Transfer of genetic information from the mycoparasite Parasitella parasitica to its host Absidia glauca

Fungal evolution: major ecological adaptations and evolutionary transitions

Fungi are a highly diverse group of heterotrophic eukaryotes characterized by the absence of phagotrophy and the presence of a chitinous cell wall. While unicellular fungi are far from rare, part of the evolutionary success of the group resides in their ability to grow indefinitely as a cylindrical multinucleated cell (hypha). Armed with these morphological traits and with an extremely high metabolical diversity, fungi have conquered numerous ecological niches and have shaped a whole world of interactions with other living organisms. Herein we survey the main evolutionary and ecological processes that have guided fungal diversity. We will first review the ecology and evolution of the zoosporic lineages and the process of terrestrialization, as one of the major evolutionary transitions in this kingdom. Several plausible scenarios have been proposed for fungal terrestralization and we here propose a new scenario, which considers icy environments as a transitory niche between water and emerged land. We then focus on exploring the main ecological relationships of Fungi with other organisms (other fungi, protozoans, animals and plants), as well as the origin of adaptations to certain specialized ecological niches within the group (lichens, black fungi and yeasts). Throughout this review we use an evolutionary and comparative-genomics perspective to understand fungal ecological diversity. Finally, we highlight the importance of genome-enabled inferences to envision plausible narratives and scenarios for important transitions.

Understanding Mucor circinelloides pathogenesis by comparative genomics and phenotypical studies

The increasing number of infections by species of Mucorales and their high mortality constitute an important concern for public health. This study aims to decipher the genetic basis of Mucor circinelloides pathogenicity, which displays virulence in a strain dependent manner. Assuming that genetic differences between strains may be linked to different pathotypes, we have conducted a study to explore genes responsible for virulence in M. circinelloides by whole genome sequencing of the avirulent strain NRRL3631 and comparison with the virulent strain CBS277.49. This genome analysis revealed 773 truncated, discontiguous and absent genes in the NRRL3631 strain. We also examined phenotypic traits resulting in reduced heat stress tolerance, chitosan content and lower susceptibility to toxic compounds (calcofluor white and sodium dodecyl sulphate) in the virulent strain, suggesting the influence of cell wall on pathogenesis. Based on these results, we focused on studying extracellular protein-coding genes by gene deletion and further pathotype characterization of mutants in murine models of pulmonary and systemic infection. Deletion of gene ID112092, which codes for a hypothetical extracellular protein of unknown function, resulted in significant reduction of virulence.

Although pathogenesis is a multifactorial process, these findings highlight the crucial role of surface and secreted proteins in M. circinelloides virulence and should promote further studies of other differential genes.

Sex and the Imperfect Fungi

Approximately 20% of species in the fungal kingdom are only known to reproduce by asexual means despite the many supposed advantages of sexual reproduction. However, in recent years, sexual cycles have been induced in a series of emblematic "asexual" species. We describe how these discoveries were made, building on observations of evidence for sexual potential or "cryptic sexuality" from population genetic analyses; the presence, distribution, and functionality of mating-type genes; genome analyses revealing the presence of genes linked to sexuality; the functionality of sex-related genes; and formation of sex-related developmental structures. We then describe specific studies that led to the discovery of mating and sex in certain Candida, Aspergillus, Penicillium, and Trichoderma species and discuss the implications of sex including the beneficial exploitation of the sexual cycle. We next consider whether there might be any truly asexual fungal species. We suggest that, although rare, imperfect fungi may genuinely be present in nature and that certain human activities, combined with the genetic flexibility that is a hallmark of the fungal kingdom, might favor the evolution of asexuality under certain conditions. Finally, we argue that fungal species should not be thought of as simply asexual or sexual, but rather as being composed of isolates on a continuum of sexual fertility.

