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sequences: evidence for cryptic diversification and links to Cordyceps teleomorphs
Insect Biocontrol Laboratory, USDA-ARS, Beltsville, Maryland 20705
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Beauveria is a globally distributed genus of soil-borne entomopathogenic hyphomycetes of interest as a model system for the study of entomo-pathogenesis and the biological control of pest insects. Species recognition in Beauveria is difficult due to a lack of taxonomically informative morphology. This has impeded assessment of species diversity in this genus and investigation of their natural history. A gene-genealogical approach was used to investigate molecular phylogenetic diversity of Beauveria and several presumptively related Cordyceps species. Analyses were based on nuclear ribosomal internal transcribed spacer (ITS) and elongation factor 1-alpha (EF1-
) sequences for 86 exemplar isolates from diverse geographic origins, habitats and insect hosts. Phylogenetic trees were inferred using maximum parsimony and Bayesian likelihood methods. Six well supported clades within Beauveria, provisionally designated A–F, were resolved in the EF1- and combined gene phylogenies. Beauveria bassiana, a ubiquitous species that is characterized morphologically by globose to subglobose conidia, was determined to be non-monophyletic and consists of two unrelated lineages, clades A and C. Clade A is globally distributed and includes the Asian teleomorph Cordyceps staphylinidaecola and its probable synonym C. bassiana. All isolates contained in Clade C are anamorphic and originate from Europe and North America. Clade B includes isolates of B. brongniartii, a Eurasian species complex characterized by ellipsoidal conidia. Clade D includes B. caledonica and B. vermiconia, which produce cylindrical and comma-shaped conidia, respectively. Clade E, from Asia, includes Beauveria anamorphs and a Cordyceps teleomorph that both produce ellipsoidal conidia. Clade F, the basal branch in the Beauveria phylogeny includes the South American species B. amorpha, which produces cylindrical conidia. Lineage diversity detected within clades A, B and C suggests that prevailing morphological species concepts underestimate species diversity within these groups. Continental endemism of lineages in B. bassiana s.l. (clades A and C) indicates that isolation by distance has been an important factor in the evolutionary diversification of these clades. Permutation tests indicate that host association is essentially random in both B. bassiana s.l. clades A and C, supporting past assumptions that this species is not host specific. In contrast, isolates in clades B and D occurred primarily on coleopteran hosts, although sampling in these clades was insufficient to assess host affliation at lower taxonomic ranks. The phylogenetic placement of Cordyceps staphylinidaecola/bassiana, and C. scarabaeicola within Beauveria corroborates prior reports of these anamorph-teleomorph connections. These results establish a phylogenetic framework for further taxonomic, phylogenetic and comparative biological investigations of Beauveria and their corresponding Cordyceps teleomorphs.
Key words: Ascomycetes, Beauveria, Clavicipitaceae, Cordyceps, cryptic species, systematics
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, Dunn and Mechalas 1963, Ferron 1978, McCoy 1990, Feng et al 1994, Gillespie and Moorehouse 1989, Ferron et al 1991). Despite long term interest in developing Beauveria as a biological alternative to chemically based insecticides, progress toward this goal has been hindered in part by difficulties in recognizing and identifying species in this genus. As a result, little is known about the genetic bases and pattern(s) of variation in the determinants of host range, mode of pathogenesis, virulence and the role of toxic metabolites in entomopathogenesis by individual species of Beauveria.
Agostino Bassi (1835) first described Beauveria as the causal agent of mal del segno or the mark disease, also known as calcinaccio or cannellino in Italy and white muscardino in France, which caused economically devastating epizootics of domestic larval silkworms in southern Europe during the 18th and 19th centuries. In his studies with Beauveria, Bassi was the first to demonstrate that microbes can act as contagious pathogens of animals, providing an important antecedent to the germ theory of disease (Ainsworth 1973). The first taxonomic recognition of the muscardino fungus was proposed by Balsamo-Crivelli (1835a, b) who acknowledged Bassi’s discoveries by naming this pathogen Botrytis bassiana. The genus Beauveria, however, was not formally described until the early 20th century by Vuillemin (1912), who designated Botrytis bassiana Bals.-Criv. as the type species.
