Abstract
Orobanche cumana Wallr. (sunflower broomrape) is a holoparasitic weed that infects roots of sunflower in large areas of Europe and Asia. Two distant O. cumana gene pools have been identified in Spain, one in Cuenca province in the Center and another one in the Guadalquivir Valley in the South. Race F has been hypothesized to have arisen by separate mutational events in both gene pools. In the Guadalquivir Valley, race F spread in the middle 1990’s to become predominant and contained so far with race F hybrids. Recently, enhanced virulent populations of O. cumana have been observed in commercial fields parasitizing race F resistant hybrids. From them, we collected four independent populations and conducted virulence and SSR marker-based genetic diversity analysis. Virulence essays confirmed that the four populations studied can parasitize most of the race F resistant hybrids tested, but they cannot parasitize the differential inbred lines DEB-2, carrying resistance to race F and G, and P-96, resistant to F but susceptible to races G from other countries. Accordingly, the new populations have been classified as race GGV to distinguish them from other races G. Cluster analysis with a set of populations from the two Spanish gene pools and from other areas, mainly Eastern Europe, confirmed that race GGV populations maintain close genetic relatedness with the Guadalquivir Valley gene pool. This suggested that increased virulence was not caused by new introductions from other countries. Genetic diversity parameters revealed that the four populations had much greater genetic diversity than conventional populations of the same area, containing only alleles present in the Guadalquivir Valley and Cuenca gene pools. The results suggested that increased virulence may have resulted from admixture of populations from the Guadalquivir Valley and Cuenca followed by recombination of avirulence genes.
Highlights
Orobanche cumana Wallr. is a holoparasitic plant species with a restricted range of hosts both in the wild, where it mainly parasitizes Artemisia spp., as well as in agricultural fields, where it only grows on sunflower (Fernández-Martínez et al, 2015)
The four populations from the Guadalquivir Valley clearly differed from race G populations from Eastern Europe for their inability to parasitize P96 line, while the reaction on race F resistant hybrids followed different patterns in the race G populations from Eastern Europe
The line P-96 was selected as differential line because it was completely resistant to race F population of the Guadalquivir Valley (FGV) and to the race F predominant in Central Spain, and susceptible to the first reported race G, which was identified in the Edirne area of Turkey around 2000 (Kaya et al, 2004)
Summary
Orobanche cumana Wallr. (sunflower broomrape) is a holoparasitic plant species with a restricted range of hosts both in the wild, where it mainly parasitizes Artemisia spp., as well as in agricultural fields, where it only grows on sunflower (Fernández-Martínez et al, 2015). The parasitic interaction between sunflower and O. cumana generally follows a gene for gene model, with resistance in sunflower (Vrânceanu et al, 1980) and avirulence in O. cumana (Rodríguez-Ojeda et al, 2013b) controlled by dominant alleles at single loci. The general occurrence of a gene for gene interaction between sunflower and O. cumana and the associated development of physiological races of the parasite is an exception in parasitic systems involving Orobanche spp., which are in general under quantitative or horizontal genetic control (Pérez-Vich et al, 2013). Vrânceanu et al (1980) reported the existence of five races of O. cumana named as A to E, controlled by resistance genes Or1 to Or5. Increasingly virulent populations classified as races G and H are becoming predominant in countries around the Black Sea (Kaya, 2014)
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