Independent breakdown events of the Brassica napus Rlm7 resistance gene including via the off-target impact of a dual-specificity avirulence interaction.

Protection of many crops is achieved through the use of genetic resistance. Leptosphaeria maculans, the causal agent of blackleg disease of Brassica napus, has emerged as a model for understanding gene-for-gene interactions that occur between plants and pathogens. Whilst many of the characterized avirulence effector genes interact with a single resistance gene in the host, the AvrLm4-7 avirulence gene is recognized by two resistance genes, Rlm4 and Rlm7. Here, we report the "breakdown" of the Rlm7 resistance gene in Australia, under two different field conditions. The first, and more typical, breakdown probably resulted from widescale use of Rlm7-containing cultivars whereby selection has led to an increase of individuals in the L. maculans population that have undergone repeat-induced point (RIP) mutations at the AvrLm4-7 locus. This has rendered the AvrLm4-7 gene ineffective and therefore these isolates have become virulent towards both Rlm4 and Rlm7. The second, more atypical, situation was the widescale use of Rlm4 cultivars. Whilst a single-nucleotide polymorphism is the more common mechanism of virulence towards Rlm4, in this field situation, RIP mutations have been selected leading to the breakdown of resistance for both Rlm4 and Rlm7. This is an example of a resistance gene being rendered ineffective without having grown cultivars with the corresponding resistance gene due to the dual specificity of the avirulence gene. These findings highlight the value of pathogen surveillance in the context of expanded knowledge about potential complexities for Avr-R interactions for the deployment of appropriate resistance gene strategies.

Mitochondrial microsatellite markers for the Australian ectomycorrhizal fungus Laccaria sp. A (Hydnangiaceae).

PREMISE OF THE STUDY: Microsatellite loci were developed for the ectomycorrhizal fungus Laccaria sp. A to investigate the population genetic structure of this fungal symbiont across its fragmented distribution in southeastern Australia. • METHODS AND RESULTS: A partial genome sequence from an individual collection of Laccaria sp. A was obtained by 454 genome sequencing. Eight microsatellite markers were selected from 66 loci identified in the genome. The selected markers were highly polymorphic (4-19 alleles per locus, average 13 alleles) and amplified reproducibly from collections made across the distribution of this species. Five of these markers also amplified reproducibly in the sister species Laccaria sp. E (1). All eight of the selected microsatellite loci were from the mitochondrial genome. • CONCLUSIONS: The highly polymorphic markers described here will enable population structure of Laccaria sp. A to be determined, contributing to research on mycorrhizal fungi from a novel distribution.

Multigene sequence data reveal morphologically cryptic phylogenetic species within the genus Laccaria in southern Australia.

Laccaria (Hydnangiaceae, Agaricales, Basidiomycota) is one of the more intensively studied ectomycorrhizal genera; however, species boundaries within Laccaria and the closely related Hydnangium and Podohydnangium in Australia have not yet been examined with molecular sequence data. Based on morphological characters, eight native species of Laccaria are currently recognized in Australia, as well as three Hydnangium species and the monotypic Podohydnangium australe. Sequences of the internal transcribed spacer region of nuclear rDNA (ITS), RNA polymerase beta subunit II (rpb2) and translation elongation factor 1 alpha (tef-1α) were generated from 77 collections of Laccaria, Hydnangium and Podohydnangium from Australia. Ten phylogenetic species and a further 11 potential species (represented by singletons) of Laccaria in Australia are delimited from sequence analyses. Most of the morphological species contained cryptic phylogenetic species, but these species were always nested entirely within a given morphological species, although not always as sister taxa. The rpb2 locus performed best as a species barcode with pairwise and patristic distance measures. The ITS sequence region returned the least resolved gene tree of the three regions examined and was the least useful as a barcode region. Based on the phylogenetic topology, there appears to have been multiple gains and/or losses of the ectomycorrhizal association of some species with the myrtle beech, Nothofagus cunninghamii as well as of sequestrate basidiocarps and two-spored basidia.