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Risk assessment for transgenic sorghum in Africa: crop-to-crop gene flow in Sorghum bicolor (L.) moench.

Article Excerpt
THE MAJORITY of world crops originated in today's developing countries in so-called Vavilov centers, regions that hold a high diversity of traditional crop varieties and crop wild relatives. These regions hold the main genetic resources for crops and are especially important to secure current and future plant breeding programs. They serve as "genetic insurance" against upcoming pests or changing abiotic conditions (Swanson, 1995; Tanksley and McCouch, 1997). In light of the increasing deployment of GM crops in developing countries (James, 2003), scientific risk assessment of transgenic crops and its impact on conventionally bred crops and crop wild relatives is needed to further establish adequate biosafety regulations. Information on probable introgression of transgenes into wild relatives and its economic and ecological consequences is still rare despite increasing study efforts. Therefore, data are needed on outcrossing rates, outcrossing distance, and the consequences of gene flow. Gene flow has been investigated for some crops, such as rice (Oryza sativa L.), rapeseed (Brassica napus L.), sunflower (Helianthus annuus L.), beet (Beta vulgaris L.)and maize (Zea mays L.) (Ellstrand et al., 1999; Hall et al., 2000; Lavigne et al., 1998; Reboud, 2003; Snow et al., 2003; Song et al., 2004; St. Amand et al., 2000). Nonetheless, for one important crop, sorghum, little information on gene flow and especially on the outcrossing distance is available, despite the fact that transgenic, herbicide-resistant sorghum could be developed soon (Arriola, 1995; Arriola and Ellstrand, 1996; Arriola and Ellstrand, 1997; Tadesse et al., 2003). If this gene were to "escape" into the environment to a weed such as johnsongrass, it would have a major impact on the control of this weed. To assess the environmental risk of possible future deployment of transgenic sorghum, data on the outward flow of genes through sorghum pollen to nontarget relatives is needed. This study investigates crop-to-crop gene flow in sorghum. In the case of sorghum, a special emphasis must be placed on Africa--its center of origin--where a large number of sexually compatible weeds, wild relatives, strains, and races of cultivated sorghum occur (Harlan, 1976; Doggett, 1988). Also, sorghum is an important staple food crop in Africa, South Asia, and Central America and was ranked the seventh most important crop worldwide in terms of harvested area. It is the dietary staple for more than 500 million people in more than 30 countries and more than 50% of the global harvest takes place in Africa (FAOSTAT, 2002; NRC, 1996). In terms of plant-derived energy uptake, only rice, wheat (Triticum aestivum L.), maize, and potatoes (Solanum tuberosum L.) surpass sorghum in feeding humans worldwide. Sorghums are remarkably drought-resistant and vitally important where the climate is too dry for maize, i.e., at annual rainfalls ranging from 350 to 750 mm (FAO, 2003; Wenzel, 2003), and it tolerates an astounding array of soils. These characteristics make sorghum an ideal food crop in semiarid areas of Africa (and elsewhere) where other crops, such as maize, would fail.

Sorghum Taxonomy and Outcrossing

Sorghum belongs to the family Poaceae, and the genus is subdivided into five sections. The section sorghum includes three species (de Wet, 1978; Doggett, 1988; Smith and Frederiksen, 2000; Raemakers, 2001; McGuire, 2004): S. halepense (weed, johnsongrass), S. propinquum (Kunth) Hitchc. (perennial, fully fertile with S. bicolor), and S. bicolor with three subspecies. The three S. bicolor subspecies are S. bicolor subsp, bicolor (cultivated species, five main races), S bicolor subsp, arundinaceum (Desv.) de Wet & J. R. Harlan ex Davidse [synonym to subsp, verticalliflorum (Steud.) Stapf] (the wild progenitor of cultivated sorghum, four main races), and S. bicolor subsp, drummondii (Steud.) de Wet ex Davidse (sudangrass, weed).

No reproduction barrier exists between cultivated S. bicolor subsp. bicolor and its wild progenitor S. bicolor subsp. arundinaceum (hybrids form shattercane-type weeds) and the weed S. bicolor subsp. drummondii (de Wet, 1978; Doggett, 1988). Sorghum bicolor x almum Parodi is a rhizomatous hybrid between S. bicolor and S. halepense and is occasionally cultivated as a fodder grass. If backcrossed to S. halepense, it can give rise to aggressive weeds, including johnsongrass, which was classified as one of the world's most noxious weeds (Pope and Martins, 2002; Holm et al., 1977). Sorghum is largely self pollinated, but wind pollination between plants does occur (McGuire, 2004). Subspecies or varieties of sorghum with open, grass-like panicles, such as sudangrass (S. bicolor), have a higher rate of outcrossing than sorghum, with compact heads typical of commercial hybrids. Outcrossing also varies by location on the panicle, with much higher rates at the top of the panicle, where flowering initiates (Maunder and Sharp, 1963).

Gene Flow

Gene flow can be expected to occur in many crop-weed complexes in cases where the crop and the weed have sympatric ranges, are sexually compatible, have overlapping flowering times, and share a common pollination mechanism. Several studies demonstrated that hybridization in the sorghum crop-weed-wild relatives complex takes place (Arriola and Ellstrand, 1997; Baker, 1972; Doggett, 1988; Ellstrand et al., 1999; ICRISAT, 2002). Molecular and genetic analyses revealed crop-specific alleles in sorghum wild relatives when it co-occurs with the crop in Africa, suggesting that intraspecific hybridization and introgression are common (Aldrich and Doebley, 1992; Aldrich et al., 1992; Smith and Frederiksen, 2000). Arriola and EUstrand (1997) concluded that a transgene that is either neutral or even beneficial to johnsongrass is likely to persist in populations growing under agricultural conditions under continued gene flow from the crop. Although the triploid progeny of johnsongrass x sorghum hybrids are usually sterile, such hybrids are expected to propagate vegetatively through rhizomes and to persist under agricultural conditions. Furthermore, limited viable seed production on johnsongrass x sorghum hybrids has been reported (Hoang-Tang and Liang, 1988). Introgression from crop sorghum has been implicated in the evolution of increased weediness in one of the world's worst weeds, S. halepense (Holm et al., 1977). Although not classified as a noxious weed, weed-like...