Hybridization of the seedling-infecting Ustilago spp. pathogenic on barley and oats, and a study of the genotypes conditioning the morphology of their spore walls

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Abstract
The Ustilago spp. pathogenic on barley, U. nigra and U. hordei, were successfully hybridized with the species pathogenic on oats, U. avenae and U. kolleri, using the common host Agropyron tsukushiense var. transiens. The genetic relationship of these species was studied based on their crossability and the distribution of genes for spore wall morphology. The echinulation of the spore walls of U. nigra was conditioned by the two dominant and complementary genes E1 and E2 that had previously been identified in U. avenae. Ten collections of U. hordei from 10 countries of five continents had genotype E1e2, resulting in smooth spores. The 11 collections of U. kolleri from seven countries had smooth walls, and their genotypes were either e1e2 (Canada), e1E2 (Spain, Morocco, Tunisia, Turkey, and Australia), or E1e2 (Portugal). Hybrids between genotypes e1E2 and E1e2 of U. kolleri had echinulate spores, and would thus belong to another taxon: U. avenae. Similarly, hybrids between U. hordei genotype E1e2, and U. kolleri genotype e1E2 had echinulate spores. Intra- and inter-specific crosses of U. avenae and U. kolleri showed that incompatibility between collections can occur within as well as between these species. The suggested causes of such incompatibility are multigene differences or chromosomal rearrangement, an effect of long-term isolation of populations of these species. The results are discussed in view of proposals of how to taxonomically delimit the four seedling-infecting Ustilugo species parasitic on barley and oats.

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