Beauveria is characterized morphologically by its sympodial to whorled clusters of short-globose to flask-shaped conidiogenous cells, which give rise to a succession of one-celled, hyaline, holoblastic conidia that are borne on a progressively elongating sympodial rachis. Although morphologically distinctive as a genus, species identification in Beauveria is difficult because of its structural simplicity and the lack of distinctive phenotypic variation. Conidia are the principal morphological feature used for species identification in Beauveria. In shape conidia may be globose, ellipsoidal, reniform to cylindrical, or comma-shaped, and range in size from 1.7 to 5.5 µm. Species identification in Beauveria has been complicated by the proliferation of new species described between the late 19th to mid-20th centuries, few of which are morphologically distinct from previously described species.
Several revisionary studies of Beauveria have been conducted to evaluate morphological species concepts. Petch (1926) recognized two species, B. bassiana and B. densa (Link) F. Picard and concluded that cultural data were uninformative for delimiting species. MacLeod (1954) monographed Beauveria and, like Petch, recognized only two species, which he classified in B. bassiana and B. brongniartii (Sacc.) Petch (= B. densa). Hoog (1972) concurred with MacLeod but recognized an additional species, B. alba (Limber) Saccas, which was later transferred to Engyodontium (Limber) Hoog (Hoog 1978). More recently, Hoog and Rao (1975) and Samson and Evans (1982) described several new species. In all, forty-nine species have been placed in Beauveria and 22 epithets are currently valid. Today, researchers generally follow Macleod (1954) and Hoog (1972) and classify most environmental isolates of Beauveria in either B. bassiana or B. brongniartii, a practice reflected in contemporary texts and keys to species identification (Humber 1997, Tanada and Kaya 1993).
Ongoing difficulties in applying morphological approaches to species recognition in Beauveria have spurred the search for additional sources of taxonomic characters. Alternative character systems that have been investigated include isozymes (St. Leger et al 1992, Poprawski et al 1988), chemotaxonomic characters (Mugnai et al 1989), mitochondrial RFLP (Hegedus et al 1993), immunological approaches (Shimizu and Aizawa 1988; Tan and Ekramoddoullah 1991), rRNA sequencing (Rakotonirainy et al 1991), RFLP (Kosir et al 1991, Maurer et al 1997), introns in the large subunit rDNA (Neuveglise and Brygoo 1994, Neuveglise et al 1996), RFLP and nucleotide sequences of ITS (Neuveglise et al 1994, Coates et al 2002), SSCP analysis of taxon specific markers (Hegedus and Khachatourians 1993, 1996), RAPD markers (Bidochka et al 1994, Cravanzola et al 1997, Maurer et al 1997), and the combined use of morphology and RAPD markers (Glare and Inwood 1998). Although all character systems investigated in these studies were effective in detecting genetic variation within Beauveria, none have been applied directly to taxonomic investigations in this genus.
Although biologically relevant species concepts and explicit species recognition criteria have yet to be defined for Beauveria, recent molecular and cultural studies have provided insight regarding the phylogenetic position and reproductive biology of several species. An rDNA phylogeny by Sung et al (2001) supports a single evolutionary origin of Beauveria within the subfamily Cordycipitoideae of the Clavicipitaceae, and that the teleomorph C. scarabaeicola is nested within Beauveria and is the sister to B. caledonica Bissett & Widden. Second, strains isolated from stromata of several Cordyceps species produce Beauveria anamorphs, clearly demonstrating that some Beauveria species are sexual. These Cordyceps species include C. bassiana Li, Li, Huang & Fan (Li et al 2001), C. brongniartii Shimazu (Shimazu et al 1988), C. staphylinidaecola Kobayasi & Shimazu (1982), and C. sobolifera Berk. (Liu et al 2001). Together the molecular phylogenetic and cultural data support a Cordyceps origin to the Beauveria lineage.
