Genetic and molecular analyses of factors defining host-specificity and virulence in Striga species (Scrophulariaceae)

The genus Striga is composed of about 50 species, all obligate parasites of tropical cereals and legumes. The genetic structure of S. asiatica of Benin and S. gesnerioides (SG) were analyzed using AFLP. In the former, the mean within population genetic distances ranged from 0.028 to 0.038, while the mean among-population genetic distances ranged from 0.019 to 0.088, with an assumed minimum genetic distance of 0.01 in each case. The intra- and inter-population variation was reflected by a highly significant R2 value of 0.61 (Mantel's test). Analysis of SG from across West Africa parasitic on cowpea (Vigna unguiculata ) and a strain from Central Florida (SGFL) parasitic on Indigofera hirsuta showed that isolates of this parasite are very uniform genetically. However, there were sufficient differences among the different races of the parasite from West Africa and the strain from Florida that allowed for the identification of molecular markers that were specific to each category using specific EcoRI and MseI selective primer combinations. A phenetic relationship among the different races of S. gesnerioides from West Africa showed that SG race 1 (SG1) and SG5 were most closely related, while SG4 and SG3 were most diverged. A race of the parasite not previously characterized was recognized from Senegal. Although the data seem to suggest that geographic isolation contributes to race formation in this species, host-driven selection is obviously the most critical factor defining race and host-specificity in SG . This species is capable of transgressing hosts. The development of attached SGFL seedlings on cowpea roots were arrested after tubercle formation, while all collections of SGFL failed to parasitize cowpea genotypes known to be susceptible to all cowpea- SG races from West Africa, although root exudates from these cowpea genotypes stimulated the germination of SGFL.; mRNA differential display using total RNA from tubercles of different sizes for two classes of compatible interactions and one class of incompatible interaction of SG with its host were evaluated and differentially expressed cDNA profiles were cloned and sequenced. One cDNA clone from each class of the compatible interaction showed a significant match with ubiquitin-specific proteases, while a clone from an incompatible interaction showed a significant match with organic cation transporter. The cDNA differential display profiles were identical for all sizes of tubercles within each category, suggesting that genes are constitutively expressed in SG tubercles. (Abstract shortened by UMI.)...