The fate of mitochondria after infection of the Mucoralean fungus Absidia glauca by the fusion parasite Parasitella parasitica: comparison of mitochondrial genomes in zygomycetes

Absidia glauca and Parasitella parasitica constitute a versatile experimental system for studying horizontal gene transfer between a mucoralean host and its fusion parasite. The A. glauca chondriome has a length of approximately 63 kb and a GC content of 28%. The chondriome of P. parasitica is larger, 83 kb, and contains 31% GC base pairs. These mtDNAs contain the standard fungal mitochondrial gene set, small and large subunit rRNAs, plus ribonuclease P RNA. Comparing zygomycete chondriomes reveals an unusually high number of homing endonuclease genes in P. parasitica, substantiating the mobility of intron elements independent of host-parasite interactions.

Complete Mitochondrial DNA Sequence of the Mucoralean Fungus Absidia glauca, a Model for Studying Host-Parasite Interactions

The mitochondrial DNA (mtDNA) ofAbsidia glaucahas been completely sequenced. It is 63,080 bp long, has a G+C content of 28%, and contains the standard fungal gene set.A. glaucais the recipient in a laboratory model for horizontal gene transfer withParasitella parasiticaas a donor of nuclei and mitochondria.

Complete Mitochondrial DNA Sequence of the Mucoralean Fusion Parasite Parasitella parasitica

The complete mitochondrial DNA sequence of the Mucor-related fungus Parasitella parasitica has been sequenced. It has a G+C content of 30% and a total length of 83,361 bp. All protein-coding genes normally found in fungi are present in the sequence. A special feature is the remarkably high number of 27 homing endonucleases.

The transcription factor Ste12 mediates the regulatory role of the Tmk1 MAP kinase in mycoparasitism and vegetative hyphal fusion in the filamentous fungus Trichoderma atroviride

Mycoparasitic species of the fungal genus Trichoderma are potent antagonists able to combat plant pathogenic fungi by direct parasitism. An essential step in this mycoparasitic fungus-fungus interaction is the detection of the fungal host followed by activation of molecular weapons in the mycoparasite by host-derived signals. The Trichoderma atroviride MAP kinase Tmk1, a homolog of yeast Fus3/Kss1, plays an essential role in regulating the mycoparasitic host attack, aerial hyphae formation and conidiation. However, the transcription factors acting downstream of Tmk1 are hitherto unknown. Here we analyzed the functions of the T. atroviride Ste12 transcription factor whose orthologue in yeast is targeted by the Fus3 and Kss1 MAP kinases. Deletion of the ste12 gene in T. atroviride not only resulted in reduced mycoparasitic overgrowth and lysis of host fungi but also led to loss of hyphal avoidance in the colony periphery and a severe reduction in conidial anastomosis tube formation and vegetative hyphal fusion events. The transcription of several orthologues of Neurospora crassa hyphal fusion genes was reduced upon ste12 deletion; however, the Δste12 mutant showed enhanced expression of mycoparasitism-relevant chitinolytic and proteolytic enzymes and of the cell wall integrity MAP kinase Tmk2. Based on the comparative analyses of Δste12 and Δtmk1 mutants, an essential role of the Ste12 transcriptional regulator in mediating outcomes of the Tmk1 MAPK pathway such as regulation of the mycoparasitic activity, hyphal fusion and carbon source-dependent vegetative growth is suggested. Aerial hyphae formation and conidiation, in contrast, were found to be independent of Ste12.

The family structure of the Mucorales: a synoptic revision based on comprehensive multigene-genealogies

The Mucorales (Mucoromycotina) are one of the most ancient groups of fungi comprising ubiquitous, mostly saprotrophic organisms. The first comprehensive molecular studies 11 yr ago revealed the traditional classification scheme, mainly based on morphology, as highly artificial. Since then only single clades have been investigated in detail but a robust classification of the higher levels based on DNA data has not been published yet. Therefore we provide a classification based on a phylogenetic analysis of four molecular markers including the large and the small subunit of the ribosomal DNA, the partial actin gene and the partial gene for the translation elongation factor 1-alpha. The dataset comprises 201 isolates in 103 species and represents about one half of the currently accepted species in this order. Previous family concepts are reviewed and the family structure inferred from the multilocus phylogeny is introduced and discussed. Main differences between the current classification and preceding concepts affects the existing families Lichtheimiaceae and Cunninghamellaceae, as well as the genera Backusella and Lentamyces which recently obtained the status of families along with the Rhizopodaceae comprising Rhizopus, Sporodiniella and Syzygites. Compensatory base change analyses in the Lichtheimiaceae confirmed the lower level classification of Lichtheimia and Rhizomucor while genera such as Circinella or Syncephalastrum completely lacked compensatory base changes.