Here we present a molecular phylogenetic analysis based on 75 exemplar isolates of Beauveria representative of its known taxonomic diversity, geographic distributions and insect host ranges, plus eleven presumptively related Cordyceps teleomorph accessions. We compared and combined reconstructed phylogenies of two nuclear loci, the ribosomal internal transcribed spacer (ITS) and elongation factor 1-alpha (EF1-), to infer an organismal phylogeny. We then used the combined gene phylogeny to address the following questions: 1) What is the pattern of morphological variation with respect to this phylogeny and how do phylogenetic groupings correspond to prevailing morphological species concepts in Beauveria? 2) Are morphological species in Beauveria cryptically diverse? 3) What are the geographic distributions of species lineages within this genus? 4) What is the coevolutionary pattern of association between Beauveria and its insect hosts? 5) What are the phylogenetic affinities of C. bassiana, C. scarabaeicola, C. sobolifera and C. staphylinidaecola, each of which has been directly linked to Beauveria?
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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), and are listed in TABLE I. Beauveria species sampled in this study include Beauveria amorpha (Höhn.) Samson & H. C. Evans, B. bassiana, B. brongniartii (Sacc.) Petch, B. caledonica Bissett & Widden, B. vermiconia de Hoog & V. Rao, and multiple unidentified Beauveria accessions. Isolates were selected to represent diverse agricultural and non-agricultural habitats and different geographic regions including North, South and Central America, Europe, North Africa, Asia and Australia, and from different insect orders, including Coleoptera, Dermaptera, Hemiptera, Homoptera, Hymenoptera, Lepidoptera, Orthoptera, and Thysanoptera (TABLE I). Nine living isolates of Cordyceps, accessioned as C. bassiana, C. staphylinidaecola, and C. scarabaeicola, were also included. Additionally, portions of stromata from two dried specimens of C, scarabaeicola, originating from the Entomopathogenic Fungal Culture Collection (EFCC, Korea), were provided by J. Spatafora and G.-H. Sung (TABLE I). Cultures of the outgroup taxa, Cordyceps militaris (L.) Link ( JWS 00-293) and Paecilomyces farinosus (Holmsk.) A.H.S. Br. & G. Sm. ( JWS 00-224) were provided by J. Spatafora. Isolates were stored in 10% glycerol at –70 C. Isolates were grown on quarter strength SDY medium (Goettel and Inglis 1997). Mycelium for DNA extraction was produced by culturing in quarter strength SDY broth at 100 rpm on a rotary shaker for 2–3 days at 25 C. Mycelium was harvested from broth cultures by centrifugation, washed twice with sterile distilled water, then lyophilized and stored at –20 C.
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Two nuclear gene regions, ITS and EF1-, were sequenced and analyzed. The ITS was amplified and sequenced with primers ITS5 (5'-GGAAGTAAAAGTCGTAA-CAAGG) and ITS4 (5'-TCCTCCGCTTATTGATATGC) (White et al 1990). The nearly complete coding region of EF1- was amplified and sequenced using a combination of primers designed in our laboratory using the computer program Oligo 6 (MBI, Cascade, Colorado). An ~1200 bp segment spanning the 5' 
of EF1- was amplified with primers EF1T (5'-ATGGGTAAGGARGACAAGAC) and 1567R (5'-ACHGTRCCRATACCACCSATCTT). The EF1T x 1567R fragments were sequenced with the amplification primers and two internal primers, EFjR (5'-TGYTCNCGRG-TYTGNCCRTCYTT) and 983F (5'-GCYCCYGGHCAYCGT-GAYTTYAT). An overlapping fragment of approximately 1000 bp that extends nearly to the 3' end of EF1- was amplified with primers 983F and 2218R (5'-ATGACACCRA-CRGCRACRGTYTG). The amplification primers and three additional internal primers, 1577F (5'- CARGAYGTBTA-CAAGATYGGTGG), 1567RintB (5'-ACHGTRCCRATAC-CACCRAT) and 2212R (5'-CCRAACRGCRACRGTYYGTCT-CAT) were used for sequencing the 983F x 228R amplicon.
