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Interspecific hybridization involving a rare parental forest pathogen leads to asymmetrical accelerated evolution

Matteo Garbelotto: UC Berkeley

<div>Hybridization is recognized as a major force in the rapid adaptive evolution of plant pathogens. In a high elevation alpine larch stand in Montana, the dominant pathogenic species <em>Heterobasidion occidentale</em> has hybridized with a rare Eastern North American population of <em>H. irregulare</em>. Based on sequence analyses of five loci, we show that: 1)-Hybridization increased allelic diversity, generating alleles absent from either parental populations; 2)-Hybrid genotypes are viable and consequential in evolutionary terms; 3)-Because <em>H. irregulare</em> is rare, hybrid progeny has been backcrossing with <em>H. occidentale</em>, and thus hybrids carry <em>occidentale</em> mitochondria; 4)-A further consequence of hybrids solely backcrossing with <em>H. occidentale</em> has been the isolation and rapid evolution of the <em>irregulare</em> genome, while the same is not true for the <em>occidentale</em> genome; 5)-Based on phylogenetic, Bayesian assignment, and ordination tests, <em>irregulare</em> alleles in hybrids are clearly distinct from all known <em>irregulare</em> alleles from North America and may represent a new species. This discovery demonstrates that larch can be a universal host favoring hybridization, and that hybridization between one abundant and one rare species can result in asymmetrical accelerated evolution of the nuclear genome of the rare parental species leading to speciation.</div>