Transformation of the fungus Absidia glauca by complementation of a methionine-auxotrophic strain affected in the homoserine-acetyltransferase gene

Transformation of fungi by complementation of auxotrophs is generally much more reliable than usage of antibiotic resistance markers. In order to establish such a system for the model zygomycete Absidia glauca, a stable methionine auxotrophic mutant was isolated after X-ray mutagenesis of the minus mating type and characterized at the molecular level. The mutant is disrupted in the coding region of the Met2-1 gene, encoding homoserine O-acetyltransferase. The corresponding wild type gene was cloned, sequenced and inserted into appropriate vector plasmids. Transformants are prototrophs and show restored methionine-independent growth, based on complementation by the autonomously replicating plasmids.

Carotene derivatives in sexual communication of zygomycete fungi

Recognition between mating partners, early sexual morphogenesis and development are regulated by a family of beta-carotene derived signal compounds, the trisporoids, in zygomycete fungi. Mating type-specific precursors are released from the hyphae and exert their physiological effects upon compatible mating partners. In a cooperative synthesis pathway, later intermediates and finally trisporic acid are formed. All trisporoids occur in a number of derivatives. Trisporic acid and some precursors directly influence the transcription of genes involved in sexual development. This has been demonstrated for TSP3, encoding the carotene oxygenase involved in sexually induced cleavage of beta-carotene. Species specificity of mating despite a common and commonly recognized signaling system is maintained by several factors. Specific distribution and recognition patterns of the trisporoid derivatives and the proposed divergence in trisporoid synthesis pathways in diverse species play a role. The derivatives elicit vastly differing, partially mating type-specific responses during early sexual development. Another specificity factor is the realization of different regulation levels for the trisporoid synthesis enzymes in different species. Enzymes in the trisporoid synthesis pathway show remarkable variations in mating type-specific activity and the exact activation time during sexual development. This allows for the observed complex network of possible interactions, but at the same time forbids successful mating between dissimilar partners because the necessary transcripts or gene products are not available at the appropriate developmental stage.

Absidia parricida plays a dominant role in biotrophic fusion parasitism among mucoralean fungi (Zygomycetes): Lentamyces, a new genus for A. parricida and A. zychae

Within the order Mucorales (Zygomycetes), the facultative parasites Parasitella parasitica, Chaetocladium brefeldii, Chaetocladium jonesii and Absidia parricida are known to initiate biotrophic fusion parasitic interactions on a wide variety of other mucoralean hosts. Their phylogenetic relationship within the Mucorales and their ability to form parasitic structures with several known host species was examined. Together with interspecific reactions between the mycoparasites, this study found: (i) no evident differences in the spectrum of non-parasitic hosts tested within the study; (ii) A. parricida parasitises all other fusion parasites; (iii) A. parricida is ancestral to all other parasites; (iv) A. parricida is reported for the first time as phylogenetically basal to all other mucoralean fungi except the Umbelopsidaceae and (v) based on phylogenetic analyses and physiological and morphological characteristics, the slow-growing species A. parricida and Absidia zychae are removed from the genus Absidia and reclassified in the newly described genus Lentamyces.