PCR amplifications were performed in a total volume of 50 µL, which included 5µL of 10x PCR buffer (10 mM Tris/HCl pH 8.0, 50 mM KCl, 1.5–2.0 mM MgCl2), 4 µL of dNTP mix (1.25 mM each dATP, dCTP, dGTP, and dTTP), 10 pmol each of the opposing amplification primers, 0.5 ul Taq polymerase (Promega, Madison WI), and 5–20 ng genomic DNA. PCR for both loci was performed using a touchdown PCR procedure (Don et al 1991). Touchdown PCR amplifications were initiated with a 2 min denaturation at 94 C. The annealing temperature in the first amplification cycle was 66 C, which was subsequently incrementally reduced by 1 C per cycle over the next 9 cycles. An additional 36 amplification cycles were then performed, each consisting of 30 s denaturation at 94 C, a 30 s annealing step at 56 C, and a 1 min extension at 72 C, concluding with a 10 min incubation at 72 C. PCR reaction volumes were reduced to approximately 10 µL by lyophilization, then separated on a 1.5% NuSieve agarose gel (Bio-Whittaker, Rockland, Maine) in a low EDTA Tris-acetate buffer (40 mM Tris-acetate, 0.1 mM EDTA). PCR products were cut from the gel, frozen and thawed and the DNA extruded from the gel slice by centrifugation for 10 min at 20 000 x g.
Miniaturized sequencing reactions were performed with ABI BigDye 2.0 (Applied Biosystems, Foster City, CA) using 0.5 µL BigDye diluted in 1.5 µL dilution buffer (400 mM Tris/HCl pH 9.0, 10 mM MgCl2), 3 pmol primer, 75–100 ng gel-purified PCR template in a total volume of 5 µL. Cycle sequencing was performed in 96-well microtiter plates according to the manufacturer’s instructions except that the total number of cycles was increased to 35. Cycle sequencing products were separated from residual reaction components by ethanol precipitation. The sequencing reactions were suspended in deionized formamide, heat denatured, and run on an ABI 3100 Genetic Analyzer (Applied Biosystems, Foster City, CA).
DNA sequences were assembled and edited using Sequencher 4.1 (Gene Codes Corp., Ann Arbor, Michigan) and multiple sequence alignments were constructed with the MegAlign module of DNASTAR 5 (LaserGene, Madison, Wisconsin) and output in Nexus format for phylogenetic analysis. Multiple sequence alignments for ITS and EF1- were concatenated into a single file using MacClade 4.0 (Maddison and Maddison 2000).
Phylogenetic analysis.— –
ITS and EF1- data sets were analyzed separately and in combination under maximum parsimony (MP) and Bayesian-likelihood criteria. Bayesian inference was used because it enables relatively rapid analysis (as compared to maximum likelihood) of large data sets under complex evolutionary models of nucleotide substitution (Larget and Simon 1999) and yields posterior probabilities supporting phylogenetic hypotheses (Lewis 2001).
Parsimony analyses were implemented in PAUP 4.0b10 (Swofford 2001) using the heuristic search option with TBR branch swapping under equal character weighting, excluding both gapped and uninformative characters. To increase the probability that all islands of most-parsimonious trees were identified (Maddison 1991, Stewart 1993, Swofford et al 1996), 500 random-addition replicate analyses were executed. A heuristic MP bootstrap analysis (Felsenstein 1985) consisted of 1000 pseudoreplicates (TBR branch swapping), with 10 random-addition replicates per pseudoreplicate, and with gapped and parsimony-uninformative characters excluded. Clades with bootstrap values 
70% were considered strongly supported by the data.
Bayesian analyses, started from a random tree using the program’s default values for the prior probabilities, consisted of four simultaneous Markov chains, three heated and one cold, which were run for 106 generations. The Bayesian analyses were repeated four times and a single tree was sampled randomly every 100th generation. The log-likelihood scores for all generations were examined to identify the burn-in phase, or those initial generations in which likelihood scores progressively improve until they fluctuate narrowly around a stable value. In all Bayesian analyses, the latter 5000 sampled trees from each analysis were pooled (after confirming they had converged on similar log-likelihood values) and imported into PAUP 4.0b and a 50% consensus tree computed, with the support values for each branch constituting their posterior probability. Clades with posterior probabilities 95% were considered as significantly supported by the data (Huelsenbeck et al 2002).
