Collection of wild rice (Oryza L.) in east and southern Africa in response to genetic erosion

Abstract

Collecting germplasm is the first step in ex situ conservation and clearly an important prerequisite for the use of the material by breeders. The reasons for collecting germplasm include danger of genetic erosion or extinction, users at national and international level have expressed a clear need for the germplasm, the genetic diversity is missing or insufficiently represented in existing ex situ germplasm conservation, and that more needs to be known about it (Engels et al. 1995). Floristic studies, plant inventories and herbarium specimens indicate that the east and southern Africa region is home to five wild species of Oryza. These are Oryza barthii, O. brachyantha, O. eichingeri, O. longistaminata and O. punctata (Fernandes et al. 1971; FAO 1987; Vaughan 1989, 1994; Leistner 1990). Clayton (1968, 1970) clarified the species names for the African wild relatives of cultivated rice by distinguishing O. barthii as the annual relative of O. glaberrima and O. longistaminata as the rhizomatous perennial relative. Studies on the origin and evolution of twin microsatellites in the genus Oryza have now clearly revealed that O. barthii is the ancestor of the cultivated African rice, O. glaberrima (Akagi et al. 1998), primarily found in west Africa but has now spread to parts of east Africa, Zanzibar in particular (Vaughan 1994; WARDA 1997). Geographically, the species are widely distributed in different countries of this region. The highest inter-specific diversity is found in Zambia and Tanzania, in each of which at least four out of the five species occur (Clayton 1968, 1970; Fernandes et al. 1971; FAO 1987; Leistner 1990; Vaughan 1994; Phillips 1995). The wild relatives of rice are an important source of agronomically useful traits that have been extensively used in rice improvement programmes (IRRI 1970; Khush 1977; Lin and Yuan, 1980; Agueirro et al. 1984; Heinrichs et al. 1985; Jena and Khush 1990; Khush et al. 1990; Brar et al. 1991; Amante et al. 1992; Ishii et al. 1994; Multani et al. 1994; Brar and Khush 1995). In particular, there are reports of the invaluable contribution that the germplasm of the wild relatives of cultivated rice found in east and southern Africa has made in rice breeding programmes. For example, O. longistaminata A. Chev. et Roehr. has been reported to have genes for bacterial blight resistance, high pollen production, long stigmas and drought tolerance (Second et al. 1977; Taillebois 1983; Khush et al. 1990; WARDA 1997). Resistance to Meloidogyne graminicola, the nematode species causing significant damage to rice in west Africa, was found in an O. longistaminata accession originating from Botswana (Jones et al. 1996). Genes forresistance to brown plant hopper and bacterial blight have been transferred from O. brachyantha A. Chev. et Roehr. to O. sativa L. (Khush et al. 1990). Oryza brachyantha also has traits for adaptation to lateritic soils, and resistance to yellow stemborer, leaf folder and whorl maggot (Khush 1977; Heinrichs et al. 1985; IRRI 1990; Brar and Khush 1995). Oryza barthii A. Chev. has resistance to green leaf hopper, sheath blight and bacterial blight, and shows characteristics for drought resistance (Henrichs et al. 1985; Amante et al. 1990). Ou (1985) has reported that genes for tolerance to yellow mottle occur in O. eichingeri A. Peter, while salt tolerance was found in O. punctata Kotschy ex Steud. by Farooq et al. (1994). The tetraploid race of this last species, reported in Tanzania, was found to have resistance to zigzag leaf hopper (Heinrichs et al. 1985). Marshall and Brown (1975) and Brown and Marshall (1995) suggested that the objective of any germplasm sampling strategy is to include in the sample at least one copy of 95% of the alleles that occur in the target population at frequencies greater than 0.05, i.e. the so-called ‘common’ alleles. However, Allard (1970) recognized that collectors might have little time and resources at their disposal. Therefore, the problem is to define a sampling procedure that yields the maximum amount of useful genetic variation within a specified and limited number of samples (Marshall and Brown 1983). The diversity of the wild rice species is gradually being eroded for a multiplicity of reasons. These include destruction of natural wild rice habitats to pave the way for expanding agricultural activities resulting from increasing population pressure, overgrazing and changes in land use (Reid et al. 1988; NWCMP 1996; Emerton and Muramira 1999). The need to assess the socio-economic activities that are causing genetic erosion, as well as collect and conserve this diversity, is therefore high. In the past there have been some collecting missions in Kenya, Tanzania, Madagascar and Zambia by D. Vaughan of the International Rice Research Institute (IRRI) and others, as reported in Bezançon and Second (1979), Miezan and Second (1979) and Katayama (1987, 1990). However both the species’ and geographical coverage were narrow. Consequently, the wild rice gene pool of the region was poorly represented in gene bank collections. According to Vaughan et al. (1991) and Vaughan (1994), prior to the collection missions reported in this paper, only 52 accessions of the wild rice relatives from the region were conserved in the International Rice Genebank at IRRI. This represented only 2.75% of the total world wild rice collection and 21% of the total wild rice accessions from Africa, despite their widespread distribution and occurrence in the continent. Moreover, before the collection missions reported in this paper, there were no wild rice germplasm collections conserved in the National Plant Genetic Resources Centres of the countries that participated in the work because, at the time of earlier collections, these countries didn’t have appropriate conservation facilities. This made collection and conservation of the germplasm urgent in order to expediently respond to the erosion pressures facing the wild rice species. This paper reports some of the indicators of genetic erosion of wild rice diversity, and the systematic collection and conservation of this germplasm in nine countries of the east and southern Africa region. The collecting missions were undertaken under the auspices of acollaborative programme involving the national PGR programmes of the region, the IRRI, SADC Plant Genetic Resources Centre (SPGRC), and the International Plant Genetic Resources Centre (IPGRI). The collaborators from the national programmes were: Joseph Kemei (Kenya National Gene Bank); Charles Mhazo and A. Mafa (Department of Research and Seed Services, Zimbabwe); Paulino Munisse ( Instituto du Investigacao Agraria, Mozambique); John Wasswa ( National Agricultural Research Organization of Uganda; Edwin Chiwona(Chitedze Agricultural Research Station, Malawi); M. Nawa (National Plant Genetic Resources Centre, Zambia); Herta Kolberg (National Biodiversity Institute, Namibia) and Margaret Nkya (Tanzania NPGRC). The objective was to sample the diversity of wild rice species in all the geographic and ecological areas where they have been recorded. The target taxa were O. barthii, O. brachyantha, O. eichingeri, O. longistaminata and O. punctata. The species are all inbreeding, with the exception of O. longistaminata, which is outcrossing. This paper also reports the field observations and the overall results of collecting missions coordinated by the first author and highlights observations made in Kenya, Uganda, Zimbabwe, Mozambique and Malawi where the author participated in the collecting mission