Hybridization Potential between Cultivated Rice Oryza sativa and African Wild Rice Oryza longistaminata
Hybridization potential between cultivated rice (Oryza sativa) and wild rice (Oryza longistaminata) was studied in Kenya. At first seeds of the two parents were sown and their growth patterns established. At maturity, F1 hybrid seeds were generated from manual crosses between the two Oryza species under screen house conditions. The F1 hybrid seeds and seeds of the two parents were sown to compare the hybrids growth patterns and seed production with those of the parents. Correlations of seed production with other and morphological discrimination of hybrids from the parents were also scored. This study showed that hybridization between the two species can occur resulting in 6% hybrids seeds. On growth patterns, O. longistaminata plants grew taller than F1 plants which grew taller than O. sativa plants. The three types of plants continued to grow in height up to maturity but the gain in height in the hybrid and cultivar plants slowed down upon reaching the flowering stage, after the 10th week. It was also found that seed production in hybrids positively correlated with flag leaf length; while in O. longistaminata, seed produced positively correlated with plant height and panicle length. However, in O. sativa, number of seeds correlated with panicle exertion. The study also indicates that there were morphological differences (plant height, flag leaf length, panicle exertion and awn length) between the hybrids and the parents suggesting that these traits could be used as markers for identification of the hybrid plants from the parents.
May 16, 2012; Accepted: June 30, 2012;
Published: July 20, 2012
Asian domesticated rice (Oryza sativa) was introduced to East Africa
by Arab settlers about 600 years ago (Vaughan et al.,
2005). Cultivation of the species in Kenya was initially restricted to the
coastal areas but currently it has spread into the interior regions. In Africa,
the Asian rice got into contact with indigenous wild African rice species. Among
these, O. longistaminata and Oryza barthii are likely to have
hybridized with the introduced Oryza sativa species (Lu
and Snow, 2005), as they share the same AA genome. Both O. longistaminata
and O. barthii occur in natural populations outside rice cultivation
regions and as weeds in rice fields (Kiambi et al.,
2005). Under field conditions, O. longistaminata seems to hybridize
with the cultivated rice (Oka and Chang, 1961; Chu
and Oka, 1970; Ghesquiere, 1985; Kiambi
et al., 2005), although there is need for more studies to confirm
Hybridization between cultivated and wild rice may have a number of negative
impacts. Foremost among these may be the development of aggressive weeds, where
the wild recipients benefit from uptake of crop genes through repeated intercrossing
and introgression (Barrett, 1983; Langevin
et al., 1990). This has received increasing attention recently due
to the employment of transgenes in genetically modified crops which may potentially
also benefit weedy and wild recipients (Spencer and Snow,
2001; Ellstrand, 2003; Lovei et
In contrast, it has been demonstrated that wild relatives of rice can harbour
favourable genes that are not present in the cultivated rice (Wu
and Tanksley, 1993; Brondani et al., 2002).
For example, genes that enhance yield of cultivated rice can be introgressed
from wild relatives through Genetic Engineering (GE). There is thus also a need
to preserve the wild species germplasm for future plant improvement (Song
et al., 2005) which may include measure to avoid too much inflow
of cultivar genes into the wild gene pool.
Thus, in order to manage gene flow for avoidance of weed evolution and loss of genetic resources, we need to understand the hybridization potential between the wild and cultivar species. The present study therefore aimed at enhancing this information. Our objectives were to investigate (1) the extent to which cultivated rice and the wild African rice are able to hybridize, (2) if hybrids differ in their vigour and seed set compared to their parents and (3) whether the growth and morphology of hybrids are distinct from the parents which would allow the detection of hybrids in the field.
MATERIALS AND METHODS
Study species and material: The genus Oryza comprises 24 species,
of which 22 are wild and two, Oryza sativa and Oryza glaberrima,
are cultivated (Vaughan, 1994). Oryza sativa is
grown worldwide while O. glaberrima is grown solely in West African countries.
Cultivated rice has evolved into two eco-genetic types called indica
and japonica (Oka, 1988). The majority of rice
cultivars in the tropics are indica whereas japonica is generally
grown in temperate regions or at higher altitudes in the tropics (Dally
and Second, 1990).