Host association.— –
We performed a PTP test (Archie 1989, Faith and Cranston 1991) in PAUP 4.0b10 as described by Kelley and Farrell (1998) and Farrell et al (2001) to determine whether the pattern of fungus-insect associations differed significantly from the expectations of a randomly distributed character. Insect host order was coded as an un-weighted character and randomly permuted 1000 times while maintaining the original character frequencies. The observed number of changes for the original data was compared to the distribution of reconstructed changes determined for the permuted data mapped on a single representative most parsimonious tree (MPT) and the P value of the original data computed. A score of P < 0.05 would indicate the actual data lie outside the test distribution, leading to rejection of the null hypothesis that host association is random.
Conidial measurements.— –
Between twenty to thirty conidia from 10–20 day old cultures were suspended in 0.01% Tween 40 and mixed with an equal volume of molten (70 C) 0.1% Nusieve GTG agarose (BioWhittaker, Rockland, ME). Conidia were observed using a Nikon E600 microscope equipped with Nikon DXM 1200 digital camera and Nikon ACT-1 image capture software. Conidia size measurements are given in TABLE I
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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sequence data sets were constructed for 75 isolates of Beauveria spp. and eleven Cordyceps accessions identified as C. bassiana, C. scarabaeicola and C. staphylinidaecola (TABLE I). The ITS and EF1- sequence data sets consisted of 605 and 1729 aligned positions, respectively. Gapped and uninformative positions were excluded from all parsimony analyses and the final data set contained 198 parsimony-informative characters, with ITS and EF1- contributing 35 and 163 informative sites, respectively. EF1- contained three closely spaced introns at the 5' end of the gene whose combined length was approximately 400 bp. The EF1- intron regions yielded nearly 66% of the informative sites obtained from this locus, with the remaining variable and informative sites occurring at 3rd codon positions in the exons. ITS and EF1- sequences were also determined for Cordyceps militaris ( JS 00-224) and Paecilomyces farinosus ( JS 00-293). GenBank accession numbers for all sequence data generated in this study are listed in TABLE I.
Rooting the Beauveria phylogeny.— –
The root of the Beauveria phylogeny was inferred from an initial parsimony analysis that included all Beauveria isolates plus C. scarabaeicola accessions EFCC 2533 and EFCC 252, and single exemplar isolates of C. militaris ( JS 00-224) and Paecilomyces farinosus ( JS 00-293). These latter two species were previously determined to be closely related to but distinct from Beauveria in an 18S SSU rDNA phylogeny (Sung et al 2001). Nucleotide data for this analysis were obtained from the PCR fragment 983F x 2218R of EF1-, which spans the latter of the gene. This region of EF1- lacks introns in the Hypocreales and, except for a unique fifteen base pair insertion (5 codons) in the out-group P. farinosus, is colinear in Beauveria, thus facilitating sequence alignment and bolstering confidence in underlying assumptions of positional homology. In contrast, both the 5' portion of EF1-, which consists primarily of intron sequences, and the ITS spacers were unsuitable for this particular analysis because of extensive alignment ambiguities between C. militaris, P. farinosus and the Beauveria in-group.
The EF1- exon data set used to infer the root for Beauveria consisted of 87 3rd position parsimony-informative characters. Both C. militaris and P. farinosus were nearly equally divergent from Beauveria and from each other (data not shown). Regardless of which taxon was used as outgroup, the resulting phylogenetic analyses yielded topologically and statistically comparable sets of trees (data not shown). The parsimony analysis using C. militaris as outgroup yielded 120 equally parsimonious trees (MPT) of 191 steps with a rescaled consistency index (RC) of 0.4558 and 94% bootstrap support for Beauveria monophyly. In both analyses, C. scarabaeicola EFCC 2533 was basal to a monophyletic Beauveria (data not shown). Based on this result, the ITS and EF1- sequences from C. scarabaeicola EFCC 2533 were used to root all subsequent analyses.