Sexuality and parasitism share common regulatory pathways in the fungus Parasitella parasitica

Parasitella parasitica, a facultative mycoparasite of zygomycetous fungi, forms cytoplasmic fusions with its hosts during infection. Thus, the organism is an efficient donor of genetic material in parasexual host-parasite interactions. Recognition between parasite and host is mediated by trisporoids, which are also responsible for sexual communication. The TDH gene for one of the key enzymes of trisporic acid biosynthesis, 4-dihydromethyl-trisporate dehydrogenase, was cloned and its transcription analysed. TDH was cloned on a 6175-bp insert and was found to map in a complex cluster of genes that suggest post-transcriptional antisense regulation. Histochemical TDH analysis in developing parasitic or sexual structures shows high enzymatic activity in Parasitella. TDH is linked to a gene for a putative acyl-CoA thioesterase (ACT). Two ORFs were identified in the 5'-region of the TDH gene, a third one, coding for 176 amino acids overlaps the ACT gene in antisense direction completely. Expression levels of ACT and ORF1 depend on parasitic and sexual interactions.

Genetic damage following introduction of DNA in Phycomyces

Introduction of plasmids in Phycomyces blakesleeanus caused extensive changes in the exogenous DNA and in the resident genome. Plasmids with a bacterial gene for geneticin resistance under a Phycomyces promoter were either injected into immature sporangia or incubated with spheroplasts. An improved method produced about one viable spheroplast per cell. Colonies resistant to geneticin were rare and only about 0.1% of their spores grew in the presence of geneticin. The transformation frequency was very low, < or =1 transformed colony per million spheroplasts or per microg DNA. Few nuclei in the transformants contained exogenous DNA, as shown by a selective procedure that sampled single nuclei from heterokaryons. The exogenous DNA was not integrated into the genome and no stable transformants were obtained. The plasmids were replicated in the recipient cells, but their DNA sequences were modified by deletions and rearrangements and the transformed phenotype was eventually lost. The spores developed in injected sporangia were often inviable; a genetic test showed that spore death was caused by impaired nuclear proliferation and induction of lethal mutations. About one-fourth of the viable spores from injected sporangia formed abnormal colonies with obvious changes in shape, texture, or color. The abnormalities that could be investigated were due to dominant mutations. The results indicate that incoming DNA is not only attacked, but signals a situation of stress that leads to increased mutation and nuclear and cellular death.

Hypovirulence-Associated Double-Stranded RNA from Sclerotinia homoeocarpa Is Conspecific with Ophiostoma novo-ulmi Mitovirus 3a-Ld

ABSTRACT The nucleotide sequence of the hypovirulence-associated double-stranded RNA (dsRNA) in hypovirulent isolate Sh12B of Sclerotinia homoeocarpa, the causal agent of dollar spot of turf grass, was determined. This large dsRNA (L-dsRNA) is 2,632 bp long and is A and U rich (61.0% A+U residues). One strand of this dsRNA contains an open reading frame (ORF) with the potential to encode a protein of 720 amino acids. This ORF contains 12 UGA codons, predicted to encode tryptophan in ascomycete mitochondria, and has a codon bias typical of mitochondrial genes, which is consistent with a mitochondrial localization of this dsRNA. The amino acid sequence contains conserved motifs typical of RNA-dependent RNA polymerases (RdRps). Sequence analyses of the nucleotide and RdRp-like protein revealed that the L-dsRNA is homologous with previously characterized mitochondrial viruses and dsRNAs from other phytopathogenic fungi, and shares 92.4% nucleotide and 95.1% amino acid sequence identities with the Ophiostoma novo-ulmi mitovirus 3a-Ld from Ophiostoma novo-ulmi, the causal agent of Dutch elm disease. The results indicate that these two dsRNAs are conspecific. This is the first report that a hypovirulence-associated dsRNA virus naturally occurs in two taxonomically distinct fungi, and indicates that horizontal transmission of this dsRNA virus may have occurred between these fungi.

