Group-I intron family in the nuclear ribosomal RNA small subunit genes of Cenococcum geophilum isolates

Effects of High Temperature-Triggered Transcriptomics on the Physiological Adaptability of Cenococcum geophilum, an Ectomycorrhizal Fungus

High temperature stress caused by global warming presents a challenge to the healthy development of forestry. Cenococcum geophilum is a common ectomycorrhizal fungus (ECMF) in the forest system and has become an important fungus resource with application potential in forest vegetation restoration. In this study, three sensitive isolates of C. geophilum (ChCg01, JaCg144 and JaCg202) and three tolerant isolates of C. geophilum (ACg07, ChCg28 and ChCg100) were used to analyze the physiological and molecular responses to high temperature. The results showed that high temperature had a significant negative effect on the growth of sensitive isolates while promoting the growth of tolerant isolates. The antioxidative enzymes activity of C. geophilum isolates increased under high temperature stress, and the SOD activity of tolerant isolates (A07Cg and ChCg100) was higher than that of sensitive isolates (ChCg01 and JaCg202) significantly. The tolerant isolates secreted more succinate, while the sensitive isolates secreted more oxalic acid under high temperature stress. Comparative transcriptomic analysis showed that differentially expressed genes (DEGs) of six C. geophilum isolates were significantly enriched in "antioxidant" GO entry in the molecular. In addition, the "ABC transporters" pathway and the "glyoxylate and dicarboxylic acid metabolic" were shared in the three tolerant isolates and the three sensitive isolates, respectively. These results were further verified by RT-qPCR analysis. In conclusion, our findings suggest that C. geophilum can affect the organic acid secretion and increase antioxidant enzyme activity in response to high temperature by upregulating related genes.

Comparative Physiological and Transcriptome Analysis Provide Insights into the Response of Cenococcum geophilum, an Ectomycorrhizal Fungus to Cadmium Stress

Cadmium (Cd) displays strong toxicity, high mobility, and cannot be degraded, which poses a serious threat to the environment. Cenococcum geophilum (C. geophilum) is one of the most common ectomycorrhizal fungi (ECMF) in the natural environment. In this study, three Cd sensitive and three Cd tolerant strains of C. geophilum were used to analyze the physiological and molecular responses to Cd exposure. The results showed that Cd inhibited the growth of all strains of C. geophilum but had a less toxic effect on the tolerant strains, which may be correlated to a lower content of Cd and higher activity of antioxidant enzymes in the mycelia of tolerant strains. Comparative transcriptomic analysis was used to identify differentially expressed genes (DEGs) of four selected C. geophilum strains after 2 mg/L Cd treatment. The results showed that the defense response of C. geophilum strain to Cd may be closely related to the differential expression of functional genes involved in cell membrane ion transport, macromolecular compound metabolism, and redox pathways. The results were further confirmed by RT-qPCR analysis. Collectively, this study provides useful information for elucidation of the Cd tolerance mechanism of ECMF.

Analysis of small and large subunit rDNA introns from several ectomycorrhizal fungi species

The small (18S) and large (28S) nuclear ribosomal DNA (rDNA) introns have been researched and sequenced in a variety of ectomycorrhizal fungal taxa in this study, it is found that both 18S and 28S rDNA would contain introns and display some degree variation in size, nucleotide sequences and insertion positions within the same fungi species (Meliniomyces). Under investigations among the tested isolates, 18S rDNA has four sites for intron insertions, 28S rDNA has two sites for intron insertions. Both 18S and 28S rDNA introns among the tested isolates belong to group I introns with a set of secondary structure elements designated P1-P10 helics and loops. We found a 12 nt nucleotide sequences TACCACAGGGAT at site 2 in the 3'-end of 28S rDNA, site 2 introns just insert the upstream or the downstream of the12 nt nucleotide sequences. Afters sequence analysis of all 18S and 28S rDNA introns from tested isolates, three high conserved regions around 30 nt nucleotides (conserved 1, conserved 2, conserved 3) and identical nucleotides can be found. Conserved 1, conserved 2 and conserved 3 regions have high GC content, GC percentage is almost more than 60%. From our results, it seems that the more convenient host sites, intron sequences and secondary structures, or isolates for 18S and 28S rDNA intron insertion and deletion, the more popular they are. No matter 18S rDNA introns or 18S rDNA introns among tested isolates, complementary base pairing at the splicing sites in P1-IGS-P10 tertiary helix around 5'-end introns and exons were weak.