EF1- phylogeny.— –
Parsimony analysis of EF1- was terminated after 24 hours of CPU time. This analysis had progressed only to the 107th replicate and yielded 17 975 MPT of 353 steps with a character rescaled consistency index (RC) = 0.5751. The analysis was repeated, restricting the search to the first 100 trees encountered per replicate, for a total of 2000 replicates. The resulting trees had the same statistics as discovered in the original heuristic search. For the Bayesian analysis, four independent Markov-Chain Monte Carlo (MCMC) generations were run, each with a burn-in of 500K generations. All runs converged on approximately the same likelihood score and 5000 post burn-in trees from all four analyses were pooled. Trees inferred in both analyses were topologically compatible and a 50% consensus from the Bayesian analysis is shown in FIG. 1. In all, 30 internal branches in the parsimony analysis received bootstrap support greater than 70% and 38 branches in the Bayesian analysis had posterior probabilities equal to or greater than 95% (FIG. 1).
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. Branch resolution and support in the ITS phylogeny was low, and only five branches were supported by both the bootstrap and Bayesian analyses (FIG. 1).
Combined ITS and EF1- phylogeny.— –
No significant topological conflicts were noted between the EF1- and ITS tree topologies and the data sets were combined and analyzed together. To reduce search time, only the first 100 trees encountered in 2000 replicate searches were swapped to completion. All trees from this search had a length of 429 steps and RC = 0.5595. A single exemplar tree from the parsimony analysis, which was also present among the trees from the Bayesian analysis, is given in FIG. 2.
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. Insect-fungal associations were particularly heterogeneous in clades A and C (i.e. B. bassiana s.l.), which were obtained from insect species classified in seven and six classes, respectively. A permutation test of host affiliation performed separately for both clades A and C yielded the test statistics P = 0.40 and P = 0.46, respectively, indicating that the observed pattern of host association was indistinguishable from a random distribution. Isolates in clades B, D, E and F were isolated primarily from coleopteran hosts or from soil. However, the taxon sampling in these clades was too limited to perform meaningful statistical tests of host associations.
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). In general, isolates within each major clade (FIG. 2) had a similar conidial morphology. In clade A, the majority of isolates had globose to subglobose conidia 2.3–3.2 um in diameter. One notable exception within clade A was a monophyletic pair of Chinese isolates accessioned under B. amorpha and B. brongniartii (ARSEF 656 and 678, respectively), which had ellipsoidal conidia, 2.9–4.2 x 1.8–2.5 µm. Isolates in Clade C also produced globose to subglobose conidia similar to those produced by isolates in clade A, except they were slightly smaller and measured 2.1–2.9 µm. Clade B included isolates with ellipsoidal to subcylindrical conidia that ranged from 3.3–4.8 x 2.1–2.5 µm. Clade D isolates had either cylindrical conidia (ARSEF 1567, 2251, 2567), 3.8–5.2 x 1.9–2.3 µm, or comma-shaped conidia (ARSEF 2922), measuring 1.9–2.5 µm at their largest dimension. Isolates in clade E produced ellipsoidal conidia 3.0–4.4 x 2.5–3.2 µm. In clade F, only isolate ARSEF 2641 produced conidia and these were cylindrical in shape and 4.2–5.2 x 1.7–2.1 µm. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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and ITS. EF1- was much more informative for inference of relationships in Beauveria than ITS. The obtained phylogeny supports the monophyly of Beauveria and six principal clades within the genus, several of which encompass additional lineage diversity. We used this phylogeny as a basis to consider the taxonomy of Beauveria and to discuss patterns of variation in morphology, geographic distribution and host range in this genus. Beauveria phylogeny.— – The clades resolved within Beauveria, with one exception, correspond closely to species previously defined on the basis of conidial morphology and are provisionally referred to here as clades A–F.
