PhD in gene flow between cultivated and wild rice in East Africa: Mr. James Kanya

Beskrivelse

GENE FLOW BETWEEN CULTIVATED RICE (Oryza sativa) AND THE WILD RICE (Oryza longistaminata), AND ITS POTENTIAL ECOLOGICAL CONSEQUENCES IN KENYA

Hybridization occurs in many crops that are sexually compatible with their wild relatives, leading to crop-to-wild pollen gene flow. Gene flow may result in evolution of super weeds, loss of genetic resources or even extinction of endangered species. In this study, gene flow between cultivated rice (Oryza sativa) and wild rice (O. longistaminata) were studied in Kenya.

            For hybridization to be possible, dormancy breaking methods for O. longistamianta were investigated. Incubated (Memmert-TV 40b incubator, Germany) and non-incubated hulled seeds were germinated in 1) Gibberellic acid (GA), 2). Hydrogen peroxide (H₂O₂), and 3). Water (H₂O) at two temperature regimes (31^°C and 25^°C).  A similar set up for dehulled seeds was performed. Germination occurred in hulled but not in dehulled seeds. Highest germination occurred in Gibberellic acid for all treatments. Significant (P<0.05) differences were observed within treatments but not (P>0.05) between treatments. No interaction effects (P>0.05) were observed between germination media and incubation and also with temperature. Highest germination was observed at higher temperatures (31^°C) than at lower temperatures. (25^°C).

            Dispersal distance of rice pollen was investigated in Tana River district in the Coast Province of Kenya. O. sativa was planted in a 50m diameter experimental design. Pollen traps placed at different heights (1.2 and 1.8 m) and distances (North, East South and West) from the pollen source were used. Pollen count decreased rapidly with increasing distance from the pollen source. Differences (P<0.05) in pollen count were observed in different directions but no difference (P>0.05) between the two heights. The highest pollen count was observed between 11:00 am and 12:00 noon.

            F₁ hybrid seeds were generated from crosses between the two Oryza species under screenhouse conditions at Mwea Irrigation Scheme. Data on growth patterns, morphological traits and yield of parents and hybrids were scored. Only 6 % of the seed (F₁) set occurred with O.sativa as paternal plant but no seeds were formed in reciprocal crosses. Morphological differences (P<0.05) were observed among the plant types. In hybrids, yield correlated (r=+0.843) with flag leaf length but in O. longistaminata yield correlated with plant height (r=+0.767) and panicle length (r= +0.664). In O. sativa, yield correlated with plant height (r= +0.741) and panicle exsertion. (r= +0.854).

            In order to confirm that the hybrids identified by morphological markers were true hybrids, molecular characterization was performed. DNA extraction and Polymerase Chain (PCR) reactions were conducted at the University of Nairobi. 22 rice simple sequence repeat (SSR) primers were screened to select markers that could identify the two parents and their F₁ hybrids. Six markers were found to be polymorphic across the samples. F₁ hybrids were identified among the parents by genotyping. A total of 12 alleles were detected and the number of alleles per primer was two.

            Generally, the main findings of the study were: 1) Dehulling of O. longistaminata seeds was found to be necessary for effective germination., 2) Hybridization can occur between cultivated (O. sativa) and wild rice (O. longistaminata), 3) To prevent such hybridization, a minimum isolation distance of more than 250 m between the two species is recommended, and 4) Use of morphology followed by molecular characterization is recommended for identification of rice hybrids in the field.