Integrated evolution of ribosomal RNAs, introns, and intron nurseries

The initial components of ribosomes first appeared more than 3.8 billion years ago during a time when many types of RNAs were evolving. While modern ribosomes are complex molecular machines consisting of rRNAs and proteins, they were assembled during early evolution by the association and joining of small functional RNA units. Introns may have provided the means to ligate many of these pieces together. All four classes of introns (group I, group II, spliceosomal, and archaeal) are present in many rRNA gene loci over a broad phylogenetic range. A survey of rRNA intron sequences across the three major life domains suggests that some of the classes of introns may have diverged from one another within rRNA gene loci. Analyses of rRNA sequences revealed self-splicing group I and group II introns are present in ancestral regions of the SSU (small subunit) and LSU (large subunit), whereas spliceosomal and archaeal introns appeared in sections of the rRNA that evolved later. Most classes of introns increased in number for approximately 1 billion years. However, their frequencies are low in the most recently evolved regions added to the SSU and LSU rRNAs. Furthermore, many of the introns appear to have been in the same locations for billions of years, suggesting an ancient origin for these sequences. In this Perspectives paper, I reviewed and analyzed rRNA intron sequences, locations, structural characteristics, and splicing mechanisms; and suggest that rRNA gene loci may have served as evolutionary nurseries for intron formation and diversification.

Revisiting phylogenetic diversity and cryptic species of Cenococcum geophilum sensu lato

The fungus Cenococcum geophilum Fr. (Dothideomycetes, Ascomycota) is one of the most common ectomycorrhizal fungi in boreal to temperate regions. A series of molecular studies has demonstrated that C. geophilum is monophyletic but a heterogeneous species or a species complex. Here, we revisit the phylogenetic diversity of C. geophilum sensu lato from a regional to intercontinental scale by using new data from Florida (USA) along with existing data in GenBank from Japan, Europe, and North America. The combination of internal transcribed spacer (ITS) ribosomal DNA and the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene resolved six well-supported lineages (87-100 % bootstrap values) that are closely related to each other and a seventh lineage that is phylogenetically distinct. A multi-locus analysis (small subunit (SSU), large subunit (LSU), translational elongation factor (TEF), and the largest and second-largest subunits of RNA polymerase II (RPB1 and RPB2)) revealed that the divergent lineage is the sister group to all other known Cenococcum isolates. Isolates of the divergent lineage grow fast on nutrient media and do not form ectomycorrhizas on seedlings of several pine and oak species. Our results indicate that C. geophilum sensu lato includes more phylogenetically distinct cryptic species than have previously been reported. Furthermore, the divergent lineage appears to be a non-mycorrhizal sister group. We discuss the phylogenetic diversity of C. geophilum sensu lato and argue in favor of species recognition based on phylogenetic and ecological information in addition to morphological characteristics. A new genus and species (Pseudocenococcum floridanum gen. et sp. nov.) is proposed to accommodate a divergent and putatively non-mycorrhizal lineage.

Identification, molecular characterization, and evolution of group I introns at the expansion segment D11 of 28S rDNA in Rhizoctonia species

The nuclear ribosomal DNA of Rhizoctonia species is polymorphic in terms of the nucleotide composition and length. Insertions of 349-410 nucleotides in length with characteristics of group I introns were detected at a single insertion point at the expansion segment D11 of 28S rDNA in 12 out of 64 isolates. Eleven corresponded to Rhizoctonia solani (teleomorph: Thanatephorous) and one (AG-Q) to Rhizoctonia spp. (teleomorph: Ceratobasidium). Sequence data showed that all but AG-Q contained conserved DNA catalytic core regions (P, Q, R, and S) essential for selfsplicing. The predicted secondary structure revealed that base-paired helices corresponded to subgroup IC1. Isolates from same anastomosis group and even subgroups within R. solani were variable with regard to possession of introns. Phylogenetic analyses indicated that introns were vertically transmitted. Unfortunately, sequence data from the conserved region from all 64 isolates were not useful for delimiting species. Analyses with IC1 introns at same insertion point, of both Ascomycota and Basidiomycota indicated the possibility of horizontal transfer at this site. The present study uncovered new questions on evolutionary pattern of change of these introns within Rhizoctonia species.

