Multiple sex pheromones and receptors of a mushroom-producing fungus elicit mating in yeast

Homeodomain 1 Genes of the Different HD Subloci of Flammulina velutipes Can Activate the HD Pathway and Are Involved in Mating, Clamp Cell Formation, and Upregulation of FvClp1

Flammulina velutipes has two independent and functional mating type factors, HD and PR. The HD locus contains two separate subloci: HD-a and HD-b. In this study, we investigated the roles of Hd1 genes of the HD-a and HD-b subloci in the process of mating, clamp cell formation, and regulation of FvClp1 (F. velutipes clampless1 gene) gene expression in F. velutipes. To this end, we introduced Hd1 genes from mating compatible strains into F. velutipes monokaryon L11. Overexpression of Hd1 gene FvHd-a1-1 of the HD-a sublocus resulted in the formation of pseudoclamps in L11 monokaryons. L11 mutants overexpressing the Hd1 gene FvHd-b1-2 of the HD-b sublocus also similarly developed pseudoclamps in the L11 monokaryons. Moreover, these mutant L11 monokaryons produced complete clamps when crossed with monokaryotic strains that differed at the PR loci, i.e., when selective activation of the PR pathway was obtained through crossing. Thus, Hd1 genes of the two different HD subloci in F. velutipes can activate the HD mating type pathway and induce clamp cell formation. In addition, activation of the HD pathway resulted in upregulation of the FvClp1 gene. Finally, to complete clamp cell formation, activation of the PR pathway appears to be essential. Overall, these findings were beneficial for deepening our understanding of sexual reproduction and fruiting body development of edible fungi.

Sex Without Sexes: Can the Cost of Finding a Mate Explain Diversity in Fungal Mating Systems?

Eukaryotes have evolved myriad ways of uniting gametes during sexual reproduction. A repeated pattern is the convergent evolution of a mating system with the fusion of larger gametes with smaller gametes (anisogamy) from that of fusion between morphologically identical gametes (isogamy). In anisogamous species, sexes are defined as individuals that produce only one gamete type. Although sexes abound throughout Eukarya, in fungi there are no biological sexes, because even in anisogamous species, individuals are hermaphroditic and produce both gamete types. For this reason, the term mating types is preferred over sexes, and, thus defined, only individuals of differing mating types can mate (homoallelic incompatibility). In anisogamous fungal species, there is scant evidence that there are more than two mating types, and this may be linked to genetic constraints, such as the use of mating types to determine the inheritance of cytoplasmic genomes. However, the mushroom fungi (Agaricomycetes) stand out as having both large numbers of mating types within a species, which will allow nearly all individuals to be compatible with each other, and reciprocal exchange of nuclei during mating, which will avoid cytoplasmic mixing and cyto-nuclear conflicts. Although the limitation of mating types to two in most fungi is consistent with the cyto-nuclear conflicts model, there are many facets of the Agaricomycete life cycle that also suggest they will demand a high outbreeding efficiency. Specifically, they are mostly obligately sexual and outcrossing, inhabit complex competitive niches, and display broadcast spore dispersal. Subsequently, the Agaricomycete individual pays a high cost to being choosy when encountering a mate. Here, I discuss the costs of mate finding and choice and demonstrate how most fungi have multiple ways of reducing these costs, which can explain why mating types are mostly limited to two per species. Nevertheless, it is perplexing that fungi have not evolved multiple mating types on more occasions nor evolved sexes. The few exceptions to these rules suggest that it is dictated by both molecular and evolutionary constraints.

Engineered cell differentiation and sexual reproduction in probiotic and mating yeasts

G protein-coupled receptors (GPCRs) enable cells to sense environmental cues and are indispensable for coordinating vital processes including quorum sensing, proliferation, and sexual reproduction. GPCRs comprise the largest class of cell surface receptors in eukaryotes, and for more than three decades the pheromone-induced mating pathway in baker's yeast Saccharomyces cerevisiae has served as a model for studying heterologous GPCRs (hGPCRs). Here we report transcriptome profiles following mating pathway activation in native and hGPCR-signaling yeast and use a model-guided approach to correlate gene expression to morphological changes. From this we demonstrate mating between haploid cells armed with hGPCRs and endogenous biosynthesis of their cognate ligands. Furthermore, we devise a ligand-free screening strategy for hGPCR compatibility with the yeast mating pathway and enable hGPCR-signaling in the probiotic yeast Saccharomyces boulardii. Combined, our findings enable new means to study mating, hGPCR-signaling, and cell-cell communication in a model eukaryote and yeast probiotics.

Characterization and expression pattern analysis of pheromone receptor-like genes in Winter Mushroom Flammulina filiformis

Pheromone receptor-like genes (PRLGs) belong to the G protein-coupled receptors (GPCRs) family that interacts with biotic and abiotic stimulants and transmits signals to intracellular downstream pathways in eukaryotic cells. In this study, we investigated the structure and expressions patterns of PRLGs in Winter Mushroom Flammulina filiformis. Based on the alignment analysis, the structure of PRLGs was found conserved in F. filiformis strains expect few single-nucleotide polymorphism (SNP) sites. Six PRLGs were found at five different unlinked loci, scattered in the genomes of F. filiformis strains. These genes contain 2-5 introns; however, the introns were not found in the same relative positions regarding the encoded protein sequences in tested strains of F. filiformis. Three conserved motifs were identified in peptides structures of PRLGs, however, FfSte3.s6 contained only two types, suggests its difference in evolution and function. We have further analyzed the expression patterns of each PRLGs in different developmental stages of the fruiting body in F. filiformis by quantitative real-time polymerase chain reaction (qRT-PCR). The results exhibited expression variation of PRLGs at different developmental stages of the F. filiformis. Especially, FfSte3.s1 and FfSte3.s2 exhibited maximum expression level in mycelia stage. Other PRLGs exhibited high expression level in fruiting body stages. This study suggests that PRLGs could be vital genes involving in fruiting body development in F. filiformis. However, further studies could be performed to reveal their specific functional pathways in the fruiting body development.

Investigation of Mating Pheromone-Pheromone Receptor Specificity in Lentinula edodes

The B mating-type locus of Lentinula edodes, a representative edible mushroom, is highly complex because of allelic variations in the mating pheromone receptors (RCBs) and the mating pheromones (PHBs) in both the Bα and Bβ subloci. The complexity of the B mating-type locus, five Bα subloci with five alleles of RCB1 and nine PHBs and three Bβ subloci with 3 alleles of RCB2 and five PHBs, has led us to investigate the specificity of the PHB-RCB interaction because the interaction plays a key role in non-self-recognition. In this study, the specificities of PHBs to RCB1-2 and RCB1-4 from the Bα sublocus and RCB2-1 from the Bb sublocus were investigated using recombinant yeast strains generated by replacing STE2, an endogenous yeast mating pheromone receptor, with the L. edodes RCBs. Fourteen synthetic PHBs with C-terminal carboxymethylation but without farnesylation were added to the recombinant yeast cells and the PHB-RCB interaction was monitored by the expression of the FUS1 gene-a downstream gene of the yeast mating signal pathway. RCB1-2 (Bα2) was activated by PHB1 (4.3-fold) and PHB2 (2.1-fold) from the Bα1 sublocus and RCB1-4 (Bα4) was activated by PHB5 (3.0-fold) and PHB6 (2.7-fold) from the Bα2 sublocus and PHB13 (3.0-fold) from the Bα5 sublocus. In particular, PHB3 from Bβ2 and PHB9 from Bβ3 showed strong activation of RCB2-1 of the Bβ1 sublocus by 59-fold. The RCB-PHB interactions were confirmed in the monokaryotic S1-10 strain of L. edodes by showing increased expression of clp1, a downstream gene of the mating signal pathway and the occurrence of clamp connections after the treatment of PHBs. These results indicate that a single PHB can interact with a non-self RCB in a sublocus-specific manner for the activation of the mating pheromone signal pathways in L. edodes.