Oryza longistaminata is a wild rice species that grows throughout tropical
Africa and Madagascar (Gibbs-Russell et al., 1989)
at altitudes between sea level and 2,100 m (Lu, 1999;
Kiambi et al., 2005). In contrast to the cultigen,
O. longistaminata is perennial with extensive, branched rhizomes. It
is up to 2 m tall and grows in swampy areas, at edges of lakes and streams,
in water down to a depth of 4 m (Gibbs-Russell et al.,
Oryza sativa seeds (cv. Basmati 370) were obtained from the National Irrigation Board Nairobi, Kenya while about 1 kg of O. longistaminata seeds were obtained from Tana River District in the Coast Province of Kenya, where the plant naturally grows in abandoned rice fields. The seeds of O. longistaminata were sun-dried and placed in porous paper bags and transported to Nairobi where they were stored at room temperature (23-27°C) for 6 months. The seeds were then transferred to Mwea Irrigation Agricultural Development (MIAD) Centre (37°20 E and 0°40 S; altitude 1,159 m above sea level) where confined field experiments were conducted.
Controlled hybridization: Two sets of experiments were conducted: the
first experiment to generate F1 hybrids (O. sativaxO. longistaminata)
seeds and the second experiment to assess vigour, seed production and morphological
characteristics of the three plant types (F1 hybrids, O. sativa
and O. longistaminata).
Ninety seeds of O. sativa and 90 seeds of O. longistaminata were germinated in 6 petri dishes (three petri dishes for seeds of each of the plant species). Seeds of O. sativa were planted in March 2007. To synchronize flowering of the two plant species, one set of O. longistaminata seeds was planted in December 2006 and a second and third set at the beginning of January and February 2007, respectively; the set from December later turned out to be synchronous with O. sativa and were used for the crossings. The resulting seedlings were transplanted into 6 plastic basins (3 basins for each species), with 30 cm diameter and 40 cm depth. Basins were filled with soil collected from the paddy fields. In each basin, 20 seedlings were planted (an equivalent density of 40 plants m-2) to make a total of 120 seedlings, 60 from each Oryza species. Basins were arranged in three blocks for each Oryza species.
From each basin, fourteen seedlings were randomly selected and marked for further
measurements of morphological traits (plant height, flag leaf length and panicle
exertion, grain length and awn length). At flowering, seven plants were randomly
selected among the fourteen marked plants from each basin, for each species
and were used as maternal plants. The other seven plants were used as paternal
plants. From each of the maternal plants, all the florets (individual flowers)
of a spike were emasculated by cutting off the tips of the florets, about one-third
of the floret, as described by Kaushal and Raven (1998).
Each plant (genet) has an average of nine tillers (ramets) which carry one panicle
with an average of eight spikelets and an average of seven florets. Thus, in
each genet, 504 florets were emasculated. Since seven genets (maternal parents)
were selected for emasculation from each basin, 3,528 florets (7 genetsx504
florets) were emasculated per basin and in total approximately 10,584 florets
(3,528 floretsx3 basins) were emasculated for each species in the entire study.
The emasculated panicles were loosely covered in a porus paper to prevent contamination
with unintended pollen.
The other seven plants in each basin were used as pollen donors. Panicles from
these plants were cut off the day following emasculation of the female donors
at 8.00 h and kept in separate flasks with warm water for about 2 h to facilitate
opening of the florets, as described by Kaushal and Raven
(1998). Once the maternal panicles opened (between 10.00-11.00 h), they
were uncovered and pollen from O. longistaminata flowers in the flasks
was immediately sprinkled over the maternal flowers. All tillers of an individual
recipient (maternal plants) were pollinated with the pollen from the same donor
and only one donor was used. The porous paper was immediately returned over
the maternal panicles to prevent further pollination from other pollen sources.
The process was repeated for O. sativa as the paternal and O. longistaminata
as maternal and at the same time as that for reciprocal crosses. At maturity,
the F1 seeds were harvested from each plant, counted and stored in
separate porous paper bags. After two weeks, the seeds were incubated at 50°C
for 7 days to break dormancy. After incubation, the F1 seeds were
planted together with O. sativa and the O. longistaminata seeds
for characterization of plant vigour, seed production and morphology.