Interspecific Transmission of Double-Stranded RNA and Hypovirulence from Sclerotinia sclerotiorum to S. minor

ABSTRACT Interspecific transmission of a hypovirulence-associated double-stranded RNA (dsRNA) and hypovirulent phenotype was attempted from hypovirulent isolate Ss275 of Sclerotinia sclerotiorum to five virulent isolates of S. minor. dsRNA and the hypovirulent phenotype were successfully transmitted to one of the five isolates, Sm10. Three putative converted isolates of Sm10 were slow growing and developed atypical colony morphologies characteristic of the hypovirulent phenotype. These isolates were assayed for virulence and produced significantly smaller lesions than isolate Sm10 on detached leaves of Romaine lettuce. One of these putative converted isolates, designated Sm10T, was tested to confirm interspecific transmission of dsRNA. In northern hybridizations, dsRNA isolated from Sm10T hybridized with a digoxigenin-labeled cDNA probe prepared from dsRNA isolated from Ss275. Random amplified polymorphic DNA analysis confirmed that isolate Sm10T was derived from Sm10 and not from Ss275 or a hybrid of the two species. The dsRNA and hypovirulent phenotype were subsequently transmitted intraspecifically from Sm10T to Sm8. To our knowledge, this is the first report of interspecific transmission of dsRNA and an associated hypovirulent phenotype between fungal plant pathogens by hyphal anastomosis.

Role of Horizontal Gene Transfer in the Evolution of Fungi

Although evidence for horizontal gene transfer (HGT) in eukaryotes remains largely anecdotal, literature on HGT in fungi suggests that it may have been more important in the evolution of fungi than in other eukaryotes. Still, HGT in fungi has not been widely accepted because the mechanisms by which it may occur are unknown, because it is usually not directly observed but rather implied as an outcome, and because there are often equally plausible alternative explanations. Despite these reservations, HGT has been justifiably invoked for a variety of sequences including plasmids, introns, transposons, genes, gene clusters, and even whole chromosomes. In some instances HGT has also been confirmed under experimental conditions. It is this ability to address the phenomenon in an experimental setting that makes fungi well suited as model systems in which to study the mechanisms and consequences of HGT in eukaryotic organisms.

Cellular and molecular mechanisms of sexual incompatibility in plants and fungi

Plants and fungi show an astonishing diversity of mechanisms to promote outbreeding, the most widespread of which is sexual incompatibility. Sexual incompatibility involves molecular recognition between mating partners. In fungi and algae, highly polymorphic mating-type loci mediate mating through complementary interactions between molecules encoded or regulated by different mating-type haplotypes, whereas in flowering plants polymorphic self-incompatibility loci regulate mate recognition through oppositional interactions between molecules encoded by the same self-incompatibility haplotypes. This subtle mechanistic difference is a consequence of the different life cycles of fungi, algae, and flowering plants. Recent molecular and biochemical studies have provided fascinating insights into the mechanisms of mate recognition and are beginning to shed light on evolution and population genetics of these extraordinarily polymorphic genetic systems of incompatibility.

Tetrapolar fungal mating types: sexes by the thousands

In order to achieve genetic rearrangement in a sexual cycle, eukaryotes go through the processes of meiosis and mating. Different mating types assure that mating is only possible between two genetically diverse individuals. Basidiomycetous fungi display thousands of different mating types that are determined by two genetically unlinked loci. One locus is multiallelic and contains genes for homeodomain transcription factors which are able to form heterodimers. The activation of target genes is dependent on heterodimers formed from the monomeric transcription factor proteins originating from different alleles of this genetic locus. The interactions between the two monomeric transcription factors and the activation of target genes by the heterodimeric proteins make this regulatory system both complex and interesting. The second locus contains a pheromone receptor system: the pheromone receptor is similar to the G protein-linked serpentine receptors in Saccharomyces cerevisiae that activate the pheromone response via a phosphorylation signal transduction cascade in S. cerevisiae. This pheromone perception is a trigger of sexual development and not, as with yeast, itself under control of mating type genes. Rather it directly senses diversity at the mating type loci. Whereas heterobasidiomycetes display a bi-allelic structure for this locus with recognition between one receptor and the opposite pheromone, homobasidiomycetes contain multiple specificities for pheromone receptors and pheromones.