The Farnesyltransferase β-Subunit Ram1 Regulates Sporisorium scitamineum Mating, Pathogenicity and Cell Wall Integrity

The basidiomycetous fungus Sporisorium scitamineum causes a serious sugarcane smut disease in major sugarcane growing areas. Sexual mating is essential for infection to the host; however, its underlying molecular mechanism has not been fully studied. In this study, we identified a conserved farnesyltransferase (FTase) β subunit Ram1 in S. scitamineum. The ram1Δ mutant displayed significantly reduced mating/filamentation, thus of weak pathogenicity to the host cane. The ram1Δ mutant sporidia showed more tolerant toward cell wall stressor Congo red compared to that of the wild-type. Transcriptional profiling showed that Congo red treatment resulted in notable up-regulation of the core genes involving in cell wall integrity pathway in ram1Δ sporidia compared with that of WT, indicating that Ram1 may be involved in cell wall integrity regulation. In yeast the heterodimeric FTase is responsible for post-translational modification of Ras (small G protein) and a-factor (pheromone). We also identified and characterized two conserved Ras proteins, Ras1 and Ras2, respectively, and a MAT-1 pheromone precursor Mfa1. The ras1Δ, ras2Δ and mfa1Δ mutants all displayed reduced mating/filamentation similar as the ram1Δ mutant. However, both ras1Δ and ras2Δ mutants were hypersensitive to Congo red while the mfa1Δ mutant was the same as wild-type. Overall our study displayed that RAM1 plays an essential role in S. scitamineum mating/filamentation, pathogenicity, and cell wall stability.

Engineering a Model Cell for Rational Tuning of GPCR Signaling

G protein-coupled receptor (GPCR) signaling is the primary method eukaryotes use to respond to specific cues in their environment. However, the relationship between stimulus and response for each GPCR is difficult to predict due to diversity in natural signal transduction architecture and expression. Using genome engineering in yeast, we constructed an insulated, modular GPCR signal transduction system to study how the response to stimuli can be predictably tuned using synthetic tools. We delineated the contributions of a minimal set of key components via computational and experimental refactoring, identifying simple design principles for rationally tuning the dose response. Using five different GPCRs, we demonstrate how this enables cells and consortia to be engineered to respond to desired concentrations of peptides, metabolites, and hormones relevant to human health. This work enables rational tuning of cell sensing while providing a framework to guide reprogramming of GPCR-based signaling in other systems.

Nuclear arms races: Experimental evolution for mating success in the mushroom-forming fungus Schizophyllum commune

When many gametes compete to fertilize a limited number of compatible gametes, sexual selection will favour traits that increase competitive success during mating. In animals and plants, sperm and pollen competition have yielded many interesting adaptations for improved mating success. In fungi, similar processes have not been shown directly yet. We test the hypothesis that sexual selection can increase competitive fitness during mating, using experimental evolution in the mushroom-forming fungus Schizophyllum commune (Basidiomycota). Mating in mushroom fungi occurs by donation of nuclei to a mycelium. These fertilizing 'male' nuclei migrate through the receiving 'female' mycelium. In our setup, an evolving population of nuclei was serially mated with a non-evolving female mycelium for 20 sexual generations. From the twelve tested evolved lines, four had increased and one had decreased fitness relative to an unevolved competitor. Even though only two of those five remained significant after correcting for multiple comparisons, for all five lines we found a correlation between the efficiency with which the female mycelium is accessed and fitness, providing additional circumstantial evidence for fitness change in those five lines. In two lines, fitness change was also accompanied by increased spore production. The one line with net reduced competitive fitness had increased spore production, but reduced fertilisation efficiency. We did not find trade-offs between male reproductive success and other fitness components. We compare these findings with examples of sperm and pollen competition and show that many similarities between these systems and nuclear competition in mushrooms exist.

Activation of the Mating Pheromone Response Pathway of Lentinula edodes by Synthetic Pheromones

Pheromone (PHB)-receptor (RCB) interaction in the mating pheromone response pathway of Lentinula edodes was investigated using synthetic PHBs. Functionality of the C-terminally carboxymethylated synthetic PHBs was demonstrated by concentration-dependent induction of a mating-related gene (znf2) expression and by pseudoclamp formation in a monokaryotic strain S1-11 of L. edodes. Treatment with synthetic PHBs activated the expression of homeodomain genes (HDs) residing in the A mating type locus, and of A-regulated genes, including znf2, clp1, and priA, as well as genes in the B mating type locus, including pheromone (phb) and receptor (rcb) genes. The synthetic PHBs failed to discriminate self from non-self RCBs. PHBs of the B4 mating type (B4 PHBs) were able to activate the mating pheromone response pathway in both monokaryotic S1-11 and S1-13 strains, whose B mating types were B4 (self) and B12 (non-self), respectively. The same was true for B12 PHBs in the B4 (non-self) and B12 (self) mating types. The synthetic PHBs also promoted the mating of two monokaryotic strains carrying B4-common incompatible mating types (A5B4 × A1B4). However, the dikaryon generated by this process exhibited abnormally high content of hyphal branching and frequent clamp connections and, more importantly, was found to be genetically unstable due to overexpression of mating-related genes such as clp1. Although synthetic PHBs were unable to discriminate self from non-self RCBs, they showed a higher affinity for non-self RCBs, through which the mating pheromone response pathway in non-self cells may be preferentially activated.

New approaches in bioprocess-control: Consortium guidance by synthetic cell-cell communication based on fungal pheromones

Bioconversions in industrial processes are currently dominated by single-strain approaches. With the growing complexity of tasks to be carried out, microbial consortia become increasingly advantageous and eventually may outperform single-strain fermentations. Consortium approaches benefit from the combined metabolic capabilities of highly specialized strains and species, and the inherent division of labor reduces the metabolic burden for each strain while increasing product yields and reaction specificities. However, consortium-based designs still suffer from a lack of available tools to control the behavior and performance of the individual subpopulations and of the entire consortium. Here, we propose to implement novel control elements for microbial consortia based on artificial cell-cell communication via fungal mating pheromones. Coupling to the desired output is mediated by pheromone-responsive gene expression, thereby creating pheromone-dependent communication channels between different subpopulations of the consortia. We highlight the benefits of artificial communication to specifically target individual subpopulations of microbial consortia and to control e.g. their metabolic profile or proliferation rate in a predefined and customized manner. Due to the steadily increasing knowledge of sexual cycles of industrially relevant fungi, a growing number of strains and species can be integrated into pheromone-controlled sensor-actor systems, exploiting their unique metabolic properties for microbial consortia approaches.

