Genetic control of weediness traits and the maintenance of sympatric crop–weed polymorphism in pearl millet (Pennisetum glaucum)

Abstract

Pearl millet (Pennisetum glaucum), a diploid outcrossing crop widely grown in semiarid tropics, provides a unique extant material for the study of crop-weed interactive evolution. Co-occurrence of a weedy, shattering type of pearl millet with the cultivated one is the rule in the traditional agro-ecosystem in the Sahel zone of Africa. Selfed progeny of weed-type plants invariably segregated into distinct weed and crop types in an approximately 3:1 ratio. Genetic analysis using a cleaved amplified polymorphic sequence (CAPS) marker strongly suggested that a series of differences between the crop and the weed types are determined by a single putative supergene that has two allelic types, C and W. The crop-type plants are CC homozygotes, and the weed-type plants are CW heterozygotes. WW homozygotes are sterile and rare in the field. Thus, the CW weed plants recurrently arise from crosses between the crop and the weed, as well as from crosses among the weed-type plants. The weed type appears to have a sufficiently high fitness to maintain the W allele in the pearl millet population, resulting in the perpetuation of this unique crop-weed polymorphism.

Biogeography and Genetic Diversity of Pearl Millet (Pennisetum glaucum) from Sahelian Africa

“…The phenomenon of hybridization through gene flow between wild and domesticated material in the Sahel appears to perpetually add new alleles and broaden the genetic base of this African crop complex. Because pearl millet is unusual in maintaining an active exchange of genes with its wild relatives (Brunken, de Wet, and Harlan 1977;Amoukou and Marchais 1993;Miura and Terauchi 2005), it offers a unique contrast to the predominant view of the domestication syndrome and an opportunity to refine our understanding of the domestication process and its genetic consequences. Domestication of pearl millet should be viewed as an ongoing evolutionary process, rather than an event that took place historically.…”

“…The specific consequences of recurrent hybridization among various intraspecific populations are (1) possible reduction or loss of genetic diversity in wild populations (e.g., beans; ZizumboVillarreal et al 2005), (2) reduction or loss of domesticated traits in managed populations (e.g., cassava; Elias, Rival, and McKey 2000), and (3) the creation of intermediate types that can become fixed and stable within the agroecosystem (e.g., pearl millet; Couturon et al 2003;Miura and Terauchi 2005). The diversity present in African pearl millet landraces might be a function of gene flow not only among domesticated types but also among wild types and weedy hybrids that germinate spontaneously throughout their native range and might reduce the risks of management under extreme environmental heterogeneity by providing agroecological functionality and benefit to farmers (Blumler 2003;Hooper et al 2005).…”

“…Such gene flow could possibly be exploited for maintenance of beneficial characteristics under extreme conditions. Previous research suggests that hybridization among wild, intermediate, and cultivated types is occurring and is likely influenced by farmers (Miura and Terauchi 2005;Mariac, Robert et al 2006), but the extent to which farmers consciously or unconsciously impact such evolutionary dynamics through their individual practices remains unknown.…”

“…Among the new varieties are a myriad of intermediate or hybrid types that spontaneously germinate the next season when the rains commence throughout the Sahel. They result from hybridization between wild and domesticated plants unique to this region of Africa (Marchais 1994;Miura and Terauchi 2005;Mariac, Luong et al 2006;Mariac, Robert et al 2006).…”

Competition alters life history and increases the relative fecundity of crop–wild radish hybrids (Raphanusspp.)

“…Meanwhile, little is known about the potential for complex, quantitative crop traits to enhance the fecundity of wild relatives. Although hybridization with some cultivated plants may reduce the fitness of weeds in natural environments (Stewart et al ., 2003), possible benefits of cropwild hybridization for weeds suggest this phenomenon should not be overlooked (Boudry et al ., 1993;Miura & Terauchi, 2005;Campbell et al ., 2006). Studies of advanced-generation hybrids find that some interspecific hybrid genotypes persist for multiple generations under certain environmental conditions and therefore long-term gene introgression is possible (Lexer et al ., 2003).…”