Whether new combinations of genes that result from hybridization and introgression between wild and cultivated taxa are maintained, with the resultant development of populations with new characteristics, depends on natural selection, and in the case of crops, on human selection. While many cases of deliberate introgression of desirable traits into crop cultivars as part of breeding programmes are known, the extent and significance of natural or farmer-assisted introgression is uncertain. A range of techniques have been used to document natural hybridization and introgression of agricultural crops and their wild relatives in many crops including maize, wheat, barley, oats, pearl millet, foxtail millet, quinoa, hops, hemp, potato, cocona, casava, common bean, cowpea, pigeon pea, carrots, squash, tomato, radish, letuce, chilli, beets, sunflower, cabbage, and rasberries. However, the majority of these studies are based on morphological characters, and few have investigated the frequency with which such new types are produced and retained in natural and agroecosystems for farmer selection. Even more limited is information on the role of farmers in recognizing and selecting new genetic variation from the natural introgression of crops with their wild relatives, and the impact, once selected, of these new genetic combinations on the crop diversity. Molecular evaluation of natural introgression linked to investigations of farmer recognition and use of introgressed types provide ways of evaluating whether farmer selection for introgressed types is a significant process in increasing the genetic diversity of crop plants.

Wild relatives and crop cultivars: detecting natural introgression and farmer selection of new genetic combinations in agroecosystems

When de Candolle (1882) initiated the study of crop evolution, his major concern was to identify the geographic origin, i.e., the domestication center, of individual crops. In his opinion, the following four types of data would shed light on this question: (1) archaeology; (2) botany; (3) philology or linguistics; and (4) history. Of these four types of data, the first two are the most reliable (Harlan and de Wet, 1973). Archaeological remains attested to the antiquity of the cultivation of a crop in a region compared to other regions devoid of archaeological remains. Botanical arguments referred to the existence in a defined region of a wild-growing form that was sufficiently similar morphologically to the crop that it could represent its ancestral form (or at least the immediate descendant of the ancestral form). The existence of words designating a particular crop, particularly in native languages, was considered a testimony to the relative antiquity of cultivation of the crop. Finally, historical description, such as the treatises of Dioscorides and Theophrastus, the herbals of the 16th and 17th centuries, or descriptions of the New World shortly after the Spanish Conquista by, for example, Cieza de Leon (1541 –1550) or Acosta (1590), also would provide some evidence toward identifying the area of origin of crops.

The Use of Molecular and Biochemical Markers in Crop Evolution Studies

A strategy of DNA pooling aimed at identifying markers linked to quantitative trait loci (QTLs), ‘Sequential Bulked Typing’ (SBT), is presented. The method proposed consists in pooling DNA from consecutive pairs of individuals ranked phenotypically, i.e., pools are formed with individuals ranked (1st, 2nd), (3rd, 4th),…, (N-1st, Nth). The N/2 pools are subsequently amplified using the polymerase chain reaction (PCR). If the whole population is typed the number of PCRs per marker is halved with respect to individual typing (IT). But if this strategy is combined with selective genotyping of extreme individuals savings can be further increased. Two extreme cases are considered: in the first one (SBT0), it is assumed that only presence or absence of a given allele can be ascertained in a pool; in the second one (SBT1), it is further assumed that differences between allele band intensities can be distinguished. The theory to estimate by maximum likelihood the QTL effect and its position with respect to flanking markers is presented. The behaviour of IT and SBT was studied using stochastic computer simulation in backcross and F2 populations. Three percentages of subpattern distinction (0, 50 and 100%) two population sizes (n=1200 and 600) and two QTL effects (a=0.1 and 0.25 standard deviations) were considered. SBT1 had the same power as individual genotyping at half the genotyping costs in all situations studied. Accuracy of QTL location is not increased with a dense number of markers, as opposed to individual typing. As a result DNA pooling is not useful for accurate location of the QTL but rather to pick up genome regions containing QTLs of at least moderate effect. The theory developed provides the general theoretical framework to deal with any DNA pooling strategy aimed at detecting QTLs.