The cacao pathogen Moniliophthora roreri (Marasmiaceae) possesses biallelic A and B mating loci but reproduces clonally

The cacao pathogen Moniliophthora roreri belongs to the mushroom-forming family Marasmiaceae, but it has never been observed to produce a fruiting body, which calls to question its capacity for sexual reproduction. In this study, we identified potential A (HD1 and HD2) and B (pheromone precursors and pheromone receptors) mating genes in M. roreri. A PCR-based method was subsequently devised to determine the mating type for a set of 47 isolates from across the geographic range of the fungus. We developed and generated an 11-marker microsatellite set and conducted association and linkage disequilibrium (standardized index of association, IA(s)) analyses. We also performed an ancestral reconstruction analysis to show that the ancestor of M. roreri is predicted to be heterothallic and tetrapolar, which together with sliding window analyses support that the A and B mating loci are likely unlinked and follow a tetrapolar organization within the genome. The A locus is composed of a pair of HD1 and HD2 genes, whereas the B locus consists of a paired pheromone precursor, Mr_Ph4, and receptor, STE3_Mr4. Two A and B alleles but only two mating types were identified. Association analyses divided isolates into two well-defined genetically distinct groups that correlate with their mating type; IA(s) values show high linkage disequilibrium as is expected in clonal reproduction. Interestingly, both mating types were found in South American isolates but only one mating type was found in Central American isolates, supporting a prior hypothesis of clonal dissemination throughout Central America after a single or very few introductions of the fungus from South America.

Hydrophobin-Based Surface Engineering for Sensitive and Robust Quantification of Yeast Pheromones

Detection and quantification of small peptides, such as yeast pheromones, are often challenging. We developed a highly sensitive and robust affinity-assay for the quantification of the α-factor pheromone of Saccharomyces cerevisiae based on recombinant hydrophobins. These small, amphipathic proteins self-assemble into highly stable monolayers at hydrophilic-hydrophobic interfaces. Upon functionalization of solid supports with a combination of hydrophobins either lacking or exposing the α-factor, pheromone-specific antibodies were bound to the surface. Increasing concentrations of the pheromone competitively detached the antibodies, thus allowing for quantification of the pheromone. By adjusting the percentage of pheromone-exposing hydrophobins, the sensitivity of the assay could be precisely predefined. The assay proved to be highly robust against changes in sample matrix composition. Due to the high stability of hydrophobin layers, the functionalized surfaces could be repeatedly used without affecting the sensitivity. Furthermore, by using an inverse setup, the sensitivity was increased by three orders of magnitude, yielding a novel kind of biosensor for the yeast pheromone with the lowest limit of detection reported so far. This assay was applied to study the pheromone secretion of diverse yeast strains including a whole-cell biosensor strain of Schizosaccharomyces pombe modulating α-factor secretion in response to an environmental signal.

Contrasting Genomic Diversity in Two Closely Related Postharvest Pathogens: Penicillium digitatum and Penicillium expansum

Penicillium digitatum and Penicillium expansum are two closely related fungal plant pathogens causing green and blue mold in harvested fruit, respectively. The two species differ in their host specificity, being P. digitatum restricted to citrus fruits and P. expansum able to infect a wide range of fruits after harvest. Although host-specific Penicillium species have been found to have a smaller gene content, it is so far unclear whether these different host specificities impact genome variation at the intraspecific level. Here we assessed genome variation across four P. digitatum and seven P. expansum isolates from geographically distant regions. Our results show very high similarity (average 0.06 SNPs [single nucleotide polymorphism] per kb) between globally distributed isolates of P. digitatum pointing to a recent expansion of a single lineage. This low level of genetic variation found in our samples contrasts with the higher genetic variability observed in the similarly distributed P. expansum isolates (2.44 SNPs per kb). Patterns of polymorphism in P. expansum indicate that recombination exists between genetically diverged strains. Consistent with the existence of sexual recombination and heterothallism, which was unknown for this species, we identified the two alternative mating types in different P. expansum isolates. Patterns of polymorphism in P. digitatum indicate a recent clonal population expansion of a single lineage that has reached worldwide distribution. We suggest that the contrasting patterns of genomic variation between the two species reflect underlying differences in population dynamics related with host specificities and related agricultural practices. It should be noted, however, that this results should be confirmed with a larger sampling of strains, as new strains may broaden the diversity so far found in P. digitatum.

Artificial cell-cell communication as an emerging tool in synthetic biology applications

Cell-cell communication is a widespread phenomenon in nature, ranging from bacterial quorum sensing and fungal pheromone communication to cellular crosstalk in multicellular eukaryotes. These communication modes offer the possibility to control the behavior of an entire community by modifying the performance of individual cells in specific ways. Synthetic biology, i.e., the implementation of artificial functions within biological systems, is a promising approach towards the engineering of sophisticated, autonomous devices based on specifically functionalized cells. With the growing complexity of the functions performed by such systems, both the risk of circuit crosstalk and the metabolic burden resulting from the expression of numerous foreign genes are increasing. Therefore, systems based on a single type of cells are no longer feasible. Synthetic biology approaches with multiple subpopulations of specifically functionalized cells, wired by artificial cell-cell communication systems, provide an attractive and powerful alternative. Here we review recent applications of synthetic cell-cell communication systems with a specific focus on recent advances with fungal hosts.

A yeast pheromone-based inter-species communication system

We report on a pheromone-based inter-species communication system, allowing for a controlled cell-cell communication between the two species Saccharomyces cerevisiae and Schizosaccharomyces pombe as a proof of principle. It exploits the mating response pathways of the two yeast species employing the pheromones, α- or P-factor, as signaling molecules. The authentic and chimeric pheromone-encoding genes were engineered to code for the P-factor in S. cerevisiae and the α-factor in S. pombe. Upon transformation of the respective constructs, cells were enabled to express the mating pheromone of the opposite species. The supernatant of cultures of S. pombe cells expressing α-factor were able to induce a G1 arrest in the cell cycle, a change in morphology to the typical shmoo effect and expression driven by the pheromone-responsive FIG1 promoter in S. cerevisiae. The supernatant of cultures of S. cerevisiae cells expressing P-factor similarly induced cell cycle arrest in G1, an alteration in morphology typical for mating as well as the activation of the pheromone-responsive promoters of the rep1 and sxa2 genes in a pheromone-hypersensitive reporter strain of S. pombe. Apparently, both heterologous pheromones were correctly processed and secreted in an active form by the cells of the other species. Our data clearly show that the species-specific pheromone systems of yeast species can be exploited for a controlled inter-species communication.

Evolution of uni- and bifactorial sexual compatibility systems in fungi

Mating systems, that is, whether organisms give rise to progeny by selfing, inbreeding or outcrossing, strongly affect important ecological and evolutionary processes. Large variations in mating systems exist in fungi, allowing the study of their origin and consequences. In fungi, sexual incompatibility is determined by molecular recognition mechanisms, controlled by a single mating-type locus in most unifactorial fungi. In Basidiomycete fungi, however, which include rusts, smuts and mushrooms, a system has evolved in which incompatibility is controlled by two unlinked loci. This bifactorial system probably evolved from a unifactorial system. Multiple independent transitions back to a unifactorial system occurred. It is still unclear what force drove evolution and maintenance of these contrasting inheritance patterns that determine mating compatibility. Here, we give an overview of the evolutionary factors that might have driven the evolution of bifactoriality from a unifactorial system and the transitions back to unifactoriality. Bifactoriality most likely evolved for selfing avoidance. Subsequently, multiallelism at mating-type loci evolved through negative frequency-dependent selection by increasing the chance to find a compatible mate. Unifactoriality then evolved back in some species, possibly because either selfing was favoured or for increasing the chance to find a compatible mate in species with few alleles. Owing to the existence of closely related unifactorial and bifactorial species and the increasing knowledge of the genetic systems of the different mechanisms, Basidiomycetes provide an excellent model for studying the different forces that shape breeding systems.