To prevent unanticipated release of the wild rice species in this rice growing region (Mwea) of Kenya, where the wild species does not naturally occur, environmental safety issues were observed during controlled hybridization stage. All materials including pollen of the O. longistaminata were handled according to biosafety regulations in Kenya.
Measurement of seed and morphological traits: Twenty seeds of each of
plant type, O. sativa, O. longistaminata and F1 hybrids,
were planted in the same experimental design as described above. Plant height
was scored every two weeks from the time of radicle emergence as the distance
from the base of the stem to the tip of the longest leaf (Yoshida,
1981). Flag leaf length, panicle length and panicle exertion (the protrusion
of the flower head) were scored every two days from the time of panicle initiation
to the end of complete panicle formation (which took about three weeks). The
number of spikelets per plant, awn length (the terminal part of the bearded
lemma), grain length and number of seeds were noted at harvesting. For each
of the plant species self-pollination was facilitated by covering all the panicles
of each plant.
Statistical analysis: Differences between cultivar, wild and F1
plants were presented by box plots and statistically compared by analysis of
variance (ANOVA), using R statistical software version 2.6.1 (R
Development Core Team, 2006). Student-Newman-Keuls (SNK) test was
used to separate the means when ANOVA was significant.
A total of 594 F1 hybrid seeds were obtained from the controlled pollinations (i.e., approximately 6% of the pollinated flowers that developed into seeds; Table 1. All of the seeds were obtained with O. sativa as the recipient plant, despite seemingly successful emasculation and pollination. After the F1 and parental seeds were sown, O. longistaminata plants grew taller than F1 plants which grew taller than O. sativa plants (F2, 288 = 34; p<0.05; Fig. 1). The plants continued to grow in height up to maturity but the gain in height in the hybrid and cultivar plants slowed down upon reaching the flowering stage, after the 10th week (Fig. 1b).
F1 hybrids developed the longest flag leaves and O. longistaminata
the shortest (Fig. 2a, b; F2, 321
= 29; p<0.05). However, unlike in O. sativa and the F1
hybrids, where growth in plant height decreased towards flowering period, growth
of flag leaves continued to grow throughout the entire period (2b).
||(a) Final plant height at maturity and (b) Growth patterns
in plant height of the three plant types. In the box plots the figures 25,
50 and 75 percentiles are indicated by boxes, averages by broken lines,
10 and 90 percentiles by whiskers and more extreme counts by dots, respectively
|| F1 hybrid seeds formed from crosses between
O. sativa and O. longistaminata
||(a) Mean flag leaf length at maturity and (b) Growth pattern
of flag leaf in the three plant types
||(a) Mean panicle exertion at maturity and (b) Growth patterns
in panicle exertion in the three plant types
A reverse growth pattern was however observed in panicle exertion; the exertion
was greatest in O. longistaminata (29 cm) but lowest in hybrids (Fig.
3a; 5 cm; F2, 288 = 290; p<0.05). Similarly, O. longistaminata
displayed the greatest growth pattern in exertion while the hybrids had the
least (Fig. 3b). Panicles were longest in F1 hybrids
(32 cm) and shortest in O. sativa (Fig. 4; 25 cm: F2,
417 = 126; p<0.05). In contrast, awns were longest in O. longistaminata
(F2, 333 = 258; p<0.05; Fig. 5), with no substantial
difference between F1 hybrids and O. sativa.
|| Mean panicle length at maturity
|| Mean awn length between the F1 hybrids and parents
|| Mean number of seeds produced by F1 hybrids, Oryza
longistaminata and O. sativa
At maturity, F1 hybrids produced the highest number of seeds per
plant while the wild plants (O. longistaminata) produced the least (Fig.
6; F2, 501 = 413; p<0.05). Among the F1 plants,
seed production was positively correlated with flag leaf length (r = 0.843;
p<0.05; Fig. 7a), among O. longistaminata plants
seed production was positively correlated with plant height (r = 0.767; p<0.05)
and panicle length (r = 0.664; p<0.05; Fig. 7b) and among
O. sativa, seed production was positively correlated with plant height
(r = 0.741; p<0.05) and panicle exertion (r = 0.854; p<0.05; Fig.
||Correlation of seed production with, (a) Flag leaf length
in the F1 hybrids, (b) Plant height and panicle exertion in O.
longistaminata and (c) Plant height and panicle exertion in O. sativa
Our study shows that hybridization between East African wild rice, O. longistaminata
and Asian rice (O. sativa) is possible, although only when O. sativa
is the maternal plant and even then with only low success (6% seeds/pollinations).