Induction of longevity by cytoplasmic transfer of a linear plasmid in Podospora anserina

In Podospora anserina the longevity inducing linear plasmid pAL2-1 was transferred from the extrachromosomal long-lived mutant AL2 to the short-lived wild-type strain A. The resulting strain, AL2-IV, exhibited the long-lived phenotype. In the short-lived progeny of crosses between this strain and wild-type strain A, the plasmid was absent. In contrast, all long-lived progeny contained both the autonomous plasmid as well as copies of it integrated in the mitochondrial DNA (mtDNA). Molecular analysis revealed that the integrated plasmid copies most likely resulted from a de novo integration of the autonomous element and the generation of AT-linker sequences at the integration site. We conclude that once the plasmid is present in mitochondria of a particular genetic background, it is able to integrate into the mtDNA and to induce longevity.

Biology and ecology of mycoparasitism

The term mycoparasitism applies strictly to those relationships in which one living fungus acts as a nutrient source for another, but fungicolous relationships may also be included in which nutrient exchange has not been shown. Fungicolous fungi have a constant but indeterminate association with another fungus, and it can be difficult to demonstrate a true parasitic relationship. Mycoparasitic relationships can be necrotrophic or biotrophic, and can be classified on the basis of the host–parasite interface as contact necrotrophs, invasive necrotrophs, haustorial biotrophs, intracellular biotrophs, or fusion biotrophs depending on the intimacy of the relationship. In natural ecosystems, it is proposed that mycoparasitic relationships play an important role in the development of fungal communities. Two specific examples have been chosen to illustrate the general principles of mycoparasitism: the necrotrophic invasion of spores of arbuscular mycorrhizal fungi and the biotrophic invasion of mucoralean hosts by haustorial mycoparasites. Key words: mycoparasitism, fungicolous fungi, arbuscular mycorrhizal fungi, fungal ecology.

Relationships between sexual processes and parasitic interactions in the host–pathogen systemAbsidia glauca–Parasitella parasitica

Parasitella parasitica is a facultative parasite of many Mucorales including Absidia glauca. The infection process includes the formation of a plasmatic continuum between host and parasite, which allows the invasion of the host by nuclei of the parasite. This process gives rise to interspecies recombinants. Auxotrophic A. glauca mutants are complemented by the transfer of genes from the parasite to the host. At the molecular level, we could show that plasmid-coded genes are also transferred. The successful formation of infection structures is mating-type dependent. Parasitella parasitica exclusively infects hosts belonging to the complementary mating type. The formation of infection structures is correlated with the ability to synthesise the sex pheromone trisporic acid, which is produced in mixed cultures by compatible combinations between host and parasite. Normally, trisporic acid is formed by a cooperative biosynthesis involving both mating types. Trisporic acid seems to be involved also in mediating the recognition between P. parasitica and A. glauca. To test the hypothesis of direct hormonal correlations between sex and parasitism, we have isolated one of the key enzymes for trisporic acid synthesis, dihydromethyltrisporic acid dehydrogenase, sequenced the N-termini of some proteolytic cleavage products, and have started to isolate the corresponding gene. Key words: mycoparasitism, zygomycetes, trisporic acid, Absidia glauca, Parasitella parasitica, Mucor.

Horizontal transfer of a mitochondrial plasmid

Direct evidence for horizontal transfer of a mitochondrial plasmid from the discomycete Ascobolus immersus to the pyrenomycete Podospora anserina is presented. Southern blot hybridisation analysis, polymerase chain reaction (PCR) amplification, and DNA sequencing demonstrate transmission of a linear plasmid upon hyphal contact. DNA extraction from isolated organelles indicates a mitochondrial localisation for the plasmid in P. anserina. This is the first report of horizontal gene transfer among unrelated fungi. These results have important evolutionary implications for plasmid propagation in fungi.