Fungal fidelity: nuclear divorce from a dikaryon by mating or monokaryon regeneration

Basidiomycete fungi perform fertilizations by incorporation of nuclei into a monokaryotic mycelium to establish a dikaryon. The dikaryon cannot incorporate another type of nucleus, but can still act as a nucleus donor in a dikaryon-monokaryon (di-mon) mating, known as the Buller phenomenon. Previously, it has been observed that: (1) in a particular di-mon mating, one of the nuclear types of the dikaryon generally performs better as a donor than the other, and (2) when nuclei from a dikaryon are separated to form monokaryons again (dedikaryotisation), recovery of monokaryons of the two nuclear types is usually unequal. In this study, we investigated if these two observations of asymmetry are functionally related. We tested this hypothesis by performing both di-mon matings and dedikaryotisation of dikaryons derived from five different monokaryons. When a single mechanism controls both processes, the nucleus better at fertilizing a monokaryon in a Buller pairing should also be recovered upon dedikaryotisation with a higher frequency. The results showed a hierarchical structure for recovery among nuclei in dedikaryotisation, but this hierarchy did not correspond to the fertilization success during di-mon mating. These findings thus show that the mechanism causing asymmetric regeneration of nuclei, is most likely not the same as the mechanism responsible for increased chance of fertilization in di-mon matings. We discuss the complexity of the interactions that occur during di-mon matings with regards to the mating type loci.

Sexual reproduction and mating-type-mediated strain development in the penicillin-producing fungus Penicillium chrysogenum

Penicillium chrysogenum is a filamentous fungus of major medical and historical importance, being the original and present-day industrial source of the antibiotic penicillin. The species has been considered asexual for more than 100 y, and despite concerted efforts, it has not been possible to induce sexual reproduction, which has prevented sexual crosses being used for strain improvement. However, using knowledge of mating-type (MAT) gene organization, we now describe conditions under which a sexual cycle can be induced leading to production of meiotic ascospores. Evidence of recombination was obtained using both molecular and phenotypic markers. The identified heterothallic sexual cycle was used for strain development purposes, generating offspring with novel combinations of traits relevant to penicillin production. Furthermore, the MAT1-1-1 mating-type gene, known primarily for a role in governing sexual identity, was also found to control transcription of a wide range of genes with biotechnological relevance including those regulating penicillin production, hyphal morphology, and conidial formation. These discoveries of a sexual cycle and MAT gene function are likely to be of broad relevance for manipulation of other asexual fungi of economic importance.

Biogenesis of the Saccharomyces cerevisiae pheromone a-factor, from yeast mating to human disease

The mating pheromone a-factor secreted by Saccharomyces cerevisiae is a farnesylated and carboxylmethylated peptide and is unusually hydrophobic compared to other extracellular signaling molecules. Mature a-factor is derived from a precursor with a C-terminal CAAX motif that directs a series of posttranslational reactions, including prenylation, endoproteolysis, and carboxylmethylation. Historically, a-factor has served as a valuable model for the discovery and functional analysis of CAAX-processing enzymes. In this review, we discuss the three modules comprising the a-factor biogenesis pathway: (i) the C-terminal CAAX-processing steps carried out by Ram1/Ram2, Ste24 or Rce1, and Ste14; (ii) two sequential N-terminal cleavage steps, mediated by Ste24 and Axl1; and (iii) export by a nonclassical mechanism, mediated by the ATP binding cassette (ABC) transporter Ste6. The small size and hydrophobicity of a-factor present both challenges and advantages for biochemical analysis, as discussed here. The enzymes involved in a-factor biogenesis are conserved from yeasts to mammals. Notably, studies of the zinc metalloprotease Ste24 in S. cerevisiae led to the discovery of its mammalian homolog ZMPSTE24, which cleaves the prenylated C-terminal tail of the nuclear scaffold protein lamin A. Mutations that alter ZMPSTE24 processing of lamin A in humans cause the premature-aging disease progeria and related progeroid disorders. Intriguingly, recent evidence suggests that the entire a-factor pathway, including all three biogenesis modules, may be used to produce a prenylated, secreted signaling molecule involved in germ cell migration in Drosophila. Thus, additional prenylated signaling molecules resembling a-factor, with as-yet-unknown roles in metazoan biology, may await discovery.

Transcriptome and functional analysis of mating in the basidiomycete Schizophyllum commune

In this study, we undertook a functional characterization and transcriptome analysis that enabled a comprehensive study of the mating type loci of the mushroom Schizophyllum commune. Induced expression of both the bar2 receptor and the bap2(2) pheromone gene within 6 to 12 h after mates' contact was demonstrated by quantitative real-time PCR. Similar temporal expression patterns were confirmed for the allelic bbr1 receptor and bbp1 pheromone-encoding genes by Northern hybridization. Interestingly, the fusion of clamp connections to the subterminal cell was delayed in mating interactions in which one of the compatible partners expressed the bar2 receptor with a truncated C terminus. This developmental delay allowed the visualization of a green fluorescent protein (Gfp)-labeled truncated receptor at the cell periphery, consistent with a localization in the plasma membrane of unfused pseudoclamps. This finding does not support hypotheses envisioning a receptor localization to the nuclear membrane facilitating recognition between the two different nuclei present in each dikaryotic cell. Rather, Gfp fluorescence observed in such pseudoclamps indicated a role of receptor-pheromone interaction in clamp fusion. Transcriptome changes associated with mating interactions were analyzed in order to identify a role for pheromone-receptor interactions. We detected a total of 89 genes that were transcriptionally regulated in a mating type locus A-dependent manner, employing a cutoff of 5-fold changes in transcript abundance. Upregulation in cell cycle-related genes and downregulation of genes involved in metabolism were seen with this set of experiments. In contrast, mating type locus B-dependent transcriptome changes were observed in 208 genes, with a specific impact on genes related to cell wall and membrane metabolism, stress response, and the redox status of the cell.

The phosducin-like protein PhLP1 impacts regulation of glycoside hydrolases and light response in Trichoderma reesei

BACKGROUND

In the biotechnological workhorse Trichoderma reesei (Hypocrea jecorina) transcription of cellulase genes as well as efficiency of the secreted cellulase mixture are modulated by light. Components of the heterotrimeric G-protein pathway interact with light-dependent signals, rendering this pathway a key regulator of cellulase gene expression.

RESULTS

As regulators of heterotrimeric G-protein signaling, class I phosducin-like proteins, are assumed to act as co-chaperones for G-protein beta-gamma folding and exert their function in response to light in higher eukaryotes. Our results revealed light responsive transcription of the T. reesei class I phosducin-like protein gene phlp1 and indicate a light dependent function of PhLP1 also in fungi. We showed the functions of PhLP1, GNB1 and GNG1 in the same pathway, with one major output being the regulation of transcription of glycoside hydrolase genes including cellulase genes in T. reesei. We found no direct correlation between the growth rate and global regulation of glycoside hydrolases, which suggests that regulation of growth does not occur only at the level of substrate degradation efficiency.Additionally, PhLP1, GNB1 and GNG1 are all important for proper regulation of light responsiveness during long term exposure. In their absence, the amount of light regulated genes increased from 2.7% in wild type to 14% in Δphlp1. Besides from the regulation of degradative enzymes, PhLP1 was also found to impact on the transcription of genes involved in sexual development, which was in accordance with decreased efficiency of fruiting body formation in Δphlp1. The lack of GNB1 drastically diminished ascospore discharge in T. reesei.

