ABSTRACT Two types of genetic resistance to Leptosphaeria maculans usually are distinguished in Brassica napus: qualitative, total resistance expressed at the seedling stage and quantitative, partial ...resistance expressed at the adult plant stage. The latter is under the control of many genetic factors that have been mapped through quantitative trait loci (QTL) studies using 'Darmor' resistance. The former usually is ascribed to race-specific resistance controlled by single resistance to L. maculans (Rlm) genes. Three B. napus-originating specific Rlm genes (Rlm1, Rlm2, and Rlm4) previously were characterized. Here, we report on the genetic identification of two novel resistance genes, Rlm3 and Rlm7, corresponding to the avirulence genes AvrLm3 and AvrLm7. The identification of a novel L. maculans- B. napus specific interaction allowed the detection of another putative new specific resistance gene, Rlm9. The resistance genes were mapped in two genomic regions on LG10 and LG16 linkage groups. A cluster of five resistance genes (Rlm1, Rlm3, Rlm4, Rlm7, and Rlm9) was strongly suggested on LG10. The relation between all these specific resistance genes and their potential role in adult-plant field resistance is discussed. These two Rlm-carrying regions do not correspond to major QTL for Darmor quantitative resistance.
Gene loss, inversions, translocations, and other chromosomal rearrangements vary among species, resulting in different rates of structural genome evolution. Major chromosomal rearrangements are rare ...in most eukaryotes, giving large regions with the same genes in the same order and orientation across species. These regions of macrosynteny have been very useful for locating homologous genes in different species and to guide the assembly of genome sequences. Previous analyses in the fungi have indicated that macrosynteny is rare; instead, comparisons across species show no synteny or only microsyntenic regions encompassing usually five or fewer genes. To test the hypothesis that chromosomal evolution is different in the fungi compared to other eukaryotes, synteny was compared between species of the major fungal taxa.
These analyses identified a novel form of evolution in which genes are conserved within homologous chromosomes, but with randomized orders and orientations. This mode of evolution is designated mesosynteny, to differentiate it from micro- and macrosynteny seen in other organisms. Mesosynteny is an alternative evolutionary pathway very different from macrosyntenic conservation. Surprisingly, mesosynteny was not found in all fungal groups. Instead, mesosynteny appears to be restricted to filamentous Ascomycetes and was most striking between species in the Dothideomycetes.
The existence of mesosynteny between relatively distantly related Ascomycetes could be explained by a high frequency of chromosomal inversions, but translocations must be extremely rare. The mechanism for this phenomenon is not known, but presumably involves generation of frequent inversions during meiosis.
Stem canker of oilseed rape (canola, Brassica napus) is associated with a species complex of two closely related fungal species, Leptosphaeria maculans and L. biglobosa. Of these, L. maculans is the ...most damaging and develops gene-for-gene relationships with the host. Here, a wide scale analysis of the L. maculans - L. biglobosa species complex was performed throughout the American continent (23 locations from Chile to Canada) plus several locations in Western Australia for comparison purposes, based on a collection of 1132 isolates from infected tissues of a susceptible cultivar. Fungal species were discriminated on the basis of morphological, phytopathological and molecular criteria and showed that L. biglobosa was closely associated with L. maculans in most of the locations. Multiple gene phylogeny using sequences of ITS, actin and β-tubulin confirmed the prevalence of the L. biglobosa'canadensis' sub-clade in Canada, whereas up to three different sub-clades of L. biglobosa were found in Georgia (USA). Race structure of L. maculans was investigated using a combination of pathogenicity tests and PCR amplification of avirulence alleles AvrLm1, AvrLm4 and AvrLm6. Three contrasting situations were observed: (i) race structure in Ontario, Chile and Georgia was related to that of European and Western Australian populations, with a low race diversity; (ii) only one race was found in Mexico, and not found outside of this country; (iii) a large diversity of races was observed in central Canada (Manitoba, Alberta and Saskatchewan) with very specific features including maintenance of avirulence alleles absent from Europe, absence of the AvrLm7 allele common in Europe (or eastern Canada) and wide location-to-location variability.
