Abstract
Resistance to downy mildew (Plasmopara halstedii) in sunflower (Helianthus annuus L.) is conferred by major resistance genes, denoted Pl. Twenty-two Pl genes have been identified and genetically mapped so far. However, over the past 50 years, wide-scale presence of only a few of them in sunflower crops led to the appearance of new, more virulent pathotypes (races) so it is important for sunflower varieties to carry as wide a range of resistance genes as possible. We analyzed phenotypically 12 novel resistant sources discovered in breeding pools derived from two wild Helianthus species and in eight wild H. annuus ecotypes. All were effective against at least 16 downy mildew pathotypes. We mapped their resistance genes on the sunflower reference genome of 3,600 Mb, in intervals that varied from 75 Kb to 32 Mb using an AXIOM® genotyping array of 49,449 SNP. Ten probably new genes were identified according to resistance spectrum, map position, hypersensitive response to the transient expression of a P. halstedii RXLR effector, or the ecotype/species from which they originated. The resistance source HAS6 was found to carry the first downy mildew resistance gene mapped on chromosome 11, whereas the other resistances were positioned on chromosomes 1, 2, 4, and 13 carrying already published Pl genes that we also mapped physically on the same reference genome. The new genes were designated Pl23–Pl32 according to the current nomenclature. However, since sunflower downy mildew resistance genes have not yet been sequenced, rules for designation are discussed. This is the first large scale physical mapping of both 10 new and 10 already reported downy mildew resistance genes in sunflower.
Highlights
Downy mildew of sunflower, caused by Plasmopara halstedii can occur in most countries where sunflowers (Helianthus annuus L.) are grown
Sunflower was domesticated by Native Americans from wild H. annuus but the first large scale developments of the crop were in Russia between 1900 and 1950
P. halstedii originated in North America (Leppik, 1962) but the first important disease attacks occurred in Russia (Pustovoit, 1966) when sunflower became an important crop
Summary
Downy mildew of sunflower, caused by Plasmopara halstedii can occur in most countries where sunflowers (Helianthus annuus L.) are grown It is an obligate parasitic oomycete belonging to the Peronosporales showing physiological races or pathotypes (Gascuel et al, 2015) which interact with sunflower genotypes according to the resistance genes they contain. Several countries have developed collections of wild H. annuus and other Helianthus species, and research for new downy mildew resistance genes has been made in crosses of this material with cultivated sunflower. This paper reports genetic mapping of the broad spectrum resistance genes in 12 sunflower lines developed from crosses of cultivated and wild Helianthus spp., and their physical mapping on the sunflower reference genome using an AXIOM R SNP genotyping array. The question of what constitutes a new resistance gene is discussed
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