) in China, and its genetic diversity was
comprehensively evaluated. D.
opposita was the most important with the largest cultivation area and
widest distribution in China. It was distributed from Heilongjiang
Province in the north, Hainan Province in the south, Shanghai in the
east, and Sichuan Province in the west. And its growth environment was
also more complicated, including plains, mountainous region and coastal
areas. Therefore, this may be the reason for the larger genetic
difference compared with other species to adapt to their respective
growth environment and climate change and climatic conditions. Moreover,D. opposita had the shortest growth period and the smallest
leaves compared with other species, indicating that the aboveground
biomass was small. In addition, a large number of wild resources ofD. opposita were distributed
in the northern and southern provinces of China. In the present study,
wild resources were collected in Henan, Hebei, Guizhou, Sichuan, and
Jiangxi Provinces. This may be a strong evidence that D. oppositamay have originated in China and domesticated by wild species.
Forty-one accessions of D. alata were collected in eight
provinces in Southern China, accounting for 36.61% of the total.
The
most typical features of this species were stem wings, strong growth
potential, and long growth period, and these characteristics were
similar to the previous results (Bressan et al., 2011). This may be
evidence to infer that it may have originated in tropical or subtropical
areas. With regards to previous report, D. alata may originate in
the north and east of the Bay of Bengal and spread to Southeast Asia,
Malaysia, Pacific tropical islands, Africa, and America (Nemorin, 2013).
Some authors have proposed that the ginseng potato was domesticated in
India or Yunnan Province in China (Coursey, 1976; Chaïr et al., 2016; Wu
et al. 2019). Moreover, D. alata is considered a heterozygous
species and inferred as a cross between the wild relative D.
hamiltonii and D. persimilis .
The wild resources of D.
persimilis was previously reported to be distributed in Hunan,
Guangdong, Guangxi, Guizhou and Yunnan provinces (Pei, 1985). And we
also collected the accessions of D. persimilis in Fujian and
Jiangxi provinces, and it has a long history of cultivation. In
addition, D. persimilis and D. opposita were closely
related, and some accessions were interpenetrated on phenotypic
traits.
The rDNA internal transcribed spacer (ITS) sequences also showed thatD. persimilis and D. opposita were closely related (Wu et
al., 2013; Liu et al., 2001). It is also speculated that D.
persimilis is considered to be an internal mutation of D.
oppositeita (Liu et al., 2001).
The wild resources of D. fordii are distributed in Zhejiang,
Guangdong, Guangxi, Fujian and Hunan provinces. It has been widely
cultivated for its high yield and good resistance to stress and has been
cultivated for more than 200 years so far. D. fordii may be
formed by long-term domestication of wild species. In the current study,
CY-193, CY-201, CY-206 and CY-209 from D. fordii were dispersed
in D. alata (Figs. 3-A and B), indicating that D. alataand D. fordii were the closest species genetically. Similar
results were obtained by Lei et al. (2013) and Li et al. (2016)
on genetic diversity and Wu et al. (2014) on ITS. However,
research on the origin and evolution of Dioscorea requires
additional genomic information and increased number of species for
further study.