The low hybridization success corresponds to the findings of Causse
and Ghesquiere (1991) who reported a pollination success of 3% and 5% in
crosses between the two species. The low success in producing F1 hybrid
seeds has been attributed to low pollen fertility of only 5% in O. longistaminata
(Causse and Ghesquiere, 1991) and deterioration of hybrid
embryos about 3 days after fertilization (Sano, 1989).
It could also be attributed to possible errors in emasculation and the subsequent
Seed formation only with O. longistaminata as the pollen donor has been
reported in other studies (Oka and Chang, 1961; Morishima
et al., 1992; Song et al., 2003). Unidirectional
hybridization has also been reported in crosses between O. sativa and
other wild rice species (Oka, 1988); however, in hybridizations
with Oryza punctata, Noldin et al. (2002)
demonstrated that pollen can flow in either direction but often from the tall
O. punctata to the short O. sativa plants. Studies by Oka
(1988) and Morishima et al. (1992) indicate
that floral morphological features can affect reciprocal seed set. Cultivated
rice generally has short styles and stigmas (1.5-4.0 mm in combined length),
short anthers, limited pollen viability and brief period between opening of
florets and release of pollen (between half a minute and nine minutes). On the
other hand, wild rice species differ in all of these characteristics, with longer
styles, stigmas and anthers and pollen that remain viable for up to twice as
long as in cultivated rice. This is likely the cause for the observed unidirectional
hybridization in this study. Unidirectional hybridization has also been found
in many other plant hybridizations (Arnold, 1997).
After germination of the F1 seeds and seeds of O. longistaminata
and O. sativa, growth patterns of the three plant types were similar
at the early stages of development (2-8 weeks; the vegetative stage) but started
developing differences in height from the 10th week onwards. This coincides
with the stage of profuse tillering (WARDA, 1999), when
rapid ground cover enables the rice crop to smother (rapid ground cover) and
thus out-compete weeds. As the F1 hybrids were intermediate between
the tall wild O. longistaminata and the low O. sativa, this indicates
that they may have less competitive advantage as weeds over the cultivar plants.
F1 hybrids, in contrast, produced significantly more seeds than
either their wild or cultivar parents, suggesting that this trait is affected
by heterosis (Lu and Xu, 2010) and hybrid vigour, as
has been found in other hybrids between cultivated and wild plants (Hauser
et al., 1998). This shows that the F1 hybrids, once formed,
may produce many offspring which will enhance chances of continued intercrossing
and introgression of cultivar genes into the wild gene pool. However, the following
generation, F2 or backcrosses, may be negatively affected by the
break-up of positive epistatic interaction of the pure species by recombination.
Even though this will in the majority of cases have a negative effect on hybrid
fitness, it may sometimes create plants with novel beneficial gene combinations
e.g., enhancing weediness (Arnold et al., 1999).
Seed production was thus not related to plant height. The wild species (O.
longistaminata), despite having an outstanding height, produced the lowest
number of seeds (mainly due to embryo breakdown) compared to the hybrids and
cultivar parents. However, this observation is not in agreement with hybridization
observed in other plants, where height improves access to light, hence increasing
seed production (Kende et al., 1998). However,
in modern hybrid breeding, dwarf crops have been bred to avoid logging i.e.,
falling of plants due to weak culms (Jackson, 1985; Kende
et al., 1998) and to improve translocation of resources from stems
and leaves to seeds (WARDA, 1999). Therefore, short rice
cultivars are preferred especially in ecological zones with strong winds. A
possible explanation for the low yield in relation to height is that high investment
in plant height incurs costs linked to construction and maintenance (Falster
and Westoby, 2003). However, generation of tall rice hybrids that are capable
of producing more seeds is an indication that the hybrids may have high incidence
of logging in the paddy fields. This can also be used as a good indicator for
identification of hybrids, so that they are weeded out to avoid gene flow between
crops and wild plants.