Clade A constitutes a globally distributed set of isolates that were accessioned primarily as B. bassiana. The majority of isolates in this clade produce globose to subglobose conidia 2.3–3.2 µm in diameter, which is consistent with the traditional morphological diagnosis of B. bassiana. However, the convergent morphology of conidia produced by members of clade C, which is phylogenetically distinct from clade A, exposes a previously unrecognized complication in the taxonomic circumscription of this widespread and important morphological species complex. The taxonomic recognition of each of these two clades is thus minimally required to formalize their distinct status. However, a type specimen for B. bassiana is not known to exist. Thus, a neotype for B. bassiana needs to be selected from either clade A or C and a second species described to accomodate the alternate clade. Currently, an isolate from clade A is being designated the isoneotype of B. bassiana (Humber pers comm). The designation of clade A as the source of a type for B. bassiana appears to be the better of the two available options because, as a species that is recognized throughout the world, only clade A has been shown to have a global distribution. Moreover, several commercially registered biocontrol strains and numerous research strains received from laboratories throughout the world have been placed phylogenetically in clade A (Rehner and Buckley unpubl), indicating that clade A most probably embodies what most researchers consider to be B. bassiana.
The monophyly of Clade A was strongly supported in the single and combined gene phylogenies (FIGS. 1–3), however, phylogenetic inference within this clade was determined almost entirely from nucleotide variation at EF1-. Several deep lineages were resolved, each of which included isolates from different continents (FIGS. 1–3). We believe that this intricate phylogeographic partitioning of clade A reflects past intercontinental dispersal events and that allopatric divergence has played a significant role in its phylogenetic diversification. The complex phylogenetic structure further suggests that clade A may contain multiple phylogenetic species, although this hypothesis requires further investigation. The co-occurrence of different terminal lineages, which was observed in all of the geographic regions sampled (FIG. 3), raises the interesting question of how these closely related species co-exist in sympatry. The development of methods to differentiate among cryptic sympatric lineages will be an essential first step toward elucidating the community structure of B. bassiana and the population biology of individual species.
Isolates within clade A were cultured from a wide range of insect species classified in seven insect classes (TABLE I). A permutation test of host affiliation yielded the result that the observed pattern of host associations was indistinguishable from a random distribution. This result support the longstanding view that B. bassiana s.l. is not host specific but an opportunistic entomopathogen capable of attacking a wide range of insect taxa. This conclusion is reinforced by evidence that closely related isolates originating from the same geographic region were isolated from taxonomically distant insect hosts (FIG. 3). We find no evidence to support the view that lineage diversification in clade A is due to phylogenetic tracking or host jumping, which both require a prior history of host specialization.
Clade B, the sister to clade A, includes isolates of B. brongniartii (Sacc.), a species characterized by ellipsoidal to sub-cylindrical conidia, 3.3–4.8 x 2.1–2.5 µm in size. Originally described from Europe (Petch 1926), B. brongniartii is commonly associated with Coleoptera and is used as a biocontrol agent against the European cockchafer, Melolantha melolantha (Keller et al 1989). In this study, only Asian material was available for analysis. A Japanese B. brongniartii isolate (ND1), formulated by the Nitto Denko Corporation as the mycoinsecticide BiolisaTM for control of beetles, was identical in sequence to two Asian isolates ARSEF 1678 and 4850. Together these isolates form a monophyletic, and possibly conspecific, group. Isolates of European B. brongniartii received from J. Enkerli (Enkerli et al 2001) after the present analyses were conducted, grouped in clade B but are phylogenetically distinct from the Asian isolates (Rehner and Buckley data not shown). This suggests that B. brongniartii constitutes a complex of several or more cryptic species, which available evidence suggests is distributed across Eurasia. Where host information was available, all clade B isolates were isolated from Coleoptera.
Clade C isolates examined in this study originate from North America and Europe and were identified by their collectors primarily as B. bassiana (Humber 2001). As discussed previously, apart from having slightly smaller conidia, clade C is morphologically indistinguishable from clade A. A second point of similarity between these two clades is that clade C has a wide host range, which suggests that it too is a generalist entomopathogen. Although no conspicuous morphological or cultural characteristics have been identified that consistently differentiates clades A and C, fixed nucleotide differences at both ITS and EF1-, and other genes, may provide the most direct means of differentiating isolates from these two clades (Rehner and Buckley unpubl).