CONCLUSIONS

The heterotrimeric G-protein pathway is crucial for the interconnection of nutrient signaling and light response of T. reesei, with the class I phosducin-like protein PhLP1, GNB1 and GNG1 acting as important nodes, which influence light responsiveness, glycoside hydrolase gene transcription and sexual development.

Interspecific sex in grass smuts and the genetic diversity of their pheromone-receptor system

The grass smuts comprise a speciose group of biotrophic plant parasites, so-called Ustilaginaceae, which are specifically adapted to hosts of sweet grasses, the Poaceae family. Mating takes a central role in their life cycle, as it initiates parasitism by a morphological and physiological transition from saprobic yeast cells to pathogenic filaments. As in other fungi, sexual identity is determined by specific genomic regions encoding allelic variants of a pheromone-receptor (PR) system and heterodimerising transcription factors. Both operate in a biphasic mating process that starts with PR-triggered recognition, directed growth of conjugation hyphae, and plasmogamy of compatible mating partners. So far, studies on the PR system of grass smuts revealed diverse interspecific compatibility and mating type determination. However, many questions concerning the specificity and evolutionary origin of the PR system remain unanswered. Combining comparative genetics and biological approaches, we report on the specificity of the PR system and its genetic diversity in 10 species spanning about 100 million years of mating type evolution. We show that three highly syntenic PR alleles are prevalent among members of the Ustilaginaceae, favouring a triallelic determination as the plesiomorphic characteristic of this group. Furthermore, the analysis of PR loci revealed increased genetic diversity of single PR locus genes compared to genes of flanking regions. Performing interspecies sex tests, we detected a high potential for hybridisation that is directly linked to pheromone signalling as known from intraspecies sex. Although the PR system seems to be optimised for intraspecific compatibility, the observed functional plasticity of the PR system increases the potential for interspecific sex, which might allow the hybrid-based genesis of newly combined host specificities.

Asymmetry in sexual pheromones is not required for ascomycete mating

BACKGROUND

We investigated the determinants of sexual identity in the budding yeast Saccharomyces cerevisiae. The higher fungi are divided into the ascomycetes and the basidiomycetes. Most ascomycetes have two mating types: one (called α in yeasts and MAT1-1 in filamentous fungi) produces a small, unmodified, peptide pheromone, and the other (a in yeasts and MAT1-2 in filamentous fungi) produces a peptide pheromone conjugated to a C-terminal farnesyl group that makes it very hydrophobic. In the basidiomycetes, all pheromones are lipid-modified, and this difference is a distinguishing feature between the phyla. We asked whether the asymmetry in pheromone modification is required for successful mating in ascomycetes.

RESULTS

We cloned receptor and pheromone genes from a filamentous ascomycete and a basidiomycete and expressed these in the budding yeast, Saccharomyces cerevisiae, to generate novel, alternative mating pairs. We find that two yeast cells can mate even when both cells secrete a-like or α-like peptides. Importantly, this is true regardless of whether the cells express the a- or α-mating-type loci, which control the expression of other, sex-specific genes, in addition to the pheromones and pheromone receptors.

CONCLUSIONS

We demonstrate that the asymmetric pheromone modification is not required for successful mating of ascomycete fungi and confirm that, in budding yeast, the primary determinants of mating are the specificity of the receptors and their corresponding pheromones.

A single mating-type locus composed of homeodomain genes promotes nuclear migration and heterokaryosis in the white-rot fungus Phanerochaete chrysosporium

The white-rot basidiomycete fungus Phanerochaete chrysosporium (Agaricomycetes) is a model species that produces potent wood-degrading enzymes. The mating system of the species has been difficult to characterize due to its cryptic fruiting habit and lack of clamp connections in the heterokaryotic phase. By exploiting the draft genome sequence, we reevaluated the mating system of P. chrysosporium by studying the inheritance and segregation of putative mating-type gene homologues, the homeodomain transcription factor genes (MAT-A) and the pheromone receptors (MAT-B). A pattern of mating incompatibility and fructification consistent with a bipolar system with a single MAT locus was observed, but the rejection response was much weaker than that seen in other agaricomycete species, leading to stable heterokaryons with identical MAT alleles. The homeodomain genes appear to comprise the single MAT locus because they are heterozygous in wild strains and hyperpolymorphic at the DNA sequence level and promote aspects of sexual reproduction, such as nuclear migration, heterokaryon stability, and basidiospore formation. The pheromone receptor loci that might constitute a MAT-B locus, as in many other Agaricomycetes, are not linked to the MAT-A locus and display low levels of polymorphism. This observation is inconsistent with a bipolar mating system that includes pheromones and pheromone receptors as mating-type determinants. The partial uncoupling of nuclear migration and mating incompatibility in this species may be predicted to lead to parasexual recombination and may have contributed to the homothallic behavior observed in previous studies.

Assessment of constitutive activity of a G protein-coupled receptor, CPR2, in Cryptococcus neoformans by heterologous and homologous methods

G protein-coupled receptors (GPCRs) comprise the largest superfamily of cell surface receptors and are primary targets for drug development. A variety of detection systems have been reported to study ligand-GPCR interactions. Using Saccharomyces cerevisiae to express foreign proteins has long been appreciated for its low cost, simplicity, and conserved cellular pathways. The yeast pheromone-responsive pathway has been utilized to assess a range of different GPCRs. We have identified a pheromone-like receptor, Cpr2, that is located outside of the MAT locus in the human fungal pathogen Cryptococcus neoformans. To characterize its function and potential ligands, we expressed CPR2 in a yeast heterologous expression system. To optimize for CPR2 expression in this system, pheromone receptor Ste3, regulator of G protein signaling (RGS) Sst2, and the cyclin-dependent kinase inhibitor Far1 were mutated. The lacZ gene was fused with the promoter of the FUS1 gene that is activated by the yeast pheromone signal and then introduced into yeast cells. Expression of CPR2 in this yeast heterologous expression system revealed that Cpr2 could activate the pheromone-responsive pathway without addition of potential ligands, suggesting it is a naturally occurring, constitutively active receptor. Mutation of a single amino acid, Leu(222), was sufficient to reverse the constitutive activity of Cpr2. In this chapter, we summarize methods used for assessing the constitutive activity of Cpr2 and its mutants, which could be beneficial for other GPCR studies.

An efficient gene deletion procedure for the mushroom-forming basidiomycete Schizophyllum commune

Gene deletion in Schizophyllum commune is hampered by a low incidence of homologous integration. As a consequence, extensive screening is required to identify a transformant with the desired genotype. To alleviate this and to facilitate the assembly of deletion plasmids, vector pDelcas was constructed. This construct has a set of restriction sites, which allows for directional cloning of the flanking sequences at both sides of a nourseothricin resistance cassette. Moreover, it contains a phleomycin resistance cassette elsewhere in the plasmid, which is used to screen for transformants with an ectopic integration of the pDelcas derivative. The use of pDelcas derivatives in combination with an improved PCR screening protocol permitted the efficient identification of S. commune deletion strains. This procedure may also function in other basidiomycetes.