Seed production in the F1 was instead correlated to their length
of flag leaves. This is in line with findings of Dere and
Yildirim (2006) that high seed production in rice correlated with well-
established flag leaves. Long and broad flag leaves have more surface area hence
high productivity. Flag leaves, compared to other leaves; contribute most photosynthetic
products that are important in the grain filling (Blake
et al., 2007). According to Dere and Yildirim
(2006), flag leaves in cereals contribute about 41-43% of photosynthetic
assimilates. Hasegawa and Horie (1996) found that distribution
of assimilates from leaves varied depending on the position of the leaf. About
80% of assimilate from flag leaf was translocated to the panicle and ~5% of
it came from the fifth leaf from the flag leaf. The influence of flag leaves
on seed production is also observed in the two parental Oryza species.
The wild plants (O. longistaminata) had the shortest flag leaves which
corresponded with low seed production while the cultivar plants (O. sativa)
had intermediate flag leaves that corresponded to intermediate seed production.
Although, awn length did not show any correlation with seed production among
the three plant types in this study, in other studies it has been reported to
play a major role post-harvest, as long awns reduces predatory effects of birds
and other rice feeders, thus increasing their survival rate (Satoh
et al., 1990). Short awns in hybrids in this study imply that bird
predation may be severe among the hybrids compared to parent plants. The effect
of predation on rice grains is a serious problem in Kenya (Wanjogu
and Mugambi, 2001) and therefore predation of hybrid seeds would reduce
the potential of gene flow through seeds.
Implications: Our study clearly demonstrates that hybridization between
cultivated and wild rice (O. longistaminata) is possible, despite a low
experimental crossing success and that the resulting hybrids are vigorous and
fertile. It is especially notable that hybrids are only (or preferentially)
produced by O. sativa mother plants, suggesting a scenario where hybrids
are first produced in by pollination from wild O. longistaminata donors
into fields. Here the seeds may be spilled, or if used for resowing, hybrids
could grow up and shatter their seeds within fields, as shattering is dominantly
inherited (Lin et al., 2007). Unless hybrids
are weeded out, cycles of resowing and shattering may allow evolution of weedy
types of hybrids, as is known to occur in other weedy rice types (Ferrero
and Vidotto, 1998). The high seed production of F1 hybrids may
speed up this evolution. Hybrid descendants will carry a mosaic of segments
of the O. longistaminata and genomes that are partly determined by selection
on the segment genes and epistatic interactions. Presently it is not known whether
O. longistaminata growing in and around rice fields in East Africa indeed
originate from such hybridizations but it is likely. From weedy hybrids in fields,
pollen and seeds may subsequently disperse into neighbouring wild populations,
carrying with them cultivar genes.
Weeding may stop the cycle of shattering and resowing in fields. This is particularly
possible if hybrids are easily distinguished from cultivar plants. Present study
therefore shows that F1 hybrids between the two Oryza species
can be distinguished from their parents using morphological traits. In particular,
they can be differentiated by their height (taller than cultivar plants), longer
flag leafs and longer panicles. However, recognition of the descendants in subsequent
generations may be more difficult due to segregation (Sorensen
et al., 2007). The findings of this study can be used in the assessment
of gene flow between cultivated rice and Genetically Engineered (GE) rice in
the event that the latter is introduced in the country. These findings will
also argument similar studies to ascertain the stability of the hybrids.
The authors thank the management of Mwea Irrigation Agricultural Development
(MIAD) Centre for availing land for the confined experiments, the University
of Nairobi for providing facilities and the Danish agricultural council and
Danida for additional support to Dr. Kanya. We thank Buya Thiophilus for assisting
in collection of wild seeds in Tana River District. This work was supported
by the BiosafeTrain Project and was financed by a DANIDA-ENRECA programme grant.
Arnold, M.L., 1997. Natural Hybridization and Evolution. Oxford University Press, New York, USA.
Arnold, M.L., M.R. Bulger, J.M. Burke, A.L. Hempel and J.H.Williams, 1999. Natural hybridization: How low can you go and still be important? Ecology, 80: 371-381.