Evolution of small putative group I introns in the SSU rRNA gene locus of Phialophora species

BACKGROUND

Group I introns (specifically subgroup IC1) are common in the nuclear ribosomal RNA genes of fungi. While most range in length from more than 200 to nearly 1800 nucleotides (nt) in length, several small putative (or degenerate) group I introns have been described that are between 56 and 81 nt. Although small, previously we demonstrated that the PaSSU intron in the rRNA small subunit gene of Phialophora americana isolate Wang 1046 is capable of in vitro splicing using a standard group I intron pathway, thus qualifying it as a functional ribozyme.

FINDINGS

Here, we describe eight short putative group I introns, ranging in length from 63 to 75 nt, in the rRNA small subunit genes of Phialophora isolates, a fungal genus that ranges from saprobic to pathogenic on plants and animals. All contain putative pairing regions P1, P7, and P10, as well as a pairing region formed between the middle of the intron and part of the 3' exon. The other pairing regions common in the core of standard group I introns are absent. However, parts of the 3' exon may aid in the stabilization of these small introns. Although the eight putative group I introns were from at least three species of Phialophora, phylogenetic analysis indicated that the eight are monophyletic. They are also monophyletic with the small introns of two lichen-forming fungi, Porpidia crustulata and Arthonia lapidicola.

CONCLUSIONS

The small putative group I introns in Phialophora have common features that may represent group I introns at their minima. They appear to have a single origin as indicated by their monophyly in phylogenetic analyses.

Occurrence and characteristics of group 1 introns found at three different positions within the 28S ribosomal RNA gene of the dematiaceous Phialophora verrucosa: phylogenetic and secondary structural implications

BACKGROUND

Group 1 introns (ribozymes) are among the most ancient and have the broadest phylogenetic distribution among the known self-splicing ribozymes. Fungi are known to be rich in rDNA group 1 introns. In the present study, five sequences of the 28S ribosomal RNA gene (rDNA) regions of pathogenic dematiaceous Phialophora verrucosa were analyzed using PCR by site-specific primers and were found to have three insertions, termed intron-F, G and H, at three positions of the gene. We investigated the distribution of group 1 introns in this fungus by surveying 34 strains of P. verrucosa and seven strains of Phialophora americana as the allied species.

RESULTS

Intron-F's (inserted at L798 position) were found in 88% of P. verrucosa strains, while intron-G's (inserted at L1921) at 12% and intron-H's (inserted at L2563) at 18%. There was some correlation between intron distribution and geographic location. In addition, we confirmed that the three kinds of introns are group 1 introns from results of BLAST search, alignment analysis and Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR). Prediction of secondary structures and phylogenetic analysis of intron sequences identified introns-F and G as belonging to subgroup IC1. In addition, intron-H was identified as IE.

CONCLUSION

The three intron insertions and their insertion position in the 28S rDNA allowed the characterization of the clinical and environmental isolates of P. verrucosa and P. americana into five genotypes. All subgroups of introns-F and G and intron-H were characterized and observed for the first time in both species.

Splicing and evolution of an unusually small group I intron

Introns are common in the rRNA gene loci of fungal genomes, but biochemical studies to investigate splicing are rare. Here, self-splicing of a very small (67 nucleotide) group I intron is demonstrated. The PaSSU intron (located within the rRNA small subunit gene of Phialophora americana) splices in vitro under group I intron conditions. Most group I ribozymes contain pairing regions P1-P10, with a conserved G.U pair at the 5' splice site, and a G at the 3' intron border. The PaSSU intron contains only P1, P7, and P10. While it contains the G.U pair at the 5' splice, a U is found at the 3' end of the intron instead of a G. Phylogenetic analysis places it within subgroup IC1, whose members are found in the nuclear rRNA genes of fungi. The structural elements are similar to those in the centermost regions of other group I introns. Its size can be explained by a single large deletion that removed P2 through much of P9. Part of the original P9 region has assumed the function of P7. Its small size and genealogy makes it an excellent model to study RNA catalysis and evolution.