Basidiomycete mating type genes and pheromone signaling

The genome sequences of the basidiomycete Agaricomycetes species Coprinopsis cinerea, Laccaria bicolor, Schizophyllum commune, Phanerochaete chrysosporium, and Postia placenta, as well as of Cryptococcus neoformans and Ustilago maydis, are now publicly available. Out of these fungi, C. cinerea, S. commune, and U. maydis, together with the budding yeast Saccharomyces cerevisiae, have been investigated for years genetically and molecularly for signaling in sexual reproduction. The comparison of the structure and organization of mating type genes in fungal genomes reveals an amazing conservation of genes regulating the sexual reproduction throughout the fungal kingdom. In agaricomycetes, two mating type loci, A, coding for homeodomain type transcription factors, and B, encoding a pheromone/receptor system, regulate the four typical mating interactions of tetrapolar species. Evidence for both A and B mating type genes can also be identified in basidiomycetes with bipolar systems, where only two mating interactions are seen. In some of these fungi, the B locus has lost its self/nonself discrimination ability and thus its specificity while retaining the other regulatory functions in development. In silico analyses now also permit the identification of putative components of the pheromone-dependent signaling pathways. Induction of these signaling cascades leads to development of dikaryotic mycelia, fruiting body formation, and meiotic spore production. In pheromone-dependent signaling, the role of heterotrimeric G proteins, components of a mitogen-activated protein kinase (MAPK) cascade, and cyclic AMP-dependent pathways can now be defined. Additionally, the pheromone-dependent signaling through monomeric, small GTPases potentially involved in creating the polarized cytoskeleton for reciprocal nuclear exchange and migration during mating is predicted.

A constitutively active GPCR governs morphogenic transitions in Cryptococcus neoformans

Sex in fungi is driven by peptide pheromones sensed through seven-transmembrane pheromone receptors. In Cryptococcus neoformans, sexual reproduction occurs through an outcrossing/heterothallic a- sexual cycle or an inbreeding/homothallic - unisexual mating process. Pheromone receptors encoded by the mating-type locus (MAT) mediate reciprocal pheromone sensing during opposite-sex mating and contribute to but are not essential for unisexual mating. A pheromone receptor-like gene, CPR2, was discovered that is not encoded by MAT and whose expression is induced during a- mating. cpr2 mutants are fertile but have a fusion defect and produce abnormal hyphal structures, whereas CPR2 overexpression elicits unisexual reproduction. When heterologously expressed in Saccharomyces cerevisiae, Cpr2 activates pheromone responses in the absence of any ligand. This constitutive activity results from an unconventional residue, Leu(222), in place of a conserved proline in transmembrane domain six; a Cpr2(L222P) mutant is no longer constitutively active. Cpr2 engages the same G-protein activated signalling cascade as the Ste3a/alpha pheromone receptors, and thereby competes for pathway activation. This study established a new paradigm in which a naturally occurring constitutively active G protein-coupled receptor governs morphogenesis in fungi.

Magnificent seven: roles of G protein-coupled receptors in extracellular sensing in fungi

G protein-coupled receptors (GPCRs) represent the largest family of transmembrane receptors and are responsible for transducing extracellular signals into intracellular responses that involve complex intracellular-signaling networks. This review highlights recent research advances in fungal GPCRs, including classification, extracellular sensing, and G protein-signaling regulation. The involvement of GPCRs in pheromone and nutrient sensing has been studied extensively over the past decade. Following recent advances in fungal genome sequencing projects, a panoply of GPCR candidates has been revealed and some have been documented to play key roles sensing diverse extracellular signals, such as pheromones, sugars, amino acids, nitrogen sources, and even photons. Identification and deorphanization of additional putative GPCRs may require the development of new research tools. Here, we compare research on GPCRs in fungi with information derived from mammalian systems to provide a useful road map on how to better understand ligand-GPCR-G protein interactions in general. We also emphasize the utility of yeast as a discovery tool for systemic studies of GPCRs from other organisms.

Sexual attraction: on the role of fungal pheromone/receptor systems (A review)

Pheromones have been detected in all fungal phylogenetic lineages. This came as a surprise, as the general role of pheromones in mate attraction was not envisioned for some fungi. Pheromones and pheromone receptor genes have been identified, however, in members of all true fungal lineages, and even for mycelia forming organisms of plant and amoeba lineages, like oomycetes and myxomycetes. The mating systems and genes governing the mating type are different in fungi, ranging from bipolar with two opposite mating types to tetrapolar mating systems (with four possible mating outcomes, only one of which leads to fertile sexual development) in homobasidioymcetes with more than 23,000 mating types occurring in nature. Pheromones and receptors specifically recognizing these pheromones have evolved with slightly different functions in these different systems. This review is dedicated to follow the evolution of pheromone/receptor systems from simple, biallelic bipolar systems to multiallelic, tetrapolar versions and to explain the slightly different functions the pheromone recognition and subsequent signal transduction cascades within the fungal kingdom. The biotechnological implications of a detailed understanding of mating systems for biological control and plant protection, in medicine, and in mushroom breeding are discussed.

The KlSTE2 and KlSTE3 genes encode MATalpha- and MATa-specific G-protein-coupled receptors, respectively, which are required for mating of Kluyveromyces lactis haploid cells

Mating in yeast is initiated by binding of pheromone to G-protein-coupled receptors expressed in haploid cells. We analysed the role of KlSte2p and KlSte3p receptors in the Kluyveromyces lactis mating pathway. By sequence analysis, KlSte2p and KlSte3p are the homologues of the Saccharomyces cerevisiae alpha-pheromone and a-pheromone receptors, respectively. However, by expression experiments, we determined that KlSTE2 gene is expressed in the cells typified as MATalpha and therefore is the receptor for the K. lactis a-pheromone and KlSTE3 gene is expressed in the MATa cells and binds the alpha-pheromone. The KlSTE2 gene is silent in MATa cells, while it is highly expressed in MATalpha cells, and conversely the KlSTE3 gene is expressed in MATa cells and repressed in MATalpha cells. Disruption mutants of both genes were found to be sterile, and this defect is reversed by plasmidic copies of each gene. The cytoplasmic C-terminus of KlSte3p interacts strongly with the KlGpa1p (Galpha) subunit, which is involved in the transduction of the pheromone stimulus to induce mating. Remarkably, this same domain does not interact with a constitutive active allele of the Galpha subunit, indicating that the C-terminus is able to discriminate between the active (GTP-bound) and inactive (GDP-bound) forms of the Galpha subunit.

Sensing the environment: lessons from fungi

All living organisms use numerous signal-transduction systems to sense and respond to their environments and thereby survive and proliferate in a range of biological niches. Molecular dissection of these signalling networks has increased our understanding of these communication processes and provides a platform for therapeutic intervention when these pathways malfunction in disease states, including infection. Owing to the expanding availability of sequenced genomes, a wealth of genetic and molecular tools and the conservation of signalling networks, members of the fungal kingdom serve as excellent model systems for more complex, multicellular organisms. Here, we review recent progress in our understanding of how fungal-signalling circuits operate at the molecular level to sense and respond to a plethora of environmental cues.

Ectopic expression of a constitutively active Cdc42 small GTPase alters the morphology of haploid and dikaryotic hyphae in the filamentous homobasidiomycete Schizophyllum commune

Cloning of the Cdc42 gene from Schizophyllum commune enabled investigation of the role of ScCdc42 in the regulation of vegetative growth and sexual reproduction in this fungus, which has a well-characterized hyphal cell structure, cytoskeleton, and mating system. Ectopic expression of the constitutively active Sccdc42(G12V) or Sccdc42(Q61L) alleles from native or inducible ScCel1 promoters in haploid hyphae had dramatic effects on hyphal morphology, cytoskeletal structure, and Cdc42 localization. For transformants with constitutively active Sccdc42, polar tip growth of apical cells in the leading hyphae was normal but polar tip growth in side branches was altered, implying different regulation of polarity establishment in the two groups of apical cells. Branch emergence at exceptional sites and isotropic growth of cells near the septum indicated that ScCdc42 regulates branch site selection and subsequent hyphal development. Poor dikaryotization along with irregular clamp connections in mates expressing Sccdc42(G12V) or Sccdc42(Q61L) suggested that Cdc42 also contributes to efficient mating in S. commune.

