Mating type markers reveal high levels of heterothallism in Leptographium sensu lato

Phylogenetic analyses allow species-level recognition of Leptographium wageneri varieties that cause black stain root disease of conifers in western North America

Leptographium wageneri is a native fungal pathogen in western North America that causes black stain root disease (BSRD) of conifers. Three host-specialized varieties of this pathogen were previously described: L. wageneri var. wageneri on pinyon pines (Pinus monophylla and P. edulis); L. wageneri var. ponderosum, primarily on hard pines (e.g., P. ponderosa, P. jeffreyi); and L. wageneri var. pseudotsugae on Douglas-fir (Pseudotsuga menziesii). Morphological, physiological, and ecological differences among the three pathogen varieties have been previously determined; however, DNA-based characterization and analyses are needed to determine the genetic relationships among these varieties. The objective of this study was to use DNA sequences of 10 gene regions to assess phylogenetic relationships among L. wageneri isolates collected from different hosts. The multigene phylogenetic analyses, based on maximum likelihood and Bayesian inference, strongly supported species-level separation of the three L. wageneri varieties. These results, in conjunction with previously established phenotypic differences, support the elevation of L. wageneri var. ponderosum and L. wageneri var. pseudotsugae to the species level as L. ponderosum comb. nov. and L. pseudotsugae comb. nov., respectively, while maintaining L. wageneri var. wageneri as Leptographium wageneri. Characterization of the three Leptographium species, each with distinct host ranges, provides a baseline to further understand the ecological interactions and evolutionary relationships of these forest pathogens, which informs management of black stain root disease.

Structure and number of mating pheromone genes is closely linked to sexual reproductive strategy in Huntiella

BACKGROUND

Huntiella resides in the Ceratocystidaceae, a family of fungi that accommodates important plant pathogens and insect-associated saprotrophs. Species in the genus have either heterothallic or unisexual (a form of homothallism) mating systems, providing an opportunity to investigate the genetic mechanisms that enable transitions between reproductive strategies in related species. Two newly sequenced Huntiella genomes are introduced in this study and comparative genomics and transcriptomics tools are used to investigate the differences between heterothallism and unisexuality across the genus.

RESULTS

Heterothallic species harbored up to seven copies of the a-factor pheromone, each of which possessed numerous mature peptide repeats. In comparison, unisexual Huntiella species had only two or three copies of this gene, each with fewer repeats. Similarly, while the heterothallic species expressed up to 12 copies of the mature α-factor pheromone, unisexual species had up to six copies. These significant differences imply that unisexual Huntiella species do not rely on a mating partner recognition system in the same way that heterothallic fungi do.

CONCLUSION

While it is suspected that mating type-independent pheromone expression is the mechanism allowing for unisexual reproduction in Huntiella species, our results suggest that the transition to unisexuality may also have been associated with changes in the genes governing the pheromone pathway. While these results are specifically related to Huntiella, they provide clues leading to a better understanding of sexual reproduction and the fluidity of mating strategies in fungi more broadly.

Mating genes in Calonectria and evidence for a heterothallic ancestral state

The genus Calonectria includes many important plant pathogens with a wide global distribution. In order to better understand the reproductive biology of these fungi, we characterised the structure of the mating type locus and flanking genes using the genome sequences for seven Calonectria species. Primers to amplify the mating type genes in other species were also developed. PCR amplification of the mating type genes and multi-gene phylogenetic analyses were used to investigate the mating strategies and evolution of mating type in a collection of 70 Calonectria species residing in 10 Calonectria species complexes. Results showed that the organisation of the MAT locus and flanking genes is conserved. In heterothallic species, a novel MAT gene, MAT1-2-12 was identified in the MAT1-2 idiomorph; the MAT1-1 idiomorph, in most cases, contained the MAT1-1-3 gene. Neither MAT1-1-3 nor MAT1-2-12 was found in homothallic Calonectria (Ca.) hongkongensis, Ca. lateralis, Ca. pseudoturangicola and Ca. turangicola. Four different homothallic MAT locus gene arrangements were observed. Ancestral state reconstruction analysis provided evidence that the homothallic state was basal in Calonectria and this evolved from a heterothallic ancestor.