Using the putative asexual fungus Cenococcum geophilum as a model to test how species concepts influence recombination analyses using sequence data from multiple loci

Recent studies have found that three divergent lineages of the ectomycorrhizal fungus Cenococcum geophilum may co-occur within a single soil sample. To test how inference of population structure is affected by species concept, potential recombination in this putative asexual fungus was analyzed by sequencing 10 loci from 44 isolates from within one main lineage that is potentially sub-divisible into two phylogenetic species (A and B). Phylogenetic incongruence between these loci and recombination analyses using six different methods was consistent with recombination. However, most of the incongruence was caused by an apparently reciprocal recombination event between the actin locus and the other loci studied. Extreme divergence between the two types of actin loci suggests either an ancient recombination event or a more recent horizontal inheritance. We also found that random mating could not be rejected when A and B isolates were treated as members of a single species based on multilocus disequilibrium analyses, whereas random mating was rejected when all isolates were pooled. These results are significant and demonstrate that inferences of population structure can be confounded when isolates are pooled together based entirely on a morphological species concept.

Fungal-specific PCR primers developed for analysis of the ITS region of environmental DNA extracts

BACKGROUND

The Internal Transcribed Spacer (ITS) regions of fungal ribosomal DNA (rDNA) are highly variable sequences of great importance in distinguishing fungal species by PCR analysis. Previously published PCR primers available for amplifying these sequences from environmental samples provide varying degrees of success at discriminating against plant DNA while maintaining a broad range of compatibility. Typically, it has been necessary to use multiple primer sets to accommodate the range of fungi under study, potentially creating artificial distinctions for fungal sequences that amplify with more than one primer set.

RESULTS

Numerous sequences for PCR primers were tested to develop PCR assays with a wide range of fungal compatibility and high discrimination from plant DNA. A nested set of 4 primers was developed that reflected these criteria and performed well amplifying ITS regions of fungal rDNA. Primers in the 5.8S sequence were also developed that would permit separate amplifications of ITS1 and ITS2. A range of basidiomycete fruiting bodies and ascomycete cultures were analyzed with the nested set of primers and Restriction Fragment Length Polymorphism (RFLP) fingerprinting to demonstrate the specificity of the assay. Single ectomycorrhizal root tips were similarly analyzed. These primers have also been successfully applied to Quantitative PCR (QPCR), Length Heterogeneity PCR (LH-PCR) and Terminal Restriction Fragment Length Polymorphism (T-RFLP) analyses of fungi. A set of wide-range plant-specific primers were developed at positions corresponding to one pair of the fungal primers. These were used to verify that the host plant DNA was not being amplified with the fungal primers.

CONCLUSION

These plant primers have been successfully applied to PCR-RFLP analyses of forest plant tissues from above- and below-ground samples and work well at distinguishing a selection of plants to the species level. The complete set of primers was developed with an emphasis on discrimination between plant and fungal sequences and should be particularly useful for studies of fungi where samples also contain high levels of background plant DNA, such as verifying ectomycorrhizal morphotypes or characterizing phylosphere communities.

Twelve Group I introns in the same pre-rRNA transcript of the myxomycete Fuligo septica: RNA processing and evolution

The ribosomal DNA region of the myxomycete Fuligo septica was investigated and found to contain 12 group I introns (four in the small subunit and eight in the large subunit ribosomal RNAs). We have performed molecular and phylogenetic analyses to provide insight into intron structure and function, intron-host biology, and intron origin and evolution. The introns vary in size from 398 to 943 nt, all lacking detectable open reading frames. Secondary structure models revealed considerable structural diversity, but all, except one (subclass IB), represent the common group IC1 intron subclass. In vitro splicing analysis revealed that 10 of the 12 introns were able to self-splice as naked RNA, but all 12 introns were able to splice out from the precursor rRNA in vivo as evaluated by reverse transcription PCR analysis on total F. septica RNA. Furthermore, RNA processing analyses in vitro and in vivo showed that 10 of 12 introns perform hydrolytic cleavage at the 3' splice site, as well as intron circularization. Full-length intron RNA circles were detected in vivo. The order of splicing was analyzed by a reverse transcription PCR approach on cellular RNA, but no strict order of intron excision could be detected. Phylogenetic analysis indicated that most Fuligo introns were distantly related to each other and were independently gained in ribosomal DNA during evolution.

Structural features and evolutionary considerations of group IB introns in SSU rDNA of the lichen fungus Teloschistes

Different species of the lichen-forming ascomycete fungus Teloschistes were found to contain group IB introns at position S1506 in the small subunit ribosomal RNA gene. We have characterized the structural organization and phylogeny of the Teloschistes introns Tco.S1506, Tla.S1506, and Tvi.S1506. Common features to all the introns are a small size, a compact RNA structure, and an atypical catalytic ribozyme core sequence motif. Variations in intron sizes, due to sequence extensions in the P1 and P8 loop segments, were observed in different species and isolates. Phylogenetic analyses based on the ITS1-5.8S-ITS2 region as well as the introns show that the Teloschistes S1506 introns represent a distinct evolutionary isolated cluster among the nuclear group I introns. Furthermore, introns from different lineages of Teloschistes villosus appear not strictly vertically inherited probably due to horizontal transfer in one of the lineages.

