P. Markus Wilken

DNA Loss at the Ceratocystis fimbriata Mating Locus Results in Self-Sterility

Fungi have evolved a remarkable diversity of reproductive strategies. Some of these, most notably those of the model fungi, have been well studied but others are poorly understood. The latter is also true for uni-directional mating type switching, which has been reported in only five fungal genera, including Ceratocystis. Mating type switching allows a self-fertile fungal isolate to produce both self-fertile and self-sterile offspring. This study considered the molecular nature of uni-directional mating type switching in the type species of Ceratocystis, C. fimbriata. To do this, the genome of C. fimbriata was first examined for the presence of mating type genes. Three mating genes (MAT1-1-1, MAT1-2-1 and MAT1-1-2) were found in an atypical organisation of the mating type locus. To study the effect that uni-directional switching has on this locus, several self-sterile offspring were analysed. Using a combination of next generation and conventional Sanger sequencing, it was shown that a 3581 base pair (bp) region had been completely deleted from the MAT locus. This deletion, which includes the entire MAT1-2-1 gene, results in the permanent loss of self-fertility, rendering these isolates exclusively self-sterile. Our data also suggest that the deletion mechanism is tightly controlled and that it always occurs at the same genomic position. Two 260 bp direct repeats flanking the deleted region are strongly implicated in the process, although the exact mechanism behind the switching remains unclear.

IMA Genome-F 2: Ceratocystis manginecans, Ceratocystis moniliformis, Diplodia sapinea: Draft genome sequences of Diplodia sapinea, Ceratocystis manginecans, and Ceratocystis moniliformis.

The draft nuclear genomes of Diplodia sapinea, Ceratocystis moniliformis s. str., and C. manginecans are presented. Diplodia sapinea is an important shoot-blight and canker pathogen of Pinus spp., C. moniliformis is a saprobe associated with wounds on a wide range of woody angiosperms and C. manginecans is a serious wilt pathogen of mango and Acacia mangium. The genome size of D. sapinea is estimated at 36.97 Mb and contains 13 020 predicted genes. Ceratocystis moniliformis includes 25.43 Mb and is predicted to encode at least 6 832 genes. This is smaller than that reported for the mango wilt pathogen C. manginecans which is 31.71 Mb and is predicted to encode at least 7 494 genes. The latter is thus more similar to C. fimbriata s.str., the type species of the genus. The genome sequences presented here provide an important resource to resolve issues pertaining to the taxonomy, biology and evolution of these fungi.

Both mating types in the heterothallic fungus Ophiostoma quercus contain MAT1-1 and MAT1-2 genes

In heterothallic Ascomycota, two opposite but distinct mating types control all sexual processes. Using mating crosses, mating types were assigned to ten isolates of the heterothallic fungal species Ophiostoma quercus. Primers were subsequently designed to target the MAT1-1-1, MAT1-1-3 (of the mating type 1 idiomorph), and MAT1-2-1 (of the mating type 2 idiomorph) genes in these isolates. Results showed that all isolates contained the full gene sequence for the MAT1-2-1 gene. In addition, fragments of the MAT1-1-1 and MAT1-1-3 genes were sequenced from all isolates. These results were unexpected, as each isolate from a heterothallic species would typically contain only one of the two possible MAT idiomorphs.

IMA Genome-F 1: Ceratocystis fimbriata: Draft nuclear genome sequence for the plant pathogen, Ceratocystis fimbriata

The draft nuclear genome of Ceratocystis fimbriata, the type species of Ceratocystis, is comprised of 29 410 862 bp. De novo gene prediction produced 7 266 genes, which is low for an ascomycete fungus. The availability of the genome provides opportunities to study aspects of the biology of this and other Ceratocystis species.

Draft genome of Cercospora zeina, Fusarium pininemorale, Hawksworthiomyces lignivorus, Huntiella decipiens and Ophiostoma ips

The genomes of Cercospora zeina, Fusarium pininemorale, Hawksworthiomyces lignivorus, Huntiella decipiens, and Ophiostoma ips are presented in this genome announcement. Three of these genomes are from plant pathogens and otherwise economically important fungal species. Fusarium pininemorale and H. decipiens are not known to cause significant disease but are closely related to species of economic importance. The genome sizes range from 25.99 Mb in the case of O. ips to 4.82 Mb for H. lignivorus. These genomes include the first reports of a genome from the genus Hawksworthiomyces. The availability of these genome data will allow the resolution of longstanding questions regarding the taxonomy of these species. In addition these genome sequences through comparative studies with closely related organisms will increase our understanding of how these species or close relatives cause disease.

Mutualism and asexual reproduction influence recognition genes in a fungal symbiont

Mutualism between microbes and insects is common and alignment of the reproductive interests of microbial symbionts with this lifestyle typically involves clonal reproduction and vertical transmission by insect partners. Here the Amylostereum fungus-Sirex woodwasp mutualism was used to consider whether their prolonged association and predominance of asexuality have affected the mating system of the fungal partner. Nucleotide information for the pheromone receptor gene rab1, as well as the translation elongation factor 1α gene and ribosomal RNA internal transcribed spacer region were utilized. The identification of rab1 alleles in Amylostereum chailletii and Amylostereum areolatum populations revealed that this gene is more polymorphic than the other two regions, although the diversity of all three regions was lower than what has been observed in free-living Agaricomycetes. Our data suggest that suppressed recombination might be implicated in the diversification of rab1, while no evidence of balancing selection was detected. We also detected positive selection at only two codons, suggesting that purifying selection is important for the evolution of rab1. The symbiotic relationship with their insect partners has therefore influenced the diversity of this gene and influenced the manner in which selection drives and maintains this diversity in A. areolatum and A. chailletii.

Pheromone expression reveals putative mechanism of unisexuality in a saprobic ascomycete fungus

Homothallism (self-fertility) describes a wide variety of sexual strategies that enable a fungus to reproduce in the absence of a mating partner. Unisexual reproduction, a form of homothallism, is a process whereby a fungus can progress through sexual reproduction in the absence of mating genes previously considered essential for self-fertility. In this study, we consider the molecular mechanisms that allow for this unique sexual behaviour in the saprotrophic ascomycete; Huntiella moniliformis. These molecular mechanisms are also compared to the underlying mechanisms that control sex in Huntiella omanensis, a closely related, but self-sterile, species. The main finding was that H. omanensis displayed mating-type dependent expression of the a- and α-pheromones. This was in contrast to H. moniliformis where both pheromones were co-expressed during vegetative growth and sexual development. Furthermore, H. moniliformis also expressed the receptors of both pheromones. Consequently, this fungus is likely able to recognize and respond to the endogenously produced pheromones, allowing for self-fertility in the absence of other key mating genes. Overall, these results are concomitant with those reported for other unisexual species, but represent the first detailed study considering the unisexual behaviour of a filamentous fungus.