A homolog of Ste6, the a-factor transporter in Saccharomyces cerevisiae, is required for mating but not for monokaryotic fruiting in Cryptococcus neoformans

Fungal pheromones function during the initial recognition stage of the mating process. One type of peptide pheromone identified in ascomycetes and basidiomycetes terminates in a conserved CAAX motif and requires extensive posttranslational modifications to become mature and active. A well-studied representative is the a-factor of Saccharomyces cerevisiae. Unlike the typical secretory pathway utilized by most peptides, an alternative mechanism involving the ATP-binding cassette transporter Ste6 is used for the export of mature a-factor. Cryptococcus neoformans, a bipolar human pathogenic basidiomycete, produces CAAX motif-containing lipopeptide pheromones in both MATa and MATalpha cells. Virulence studies with a congenic pair of C. neoformans serotype D strains have shown that MATalpha cells are more virulent than MATa cells. Characterization of the MATalpha pheromones indicated that an autocrine signaling loop may contribute to the differentiation and virulence of MATalpha cells. To further address the role of pheromones in the signaling loop, we identified a STE6 homolog in the C. neoformans genome and determined its function by gene disruption. The ste6 mutants in either mating-type background showed partially impaired mating functions, and mating was completely abolished in a bilateral mutant cross. Surprisingly, the MATalpha ste6 mutant does not exhibit a defect in monokaryotic fruiting, suggesting that the activation of the autocrine signaling loop by the pheromone is via a Ste6-independent mechanism. MFalpha pheromone itself is essential for this process and could induce the signaling response intracellularly in MATalpha cells. Our data demonstrate that Ste6 is evolutionarily conserved for mating and is not required for monokaryotic fruiting in C. neoformans.

Functional characterization of an alpha-factor-like Sordaria macrospora peptide pheromone and analysis of its interaction with its cognate receptor in Saccharomyces cerevisiae

The homothallic filamentous ascomycete Sordaria macrospora possesses genes which are thought to encode two pheromone precursors and two seven-transmembrane pheromone receptors. The pheromone precursor genes are termed ppg1 and ppg2. The putative products derived from the gene sequence show structural similarity to the alpha-factor precursors and a-factor precursors of the yeast Saccharomyces cerevisiae. Likewise, sequence similarity has been found between the putative products of the pheromone receptor genes pre2 and pre1 and the S. cerevisiae Ste2p alpha-factor receptor and Ste3p a-factor receptor, respectively. To investigate whether the alpha-factor-like pheromone-receptor pair of S. macrospora is functional, a heterologous yeast assay was used. Our results show that the S. macrospora alpha-factor-like pheromone precursor PPG1 is processed into an active pheromone by yeast MATalpha cells. The S. macrospora PRE2 protein was demonstrated to be a peptide pheromone receptor. In yeast MATa cells lacking the endogenous Ste2p receptor, the S. macrospora PRE2 receptor facilitated all aspects of the pheromone response. Using a synthetic peptide, we can now predict the sequence of one active form of the S. macrospora peptide pheromone. We proved that S. macrospora wild-type strains secrete an active pheromone into the culture medium and that disruption of the ppg1 gene in S. macrospora prevents pheromone production. However, loss of the ppg1 gene does not affect vegetative growth or fertility. Finally, we established the yeast assay as an easy and useful system for analyzing pheromone production in developmental mutants of S. macrospora.

TRANSPARENT LEAF AREA1 encodes a secreted proteolipid required for anther maturation, morphogenesis, and differentiation during leaf development in maize

We report the identification and functional analysis of TRANSPARENT LEAF AREA1 (TLA1), a maize (Zea mays) gene representing a novel class of secreted, extremely hydrophobic peptides (proteolipids) with a C-terminal Caax box-like motif. ZmTLA1 encodes 27 amino acid residues and is most strongly expressed in the egg cell and microspores. Lower transcript amounts were detected during vegetative development. Transgenic maize expressing an antisense transcript displayed a variety of phenotypes. The most visible phenotypes were dwarfism and transparent leaf areas resulting from defective morphogenesis of mesophyll, bundle sheath, stomatal, and epidermal cells during leaf development. Incomplete cell walls were observed, indicating a defect of cytokinesis. The accumulation of gerontoplasts was probably a secondary effect caused by defects of leaf cell morphogenesis. A defect of anther maturation was observed in approximately 30% of the plants displaying the tla phenotype. Male sterility was mainly caused by incomplete disintegration of the tapetal cell layers and tetrad callose as 90% of the microspores developed into functional pollen. Overexpression of ZmTLA1 seemed to have a lethal effect both in maize and Arabidopsis thaliana. Development of primary roots, root hairs, primary leaves, and chloroplasts was suppressed in Arabidopsis seedlings expressing an inducible ZmTLA1-green fluorescent protein (GFP) fusion protein. GFP signals were exclusively detected in cell walls. Based on our observations, we suggest that the ZmTLA1 peptide represents a class of novel plant morphogens required for the development and maturation of leaf and reproductive tissues.

Isolation and characterization of mutations that affect nuclear migration for dikaryosis in Coprinus cinereus

We identified 14 Coprinus cinereus variants defective in nuclear migration for dikaryosis after restriction enzyme-mediated integration mutagenesis. All the variants were able to donate nuclei but failed to accept nuclei in compatible matings. Of the 14 variants, six were due to single-gene mutations. We characterized num1, a gene responsible for one of the mutations. The num1 ORF interrupted by three introns is predicted to encode a protein of 217 amino acids. The Num1 protein has two leucine-zipper motifs, one at the N-terminal region and the other at the C-terminal region. Num1 is also predicted to form a coiled-coil structure in its C-terminal half. The num1-1 mutant allele is predicted to produce a truncated peptide with only one leucine-zipper motif at the N-terminal region. A plasmid carrying a sequence for the truncated peptide could inhibit nuclear migration when introduced into a wild-type strain, showing that the num1-1 mutant allele is dominant negative. The transcription of num1 is down-regulated when the pathway regulated by B mating-type genes is activated.

ESTs from the basidiomycete Schizophyllum commune grown on nitrogen-replete and nitrogen-limited media

Lambda phage cDNA libraries were constructed using mRNAs from the basidiomycete Schizophyllum commune grown on media with high or low nitrogen concentrations. A total of 440 clones were sequenced, representing 373 distinct transcripts. Of these, 166 showed significant similarity to annotated genes in GenBank. Those that could be tentatively identified using BLAST searches were classified by function using the Gene Ontology (GO) database. Genes with products involved in cell-cycle processes were more frequent in the nitrogen-limited libraries, while genes with products involved in protein biosynthesis were more frequent in the nitrogen-replete library. Overall, clones showed much greater similarity to the one publicly available basidiomycete genome, Phanerochaete chrysosporium, than to any of the ascomycete genomes.