Analysis of nrDNA sequences and microsatellite allele frequencies reveals a cryptic chanterelle species Cantharellus cascadensis sp. nov. from the American Pacific Northwest

In the Pacific Northwest, yellow chanterelles have long been referred to as Cantharellus cibarius, synonymous with the European yellow chanterelle. Broad scale genetic surveys of North American chanterelles with C. cibarius-like morphology have demonstrated that the nrDNA internal transcribed spacer exhibits length variability, suggesting that this common morphology masks a species complex. Recently researchers have used morphological and genetic data to identify the yellow chanterelle most frequently harvested from American Pacific Northwest forests as C. formosus, a species once thought to be rare in the region. We present three genetic data sets and one morphological data set that characterize a previously undescribed, species of yellow chanterelle from the central Cascade Mountains of Oregon. Phylogenetic analyses of the nrDNA large subunit and ITS regions show that C. cascadensis sp. nov., along with two other yellow chanterelle taxa (C. cibarius var. roseocanus and European C. cibarius), are more closely related to white chanterelles (C. subalbidus) than they are to C. formosus. Data from five microsatellite loci provide evidence that C. formosus, C. subalbidus, and C. cascadensis sp. nov. do not interbreed when they co-occur spatially and temporally in Douglas fir-western hemlock forests. This demonstrates that these three sympatric chanterelles are biological species with boundaries congruent with those delineated by nrDNA phylogenetic clades. Morphological data indicate that the colour of the pileus and shape of the stipe can be used to separate fresh collections of the two yellow species now known to co-occur in Douglas fir-western hemlock forests in Oregon.

Characterization of Pacific golden chanterelle (Cantharellus formosus) genet size using co-dominant microsatellite markers

We characterized five co-dominant microsatellite markers and used them to study Pacific golden chanterelle (Cantharellus formosus) genet size and its relation to forest age and disturbance. Fruit-bodies were mapped in and collected from nine replicate study plots in old-growth, recently thinned, and unthinned 40-60-year-old second-growth stands dominated by Douglas fir (Pseudotsuga menziesii). Information from microsatellite loci, combined with random fragment length polymorphism analysis of the nuclear DNA internal transcribed spacer indicates that putative 'C. formosus' fruit-body collections may include a cryptic chanterelle species. Small genets were characterized for both genetic types with mean maximum widths of 3.2 +/- 3.6 m for C. formosus and 1.5 +/- 1.7 m for the alternative genetic group. Variance in genet size was high and some multilocus genotypes were observed on multiple plots separated by 0.3 km or more, indicating that genets were not fully resolved by the loci described here. There was no evidence that genet size differed across the three disturbance treatments.

Detection and phylogenetic analysis of mating type genes ofOphiosphaerella korrae

Portions of the mating type genes from Ophiosphaerella korrae (J. Walker & A.M. Smith) R.A. Shoemaker (=Leptosphaeria korrae J. Walker & A.M. Smith), a pathogenic fungus of grasses, were examined by PCR (polymerase chain reaction). For nine isolates of O. korrae from North America, both mating type genes were amplified, demonstrating that both MAT idiomorphs are detectable in this homothallic ascomycete. Amplified fragments from three isolates were sequenced, and parsimony analyses of MAT1 nucleotide and protein sequences placed O. korrae in the basal position of a clade of Phaeosphaeriaceae and Pleosporaceae, whereas the MAT2 nucleotide and protein data placed O. korrae in a clade with Pleosporaceae. The internal transcribed spacer (ITS) and 18S ribosomal DNA of O. korrae were also sequenced. The 18S sequences had insufficient variability to resolve the placement of O. korrae, whereas the ITS data placed it in Phaeosphaeriaceae. A total evidence analysis of Dothideomycetes with 18S, ITS, and MAT data placed O. korrae alongside Phaeosphaeria species, with moderate bootstrap support. However, the Kishino–Hasegawa test did not demonstrate this topology to be significantly different from one where O. korrae was placed with Pleosporales. Although O. korrae does not belong in Leptosphaeria based on ITS data, MAT data do not strongly support its placement in Phaeosphaeriaceae.Key words: ascomycetes, mating type genes, ribosomal genes, taxonomy.

