4.2 Multi-way gene flow among Oryza types
Gene flow analysis revealed significant gene flow among all sixOryza types (Table 1). Among the rice groups, O. nivarashowed higher sensitivity to introgression, and asymmetric gene flows
were observed between wild and weedy rice groups, particularly betweenO. rufipogon and weedy rice (Table 1). The open panicle structure
of O. rufipogon makes it more receptive to foreign pollen,
resulting in higher gene flow from other Oryza groups. TheO. nivara populations studied were in natural habitats near
commercial rice fields. Long-term gene flow from DWWC components toO. nivara populations may promote evolution and local adaptation,
transitioning their reproductive system from high selfing to slight
outcrossing, increasing its sensitivity to introgression.
In Sri Lanka, rice cultivation is carried out in two seasons,Maha and Yala , coinciding with the inter-monsoon and
monsoon rains. Approximately 450,000 hectares of paddy are cultivated
during the Yala season (March-September), while 850,000 hectares
are cultivated in the Maha season (October-February)
(Punyawardana, 2008). Most inbred rice varieties and landraces have a
maturity duration of three to four and a half months, leading to overlap
in flowering periods with wild rice, particularly during the Mahaseason. Concurrently, O. rufipogon being perennial, can be
observed throughout the year, while the annual O. nivara seeds
germinate at the beginning of the Maha season. Both species
overlap in flowering time with inbred rice varieties. This increases the
possibilities of pollen-mediated gene flow facilitated by monsoon wind
currents and insects, contributing to admixtures between wild and weedy
rice groups. Further, volunteer weeds may evolve into weedy rice over
generations and interact with the DWWC components over time. Long-term
introduction and persistence within the DWWC has resulted in complex
multi-directional gene flow among its components, shaping the complex
rice ecosystem in Sri Lanka (Vaughan et al ., 2005).
Our findings support higher gene flow from weedy rice to O.
rufipogon (M=7.299) than vice versa (M=4.334), and significant gene
flow was observed from cultivated rice to feral rice (M=8.1597) (Table
1), implying an early stage of de-domestication (Ellstrand et
al., 2010). Pollen-mediated gene exchange occurs between crops and
weeds or wild relatives and between weedy and wild relatives. Reverse
gene flow from weed to the crop has been well documented (Langevinet al., 1990; Majumder et al., 1997; Song et al.,2002). The presence of wild Oryza and greater diversity of crop
varieties and landraces in the region (Song et al., 2014)
contribute to the evolutionary dynamics of South Asian weeds.
Domesticated rice can mate with wild relatives, leading to gene flow
that can influence the genetic structure of wild rice (Wang et
al., 2017). Similarly, gene flow between cultivated carrots
(Daucus carota ssp. sativus) and wild relatives (D. carotassp. carota) has caused adjacent wild populations to become genetically
similar to the cultivars, contributing to the development of more
aggressive weeds in carrot fields (Magnussen and Hauser, 2007).
Weedy rice in the U.S.A. is genetically distinct from inbred rice
varieties due to de-domestication processes during its evolution and
accidental introductions (Reagon et al., 2010; Li et al.,2017). It predominantly descends from wild rice populations in tropical
Asia (Londo and Schaal, 2007; Reagon et al ., 2010), partly
incurred by an extended flowering period and weak post-zygotic
reproductive barrier, which promotes gene flow between cultivated and
wild rice (Craig et al ., 2014). The gene flow patterns elucidated
in the present study concerning weedy, wild, and cultivated rice
corroborate the scenario (Table 1). Within the context of weedy rice
origins and evolution in Sri Lanka, multiple factors exert influence,
such as the feralization of cultivated rice, hybridization between
landraces and/or inbred rice varieties, the adaptation of wild O.
nivara , and the hybridization between domesticated and wild rice
(Rieseberg et al ., 1993; Arnold, 1997; Rosenthal et al.,2008; Ellstrand, 2009; Ellstrand et al., 2010). Wedger et
al . (2019) discovered that 23% of the studied weedy rice plants in
Thailand exhibited introgressed alleles at one or more loci, derived
from three domestication gene sequences of DWWC. Similarly, a preceding
study done in Thailand (Wedger et al., 2019; Pusadee et al., 2013;
Wongtamee et al., 2017) reported that 17.6% of 29 weedy rice plants
showed evidence of introgression from wild or cultivated rice groups,
indicating gene flow as a significant driving force for the evolutionary
dynamics of the Oryza complex. Based on pairwise migration data,
He et al . (2014) reported a significantly high level of gene flow
within distinct weedy rice populations in Sri Lanka. Similarly, gene
flow from Malaysian weedy rice to O. rufipogon substantiates the
hypothesis positing potential introgression with other indigenousOryzas within the context of wild rice populations (San Sudoet al ., 2021). These studies indicate that weedy rice in the
Asian region has undergone a complex evolutionary process involving gene
flow with wild and cultivated rice, contributing to its genetic
distinctiveness from cultivated rice varieties.