Long-term introgression of crop genes into wild sunflower populations

The development of transgenic plants has heightened concern about the possible escape of genetically engineered material into the wild. Hybridization between crops and their wild relatives provides a mechanism by which this could occur. While hybridization has been documented between several crops and wild or weedy relatives, little is known about the persistence of cultivar genes in wild populations in the generations following hybridization. Wild and weedy sunflowers occur sympatrically with cultivated sunflowers throughout much of the cultivation range, and hybridization is known to occur. We surveyed two cultivar-specific RAPD markers in 2700 progeny in a naturally occurring population of wild Helianthus annuus over five generations following a single generation of hybridization with the cultivar. Moderate levels of gene flow were detected in the first generation (42% hybrids at the crop margin) and cultivar allele frequencies did not significantly decline over four subsequent generations. These results indicate that gene flow from cultivated into wild populations of sunflowers can result in the long-term establishment of cultivar alleles in wild populations. Furthermore, we conclude that neutral or favorable transgenes have the potential to escape and persist in wild sunflower populations.

The persistence of cultivar alleles in wild populations of sunflowers five generations after hybridization

Genetic diversity of crop species in sub-Sahelian Africa is still poorly documented. Among such crops, pearl millet is one of the most important staple species. In Niger, pearl millet covers more than 65% of the total cultivated area. Analyzing pearl millet genetic diversity, its origin and its dynamics is important for in situ and ex situ germplasm conservation and to increase knowledge useful for breeding programs. We developed new genetic markers and a high-throughput technique for the genetic analysis of pearl millet. Using 25 microsatellite markers, we analyzed genetic diversity in 46 wild and 421 cultivated accessions of pearl millet in Niger. We showed a significantly lower number of alleles and lower gene diversity in cultivated pearl millet accessions than in wild accessions. This result contrasts with a previous study using iso-enzyme markers showing similar genetic diversity between cultivated and wild pearl millet populations. We found a strong differentiation between the cultivated and wild groups in Niger. Analyses of introgressions between cultivated and wild accessions showed modest but statistically supported evidence of introgressions. Wild accessions in the central region of Niger showed introgressions of cultivated alleles. Accessions of cultivated pearl millet showed introgressions of wild alleles in the western, central, and eastern parts of Niger.

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Diversity of wild and cultivated pearl millet accessions (Pennisetum glaucum [L.] R. Br.) in Niger assessed by microsatellite markers

Intergametophytic competitions with prepotency of autopollen have been previously described in some cultivated lines of pearl millet (Pennisetum typhoides (Burm.) Stapf &Hubb.). This paper reports the analysis of intergametophytic competitions between pollen from wild and cultivated plants on stigmas of both wild and cultivated plants. Three inbred lines (two cultivated and one wild) and two land races (one cultivated and one wild) were used, each of them as pollen donors as well as plant receptors. These populations represent in some instances allopatric and sympatric evolutionary situations between wild and cultivated forms. Deviations from the equiprobability of gamete encounters for almost all progeny are clearly demonstrated and reveal a large amount of male gametophytic fitness variability in land-race populations. There is no marked hierarchy between pollen sources for their competitive ability, but taking into account the pollen-pistil interactions, preferential wild-wild and cultivated-cultivated...

Gametophytic competition as influencing gene flow between wild and cultivated forms of pearl millet (Pennisetum typhoides)

Intergametophytic competitions with a polygenic inheritance trend have been previously described in pearl millet. This study puts emphasis on the effect of environmental parameters, particularly temperature, on the segregation of five genes encoding for enzyme proteins. F2 progenies of selfed F1 plants grown under three temperature regimes (22 °C constant, 27/17 °C, and 32/27 °C (day/night)) were studied. Significant segregation distortions were found for four genes. It is shown that the selection pressures placed on the free haploid phase by pollen competition are enhanced by temperature. Furthermore, genotype × environment interactions influence the gametophytic competitive ability of the parental genotypes. The hypothesis of genetic linkage between these marker loci and the chromosomal sectors responsible for pollen competitive ability is discussed. We also discuss the role these selective mechanisms and their consequences on the genetic makeup of the following generations may play in plant breeding, p...

J. Pernès

Papers

Gametophytic competition as influencing gene flow between wild and cultivated forms of pearl millet (Pennisetum typhoides)

Sélections sur la phase haploïde chez le Mil (Pennisetum typhoides (Burm.) Stapf et Hubb.): effet de la température