Three avirulence genes, AvrLm1, AvrLm6, and AvrLm4-7, were recently identified in Leptosphaeria maculans and found to be localized as solo genes within large noncoding, heterochromatin-like regions ...mainly composed of retrotransposons, truncated and degenerated by repeat-induced point mutation (RIP). The Rlm6 resistance gene has been overcome within 3 years in outdoor experiments in France and, here, we investigate the molecular basis of evolution toward virulence at the AvrLm6 locus. A region of 235 kb was sequenced in a virulent isolate and showed the deletion of AvrLm6 and three divergent mosaics of retrotransposons. AvrLm6 was found to be absent from 66% of 70 virulent isolates, with multiple events of deletion. The sequencing of virulent alleles in 24 isolates revealed a few cases of point mutations that had created stop codons in the sequence. The most frequent mutation events, however, were RIP, leading to the modification of 4 to 9% of the bases compared with the avirulent allele and generating 2 to 4 stop codons. Thus, RIP is described for the first time as an efficient mechanism leading to virulence and the multiple patterns of mutation observed suggest that multiple RIP events could occur independently in a single field population during 1 year.
Depth sensing indentation is used here to investigate the viscous behaviour of a glass in a wide range of temperatures. A specific high temperature instrumented micro-indentation device was developed ...to perform creep indentation tests with several indenter geometries (spherical, flat punch and Vickers) up to 1100
°C with a maximum load of 15
N. The displacement vs. time curves are analysed by using a method based on a linear viscoelasticity assumption. Creep indentation tests are conducted on a standard window glass around glass transition between 540 and 640
°C. Results show no influence of load and indenter geometry on measurements in the permanent regime of creep. The temperature dependence of viscosity we found is in good agreement with those from the literature. The indentation creep method is therefore a very powerful way for studying the viscoelastic behaviour of materials. Compared to classical methods it has two main advantages: the easiness of samples preparation and the small volumes required.
Summary
Leptosphaeria maculans, a Dothideomycete causing stem canker on oilseed rape (Brassica napus), develops gene‐for‐gene interactions with its host plants. To date, nine resistance genes ...(Rlm1–9) have been identified in Brassica spp. The corresponding nine avirulence genes (AvrLm1–9) in L. maculans have been mapped at four independent loci, thereby revealing two clusters of three and four linked avirulence genes. Here, we report the completion of map‐based cloning of AvrLm1. AvrLm1 was genetically delineated within a 7.3 centimorgan interval corresponding to a 439 kb BAC contig. AvrLm1 is a single copy gene isolated within a 269 kb non‐coding, heterochromatin‐like region. The region comprised a number of degenerated, nested copies of four long‐terminal repeat (LTR) retrotransposons, including Pholy and three novel Gypsy‐like retrotransposons. AvrLm1 restored the avirulent phenotype on Rlm1 cultivars following functional complementation of virulent isolates. AvrLm1 homologues were not detected in other Leptosphaeria species or in known fungal genomes including the closely related species Stagonospora nodorum. The predicted AvrLm1 protein is composed of 205 amino acids, of which only one is a cysteine residue. It contains a peptide signal suggesting extracellular localization. Unlike most other fungal avirulence genes, AvrLm1 is constitutively expressed, with a probable increased level of expression upon plant infection, suggesting the absence of tight regulation of AvrLm1 expression.
This paper demonstrates the embrittlement of a Zr-based bulk metallic glass caused by the presence of very low oxygen traces during its synthesis. Toughness studies are conducted both at room and low ...(77K) temperatures. Samples with 2000appm of oxygen contain crystalline defects (trapping all oxygen) while those with 300appm are completely glassy. Toughness results clearly indicate that these defects embrittle the glass. Therefore, careful attention must be paid during synthesis with respect to the oxygen content to get a completely glassy alloy. This issue is of paramount importance when dealing with mechanical properties such as toughness.
ABSTRACT Leptosphaeria maculans, the causal agent of stem canker of oilseed rape (Brassica napus), develops gene-for-gene interactions with oilseed rape, and four L. maculans avirulence (AVR) genes ...(AvrLm1, AvrLm2, AvrLm4, and alm1) were previously genetically characterized. Based on the analysis of progeny of numerous in vitro crosses between L. maculans isolates showing either already characterized or new differential interactions, this work aims to provide an overview of the AVR genes that may specify incompatibility toward B. napus and the related species B. juncea and B. rapa. Two novel differential interactions were thus identified between L. maculans and B. napus genotypes, one of them corresponding to a complete resistance to European races of L. maculans. In both cases, a single gene control of avirulence was established (genes AvrLm3 and AvrLm7). Similarly, a single gene control of avirulence toward a B. rapa genotype, also resistant to European L. maculans isolates, was demonstrated (gene AvrLm8). Finally, a digenic control of avirulence toward B. juncea was established (genes AvrLm5 and AvrLm6). Linkage analyses demonstrated that at least four unlinked L. maculans genomic regions, including at least one AVR gene cluster (AvrLm1-AvrLm2-AvrLm6), are involved in host specificity. The AvrLm3-AvrLm4-AvrLm7 region may correspond either to a second AVR gene cluster or to a multiallelic AVR gene.
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