Evolution of multispecific mating-type alleles for pheromone perception in the homobasidiomycete fungi

The evolution of multiple, independent and multispecific mating-type loci is a feature unique to homobasidiomycete fungi. To propose a model of evolution, data assembled for the wood-rotting fungus Schizophyllum commune were analyzed. In one mating-type locus, pheromone receptors and several pheromones are encoded which have been investigated in some detail and can be used to understand the ligand-receptor interactions and activation of signal transduction which are essential to sexual propagation. Previous models for the evolution of new alleles were complicated and involved three subsequent steps (without selectable phenotype) prior to the establishment of a new stable pheromone-receptor pair. This paper presents a model for the evolution of new specificities by recombination and selection that incorporates the multi-state receptor activation recently established for S. commune, explaining differential responses to different pheromones in one receptor molecule. The model takes into account the occurrence of multiple pheromone genes in each locus and unilateral nuclear donor/acceptor strains that may in nature act as steps in the evolution of new specificities. A second homobasidiomycete fungus, Coprinus cinereus, was similarly characterized at the molecular level. Data acquired in this system support the conclusion that the presented model can be generalized.

The little difference: in vivo analysis of pheromone discrimination in Schizophyllum commune

The B mating type of Schizophyllum commune is defined by a multi-specific pheromone/receptor system. The interaction of pheromone receptors and their ligands, encoded by the Balpha locus, triggers sexual development. The receptors belong to the family of G protein-coupled seven-transmembrane-domain receptors, while the ligands are small lipopeptide pheromones. A productive interaction is only possible between molecules derived from different specificities. There is no induction of sexual development by pheromones of self-specificity. Since there are nine versions of different specificity for pheromones and receptors in Balpha, this system can be used to study multi-ligand discrimination. We investigated pheromone discrimination using chimeric receptor molecules and the influence of single point mutations on activation profiles of the receptor.

Pheromones stimulate mating and differentiation via paracrine and autocrine signaling in Cryptococcus neoformans

Cryptococcus neoformans is a pathogenic fungus with a defined sexual cycle involving haploid MATalpha and MATa cells. Interestingly, MATalpha strains are more common, are more virulent than congenic MATa strains, and undergo haploid fruiting in response to nitrogen limitation or MATa cells. Three genes encoding the MFalpha pheromone were identified in the MATalpha mating-type locus and shown to be transcriptionally induced by limiting nutrients and coculture with MATa cells. The MFalpha1, MFalpha2, and MFalpha3 genes were mutated, individually and in combination. MATalpha strains lacking MFalpha pheromone failed to induce morphological changes in MATa cells. Pheromoneless MATalpha mutants were fusion and mating impaired but not sterile and mated at approximately 1% the wild-type level. The pheromoneless MATalpha mutants were also partially defective in haploid fruiting, and overexpression of MFalpha pheromone enhanced haploid fruiting. Overexpression of MFa pheromone also enhanced haploid fruiting of MATalpha cells and stimulated conjugation tube formation in MATa cells. A conserved G-protein activated mitogen-activated protein kinase signaling pathway was found to be required for both induction and response to mating pheromones. The MFalpha pheromone was not essential for virulence of C. neoformans but does contribute to the overall virulence composite. These studies define paracrine and autocrine pheromone response pathways that signal mating and differentiation of this pathogenic fungus.

Molecular analysis of CPRalpha, a MATalpha-specific pheromone receptor gene of Cryptococcus neoformans

The putative Cryptococcus neoformans pheromone receptor gene CPRalpha was isolated and studied for its role in mating and filamentation. CPRalpha is MATalpha specific and located adjacent to STE12alpha at the MATalpha locus. It encodes a protein which possesses high sequence similarity to the seven-transmembrane class of G-protein-coupled pheromone receptors reported for other basidiomycetous fungi. Strains containing a deletion of the CPRalpha gene exhibited drastic reductions in mating efficiency but were not completely sterile. Delta cpr alpha cells displayed wild-type mating efficiency when reconstituted with the wild-type CPRalpha gene. Hyphal production on filament agar was not affected in the delta cpr alpha strain, indicating no significant role for CPRalpha in sensing environmental cues during haploid fruiting. The wild-type MATalpha CPRalpha strain produced abundant hyphae in response to synthetic MATa pheromone; however, the hyphal response to pheromone by delta cpr alpha cells was significantly reduced. Exposure of wild-type cells to synthetic MATa pheromone for 2 h induced MFalpha pheromone expression, whereas unexposed cells showed only basal levels of the MFalpha transcript. The delta cpr alpha cells, however, exhibited only basal levels of MFalpha message with or without pheromone exposure, suggesting that CPRalpha and MFalpha are components of the same signaling pathway.

Mate recognition in fungi

The ascomycete and basidiomycete fungi have contributed much to our understanding of eukaryotic cell biology. The study of mate recognition, in particular, has provided detailed understanding of cell signalling pathways and cell-specific gene transcription. Sexual dimorphism has little relevance to mating in these organisms, indeed specialised cells for mating are found only in filamentous ascomycetes and even here, a single individual produces both male and female structures. None the less, most species have genetic barriers to prevent selfing. The genes that determine self-incompatibility divide populations into different mating types, and only individuals with different mating types can engage in sexual reproduction. Ascomycetes have just two mating types, but basidiomycetes may have several thousands. Despite apparent differences in the biology and numbers of mating types in these fungi, it is becoming increasingly apparent that many components of their mating pathways are highly conserved.

Identification of transcriptionally expressed pheromone receptor genes in filamentous ascomycetes

Detection of pheromone genes in filamentous ascomycetes implicated the presence of pheromone receptor genes. Similar to yeasts and basidiomycetes, these might be involved in a G-protein triggered signal transduction pathway during mating. We have identified two pheromone receptor genes, named pre1 and pre2, in the genome of the heterothallic filamentous ascomycete Neurospora crassa and the closely related homothallic Sordaria macrospora. The deduced pre1 gene product is a putative seven-transmembrane protein, which displays a high-level amino acid identity with the a-factor receptor Ste3p of Saccharomyces cerevisiae, and is also homologous to lipopeptide pheromone receptors of basidiomycetes. The deduced pre2 product displays significant sequence similarity with the S. cerevisiae STE2 gene product, the alpha-factor receptor. Pair-wise comparisons between pheromone receptor genes of N. crassa and S. macrospora revealed an extremely low degree of nucleotide conservation in these genes, suggesting that they evolved very rapidly. The two genes are transcriptionally expressed in both N. crassa and S. macrospora. Northern and reverse transcription-polymerase chain reaction analyses indicate that in N. crassa, expression of the receptor genes does not occur in a mating type specific manner. Thus, filamentous ascomycetes appear to posses and express pheromone receptor genes.

Changes in mate recognition through alterations of pheromones and receptors in the multisexual mushroom fungus Schizophyllum commune

Schizophyllum commune has thousands of mating types defined in part by numerous lipopeptide pheromones and their G-protein-coupled receptors. These molecules are encoded within multiple versions of two redundantly functioning B mating-type loci, B alpha and B beta. Compatible combinations of pheromones and receptors, produced by individuals of different B mating types, trigger a pathway of fertilization required for sexual development. Analysis of the B beta 2 mating-type locus revealed a large cluster of genes encoding a single pheromone receptor and eight different pheromones. Phenotypic effects of mutations within these genes indicated that small changes in both types of molecules could significantly alter their specificity of interaction. For example, a conservative amino acid substitution in a pheromone resulted in a gain of function toward one receptor and a loss of function with another. A two-amino-acid deletion from a receptor precluded the mutant pheromone from activating the mutant receptor, yet this receptor was activated by other pheromones. Sequence comparisons provided clues toward understanding how so many variants of these multigenic loci could have evolved through duplication and mutational divergence. A three-step model for the origin of new variants comparable to those found in nature is presented.