Nucleotide polymorphisms in three genes support host and geographic speciation in tree pathogens belonging toGremmeniellaspp

We detected nucleotide polymorphisms within the genus Gremmeniella in DNA sequences of β-tubulin, glyceraldehyde phosphate dehydrogenase, and mitochondrial small subunit rRNA (mtSSU rRNA) genes. A group-I intron was present in strains originating from fir (Abies spp.) in the mtSSU rRNA locus. This intron in the mtSSU rRNA locus of strains isolated from Abies sachalinensis (Fridr. Schmidt) M.T. Mast in Asia was also found in strains isolated from Abies balsamea (L.) Mill. in North America. Phylogenetic analyses yielded trees that grouped strains by host of origin with strong branch support. Asian strains of Gremmeniella abietina (Lagerberg) Morelet var. abietina isolated from fir (A. sachalinensis) were more closely related to G. abietina var. balsamea from North America, which is found on spruce (Picea spp.) and balsam fir, and European and North American races of G. abietina var. abietina from pines (Pinus spp.) were distantly related. Likewise, North American isolates of Gremmeniella laricina (Ettinger) O. Petrini, L.E. Petrini, G. Laflamme, & G.B. Ouellette, a pathogen of larch, was more closely related to G. laricina from Europe than to G. abietina var. abietina from North America. These data suggest that host specialization might have been the leading evolutionary force shaping Gremmeniella spp., with geographic separation acting as a secondary factor.Key words: Gremmeniella, geographic separation, host specialization, mitochondrial rRNA, nuclear genes.

A diverse population of introns in the nuclear ribosomal genes of ericoid mycorrhizal fungi includes elements with sequence similarity to endonuclease-coding genes

Ericoid mycorrhizal fungi form symbioses with the roots of members of the Ericales. Although only two genera have been identified in culture, the taxonomic diversity of ericoid symbionts is certainly wider. Genetic variation among 40 ericoid fungal isolates was investigated in this study. PCR amplification of the nuclear small-subunit ribosomal DNA (SSU rDNA) and of the internal transcribed spacer (ITS), followed by sequencing, led to the discovery of DNA insertions of various sizes in the SSU rDNA of most isolates. They reached sizes of almost 1,800 bp and occurred in up to five different insertion sites. Their positions and sizes were generally correlated with morphological and ITS-RFLP grouping of the isolates, although some insertions were found to be optional among isolates of the same species, and insertions were not always present in all SSU rDNA repeats within an isolate. Most insertions were identified as typical group I introns, possessing the conserved motifs characteristic of this group. However, other insertions lack these motifs and form a distinct group that includes other fungal ribosomal introns. Alignments with almost 70 additional sequences from fungal nuclear SSU rDNA introns indicate that introns inserted at the same site along the rDNA gene are generally homologous, but they also suggest the possibility of some horizontal transfers. Two of the ericoid fungal introns showed strong homology with a conserved motif found in endonuclease genes from nuclear rDNA introns.

Correlation of Haplotypes of a Fungal Plant Pathogen with their Respective Host Species of Origin

The anther smut fungus, Microbotryum violaceum, infects over 200 species of Caryophyllaceae (Pinks). However, limited published studies, as well as anecdotal evidence, suggest that each isolate of the fungus is restricted to one or a few species that it can productively infect. In the absence of physical differences, it would be useful to have molecular markers to identify individuals with specific host ranges prior to genetic analyses of host preference. With this purpose in mind, 17 isolates from eight different host species were characterized for differences in their respective γ-tubulin genes. The region of the gene including the sixth and seventh introns and some surrounding coding regions was amplified and sequenced and the results were analyzed phylogenetically. Despite the small sample size and the geographical distribution of their respective host plants, isolates from the same host species showed no differences in the DNA regions examined; isolates of closely related pathovars also grouped together. In contrast, relative to the corresponding regions from other pathovars, isolates from host species that were genetically or taxonomically more distant showed a marked number of differences in both introns and in the third (wobble) position of codons in the seventh exon. Thus, DNA sequence differences in this highly conserved gene may be used to distinguish isolates from different host species. Such information may prove useful as markers for the different formae speciales in future analyses of host preference.