Genetic Analyses of the Laurel Wilt Pathogen, Raffaelea lauricola, in Asia Provide Clues on the Source of the Clone that is Responsible for the Current USA Epidemic

Laurel wilt is caused by the fungus Raffaelea lauricola T.C. Harr., Fraedrich and Aghayeva, a nutritional symbiont of its vector the redbay ambrosia beetle, Xyleborus glabratus Eichhoff. Both are native to Asia but appeared in Georgia in the early 2000s. Laurel wilt has since spread to much of the southeastern United States killing >300 million host trees in the Lauraceae plant family. The aims of this research were to elucidate the genetic structure of populations of R. lauricola, to examine its reproductive strategy, and determine how often the pathogen had been introduced to the USA. A panel of 12 simple sequence repeat (SSR) markers identified 15 multilocus genotypes (MLGs) in a collection of 59 isolates from the USA (34 isolates), Myanmar (18), Taiwan (6) and Japan (1). Limited diversity in the USA isolates and the presence of one MAT idiotype (mating type locus) indicated that R. lauricola was probably introduced into the country a single time. MLG diversity was far greater in Asia than the USA. Only three closely related MLGs were detected in the USA, the most prevalent of which (30 of 34 isolates) was also found in Taiwan. Although more work is needed, the present results suggest that a Taiwanese origin is possible for the population of R. lauricola in the USA. Isolates of R. lauricola from Myanmar were distinct from those from Japan, Taiwan and the USA. Although both MAT idiotypes were present in Myanmar and Taiwan, only the population from Taiwan had the genetic structure of a sexually reproducing population.

Characterization of Non-coding Regions in B Mating Loci of Agrocybe salicacola Groups: Target Sites for B Mating Type Identification

Agrocybe salicacola is a delicious and cultivable mushroom. It is important to understand this species' inherent characteristics, especially to elucidate the constitution and segregation of mating genes. In this study, two compatible B mating loci in strain YAASM0711 of A. salicacola were cloned from the monokaryons, and sequence and phylogeny analyses showed two conserved genes encoding pheromone receptors maybe lost mating activity, which determined by comparing with those of other mushrooms. In the conserved regions of mating loci, partial insertion/deletion fragments made non-coding regions posses polymorphisms, and monokaryotic strains of different mating types were distinguished from each other according to the amplification profile of variable regions, which suggested mating loci were integrally assigned to offspring strains during mitosis in A. salicacola. As our known, it is the first to develop molecular markers for B mating-type identification using variable non-coding fragments of mating loci in basidiomycetes.

Genetic Variation in Native Populations of the Laurel Wilt Pathogen, Raffaelea lauricola, in Taiwan and Japan and the Introduced Population in the United States

Laurel wilt is a vascular wilt disease caused by Raffaelea lauricola, a mycangial symbiont of an ambrosia beetle, Xyleborus glabratus. The fungus and vector are native to Asia but were apparently introduced to the Savannah, GA, area 15 or more years ago. Laurel wilt has caused widespread mortality on redbay (Persea borbonia) and other members of the Lauraceae in the southeastern United States, and the pathogen and vector have spread as far as Texas. Although believed to be a single introduction, there has been no extensive study on genetic variation of R. lauricola populations that would suggest a genetic bottleneck in the United States. Ten isolates of R. lauricola from Japan, 55 from Taiwan, and 125 from the United States were analyzed with microsatellite and 28S rDNA markers, and with primers developed for two mating-type genes. The new primers identified isolates as either MAT1 or MAT2 mating types in roughly equal proportions in Taiwan and Japan, where there was also high genetic diversity within populations based on all the markers, suggesting that these populations may have cryptic sex. Aside from a local population near Savannah and a single isolate in Alabama that had unique microsatellite alleles, the U.S. population was genetically uniform and included only the MAT2 mating type, supporting the single introduction hypothesis. This study suggests the importance of preventing a second introduction of R. lauricola to the United States, which could introduce the opposite mating type and allow for genetic recombination.