Direct Link |
Barrett, S.C.H., 1983. Crop mimicry in weeds. Econ. Bot., 37: 255-282.
Direct Link |
Blake, N.K., S.P. Lanning, J.M. Martin, J.D. Sherman and L.E. Talbert, 2007. Relationship of flag leaf characteristics to economically important traits in two spring wheat crosses. Crop Sci., 47: 491-494.
Direct Link |
Brondani, C., N. Rangel, V. Brondani and E. Ferreira, 2002. QTL mapping and introgression of yield-related traits from Oryza glumaepatula to cultivated rice (Oryza sativa) using microsatellite markers. Theor. Applied Genet., 104: 1192-1203.
Causse, M. and A. Ghesquiere, 1991. Prospective use of O. longistaminata for rice breeding: In ice Genatics II. Rice Research Institute. Philippines, pp: 81-89.
Chu, Y.E. and H.I. Oka, 1970. Introgression across isolating barriers between Oryza perennis subsp: Barthiiand its related taxa. Evolution, 24: 135-144.
Direct Link |
Dally, A.M. and G. Second, 1990. Chloroplast DNA diversity in wild and cultivated species of rice (Genus Oryza, section Oryza). Cladistic-mutation and genetic-distance analysis. Theor. Applied Genet., 80: 209-222.
Dere, S. and M.B. Yildirim, 2006. Inheritance of grain yield per plant, flag leaf width and length in 8x8 Diallel cross pollination of bread wheat (T. aestivum L.). Turkey J. Agric., 30: 339-345.
Direct Link |
Ellstrand, N.C., 2003. Dangerous liaisons? When cultivated plants mate with their wild relatives. Hopkins University Press, London, pp: 288-293.
Falster, D.S. and M. Westoby, 2003. Plant height and evolutionary games. Trends in Ecology and Evolution, 18: 337-343.
Direct Link |
Ferrero, A. and F. Vidotto, 1998. Shattering ability of red rice seeds in cultural conditions. Proceedings of 50th International Symposium on Crop Protection, May 5, 1998, Gent, Belgium, pp: 839-843.
Ghesquiere, A., 1985. Evolution of Oryza longistaminata. in Rice genetics. International Rice Research Institute, Manila Philippines, pp: 15-27.
Gibbs-Russell, G.E., L. Watson, M. Koekemoer, L. Smook, N.P. Barker, H.M. Anderson and M.J. Dallwitz, 1989. Grasses of Southern Africa. Memoirs of the Botanical Survey of South Africa No. 58, National Botanical Institute, Pretoria, South Africa.
Hasegawa, T. and T. Horie, 1996. Rice leaf photosynthesis as a function of nitrogen and crop developmental stage. Crop Sci., 65: 553-554.
Hauser, T.P., R.G. Shaw and H. Ostergard, 1998. Fitness of F1 hybrids between weedy Brassica rapa and oilseed rape (B. napus). Heredity, 81: 429-435.
Jackson, M.B., 1985. Ethylene and responses of plants to soil waterlogging and submergence. Annu. Rev. Plant Phys., 36: 145-174.
Direct Link |
Kaushal, P. and S. Raven, 1998. Crossability of wild species of Oryza with O. sativa cvs PR 106 and Pusa Basmati 1 for transfer of bacterial leaf blight resistance through interspecific hybridization. Agric. Sci., 130: 423-431.
Direct Link |
Kende, H., E. Knaap and H.T. Cho, 1998. Deep water rice: A model plant to study stem elongation. Plant Physiol., 118: 1105-1110.
Direct Link |
Kiambi, D.K., H.J. Newbury, B.V. Ford-Lloyd and I. Dawson, 2005. Contrasting genetic diversity among Oryza longistaminata (A. Chev et Roehr) populations from different geographic origins using AFLP. Afr. J. Biotechnol., 4: 308-317.
Direct Link |
Langevin, S.A., K. Clay and J.B. Grace, 1990. The incidence and effects of hybridization between cultivated rice and its related weed red rice (Oryza sativa L.). Evolution, 44: 1000-1008.