Analysis of the gene for the elongation factor 1 alpha from the zygomycete Absidia glauca. Use of the promoter region for constructions of transformation vectors

The complete genomic DNA sequence was determined for one of the gene for the elongation factor 1 alpha (TEF), isolated from the zygomycete Absidia glauca. Sequence comparison with TEF genes from other fungi show the highest similarity to TEF-genes of the closely related zygomycete Mucor racemosus (Sundstrom et al. 1987). Southern-blot analysis of genomic DNA from A. glauca with the TEF gene reveals six chromosomal copies in the genome. In transformation experiments of A. glauca, vector constructions were used which allow targeting of one of the TEF loci. Several transformants of A. glauca were analyzed at the DNA level. In most cases, rearranged forms of autonomously replicated plasmids could be found in these isolates. However, some transformants show a different restriction pattern of the TEF loci if compared with the parental strains. From Southern-blot data it could be concluded that in one case the rearrangement lies downstream of one TEF locus. In a second case genetic parts following the 3'-end of the TEF gene are moved towards the 5'-end of the gene.

Foreign DNA sequences are received by a wild-type strain of Aspergillus niger after co-culture with transgenic higher plants

Different transgenic plants of Brassica napus, Brassica nigra, Datura innoxia and Vicia narbonensis expressing the hph gene under the control of the 35s promoter were co-cultivated with mycelial material of Aspergillus niger in microcosms under sterile conditions. A significantly higher number of hygromycin B-resistant colonies of re-isolated fungi was obtained if compared with co-cultures with non-transgenic plants. The hph gene and other foreign sequences could be detected in some of the resistant strains only for a short time after selection, indicating a rapid loss of foreign DNA. A more stable transgenic strain was obtained after co-culture with transgenic plants of D. innoxia including a high number of hph copies in their genome. DNA with detected pUC sequences was prepared to transform E. coli DH5 alpha. One of the recovered plasmids is shown to include pieces of the plant-transforming vector and a foreign sequence. The 35s-regulated expression of genes is studied in A. niger.

Variability in genome organization of the zygomycete Parasitella parasitica

In addition to conventional methods for the identification of fungi, molecular techniques at the DNA level are increasingly being employed. In order to check the validity of such experimental approaches, we have analyzed the well-defined species Parasitella parasitica, which belongs to the family Mucoraceae (Mucorales, Zygometes). The seven strains of this species, which are available from international strain collections, were analyzed by several molecular methods: restriction fragment length polymorphism analysis (RFLP), the random primer-dependent polymerase chain reaction (RAPD-PCR), and electrophoretic karyotyping. Unexpectedly, these strains are highly diverse at the molecular level. By these techniques they can be divided consistently into two different groups. Nevertheless, all seven strains belong to a single species. They show no morphological differences and sexual spores (zygospores) were found in all possible combinations either within or between the two groups. Southern-blot analysis of genomic DNA of all P. parasitica strains with RAPD-PCR-derived labelled probes shows the existence of repetitive elements characteristic for only one group of P. parasitica. In addition, chromosome sizes, which were separated by rotating-field electrophoresis, were highly divergent, and ranged from 3 to 6.5 Mb in one group and between 2 and 4.5 Mb in the other. The RAPD-PCR patterns also discriminate both groups of P. parasitica. However, they are very similar if strains of a single group are compared. Therefore, we propose that the determination of fungal species by molecular techniques should be vetted at least by morphological and physiological parameters and, whenever possible, by mating experiments.

Sequence comparison of a segment of the gene for 3-hydroxy-3-methylglutaryl-coenzyme A reductase in zygomycetes

In this paper we compare the sequences of a segment of the 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase gene, isolated from eleven different strains belonging to four species of the fungal order Mucorales, Parasitella parasitica, Absidia glauca, Mucor mucedo (Mucoraceae) and Blakeslea trispora (Choanephoraceae). The segment was obtained by polynucleotide-chain-reaction amplification with primers derived from conservative regions of the gene. For the species M. mucedo and P. parasitica we have obtained evidence for two different types of HMG-CoA reductase genes by hybridization of genomic DNA with the amplified fragment and by cloning and sequencing of two different fragments. The different genes from one species show a sequence similarity of around 80% at the protein sequence level, whereas sequences of the same type from different species show similarity ranging between 91-96%. The highest similarity was found between the genes of type 1 from B. trispora and M. mucedo, although these species belong to different families. Southern-blot analysis of A. glauca DNA and B. trispora DNA revealed a second copy of the genes.