Direct Link |
Lin, Z., M. Griffith, X. Li, Z. Zhu and I. Tan et al., 2007. Origin of seed shattering in rice (Oryza sativa L.). Planta, 226: 11-20.
Lovei, G.L., T. Bohn and T.A. Hilbeck, 2007. Biodiversity, ecosystem services and genetically modified organisms. In: Biosafety First: Holistic Approaches to Risk and Uncertainty in Genetic Engineering and Genetically Modified Organisms, Traavik, T. and L.C. Lim (Eds.). Tapir Academic Press, Trondheim, Norway, pp: 161-180.
Lu, B.R. and A. Snow, 2005. Gene flow from genetically modified Rice and Its Environmental Consequences. Bioscience, 55: 669-676.
Direct Link |
Lu, B.R., 1999. Taxonomy of genus Oryza (Poaceae): A historical perspectives and current status. Int. Rice Res. Notes, 24: 4-8.
Direct Link |
Lu, Z.M. and B.Q. Xu, 2010. On significance of heterotic group theory in hybrid rice breeding. Rice sci., 17: 94-98.
Morishima, H.M., Y. Sano and H.I. Oka, 1992. Evolutionary studies in cultivated rice and its wild relatives. Evolutionary Biol., 8: 135-184.
Noldin, J.A., S. Yokoyama, P. Antunes and R. Luzzardi, 2002. Outcrossing potential of glufosinate-resistant rice to red rice. Planta Daninha, 20: 243-251.
Oka, H.I. and W.T. Chang, 1961. Hybrid swarms between wild and cultivated rice species Oryza perennis and O. sativa. Evolution, 15: 418-430.
Direct Link |
Oka, H.I., 1988. Origin of cultivated rice. Experimental Genetics, 1: 45-51.
R Development Core Team, 2006. R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria.
Sano, Y., 1989. The direction of pollen flow between two co-occurring rice species, Oryza sativa and O. glaberrima. Heredity, 63: 353-357.
Satoh, H., H.M. Ching'ang'a, D. Ilaila and T.C. Katayama, 1990. On distribution and grain morphology of cultivated rice collected in Tanzania, 1988. Kagoshima University Press, Occasional Papers, 18: 73-82.
Song, Z., B. Li, J. Chen and B.R. Lu, 2005. Genetic diversity and conservation of common wild rice (Oryza rufipogon) in China. Plant Species Biol., 20: 83-92.
Song, Z.P., B.R. Lu, Y.G. Zhu and J.K. Chen, 2003. Gene flow from cultivated rice to the wild species Oryza rufipogon under experimental field conditions. New Phytol., 157: 657-665.
Direct Link |
Sorensen, B.S., L.P. Kiaer, R.B. Jorgensen and T.P. Hauser, 2007. The temporal development in a hybridizing population of wild and cultivated chicory (Cichorium intybus L.). Mol. Ecol., 16: 3292-3298.
Spencer, L.J. and A.A Snow, 2001. Fecundity of transgenic wild-crop hybrids of Cucurbita pepo: Implications for crop-to-wild gene flow. Heredity, 86: 694-702.
Vaughan, D.A., 1994. The wild Relatives of Rice. A Genetic Resource Handbook. International Rice Research Institute, Los Banos, Philippines, Pages: 137..
Vaughan, D.A., K. Kadowaki, A. Kaga and N. Tomooka, 2005. On the phylogeny and biogeography of the Genus Oryza. Breeding Sci., 55: 113-122.
Direct Link |
WARDA, 1999. Crossing African and Asian rice species: Report of Advances in Rice Research. West African Rice Development Association, Bouake, Cote d'Ivoire, pp: 13-36.
Wanjogu, R.K. and G. Mugambi, 2001. Rice research in Kenya. Improvement of rice production in eastern and Southern Africa region through research. Proceedings of a Workshop on Rice Research in Kenya, Malawi, Zambia and Tanzania, March 12-16, 2001, KATC, Moshi, Tanzania, pp: 15-23.
Wu, K.S. and S.D. Tanksley, 1993. Abundance, polymorphism and genetic mapping of microsatellite in rice. Mol. Gen. Genet., 241: 225-235.
Yoshida, S., 1981. Fundamentals of Rice Crop Science. International Rice Research Institute, Manila, Philippines.