Clade D includes isolates that have either cylindrical or comma-shaped conidia. A Brazilian isolate, AR-SEF 2251, accessioned under the name B. amorpha, produces cylindrical conidia 3.5–4.2 x 2.1–2.5 µm. These conidial dimensions are shorter and broader than those described for B. amorpha by Samson and Evans (1982), which they reported as 3.5–5.0 x 1.5–2.0 µm. For this reason we suspect that ARSEF 2251 is misidentified and may represent an undescribed species. B. caledonica, represented here by European isolates ARSEF 2567 and 1567 produce larger cylindrical conidia, 3.7–5.2 x 1.9–2.3 µm, consistent with the conidia described for this species (Bissett and Widden 1986). By contrast, B. vermiconia produced distinctive comma-shaped conidia 2.1–2.9 x 2.3–2.9 µm (Hoog and Rao 1975).
Clade E contains isolates from northeast Asia and includes one unidentified Beauveria isolate (ARSEF 1685) and four Cordyceps individuals, which were identified as either C. scarabaeicola (ARSEF 5689) or C. staphylinidaecola (ARSEF 7043, 7044; EFCC 252). All live isolates from this clade produced broadly ellipsoidal conidia in culture, measuring 3.1–4.4 x 2.5–3.1 µm. The close similarities in DNA sequence, conidial morphology and geographic origin suggest that these isolates may be conspecific.
Clade F includes two South American isolates, AR-SEF 2641 and 1969. ARSEF 2641 was accessioned as B. amorpha. The conidia produced by this isolate were 4.5–5.2 x 1.7–2.1 µm, which closely match the narrowly cylindrical conidia described in the amended description of B. amorpha by Samson and Evans (1982). Clade F is presently the basal-most lineage in the Beauveria phylogeny in which the sympodial conidiogenous cells that characterize Beauveria have been documented. The second isolate in clade F, AR-SEF 1969, was sterile in culture.
Teleomorph connections to Beauveria.— –
Four Asian Cordyceps teleomorphs identified as C. basssiana, C. scarabaeicola, and C. staphylinidaecola were linked phylogenetically to Beauveria at three discrete points in the present analysis: as the sister to Beauveria s.l., and within clade E and within clade A (FIGS. 1–3). Thus, sexual reproduction is confirmed to occur in both basal and derived lineages of Beauveria. From these results we believe it to be likely that most if not all lineages in Beauveria maintain the potential for sexual reproduction.
EFCC 2533, a fruiting body identified as C. scarabaeicola, was placed as the basal branch in the Beauveria s.l. clade. Cultures for this collection were not available, thus its mode of conidiogenesis could not be determined. Additional data are needed to determine whether EFCC 2533 is a divergent basal lineage within Beauveria or a related genus.
Clade E included four teleomorphs (ARSEF 5689, 7043, 7044; EFCC 252) and one anamorph (ARSEF 1685) isolates. Due to the close genetic relationship of these isolates and their similar anamorphs, it is probably that these isolates are conspecific.
Five teleomorph collections grouped phylogenetically in clade A and are considered to represent the sexual stage of B. bassiana (FIGS. 1–3). These collections include the ex-type culture of C. bassiana from China, ARSEF 7047, and four Korean isolates identified as C. staphylinidaecola, ARSEF 5718, 6721, 6722, and 6723. This finding corroborates the Beauveria bassiana-Cordyceps anamorph-teleomorph link proposed by Li et al (2001). The close genetic relationship of these five individuals suggests that they are conspecific, with C. staphylinidaecola possibly an older synonym of C. bassiana (R. Humber pers comm). Interestingly, these teleomorphs were isolated from three insect classes (Coleoptera, Homoptera and Hemiptera; TABLE I), a pattern that matches the diverse host associations observed among the anamorph isolates that constitute the bulk of clade A. It appears then that neither sexual nor asexual reproduction within clade A requires infection of a specific host.
Teleomorph connections have also been proposed to two additional species of Beauveria including B. brongniartii (Shimazu 1988) and C. sobolifera (Liu et al 2001). However, cultures and specimens were not available for these species at the time of this study. ITS sequences from C. sobolifera (GenBank AJ309325
[GenBank]
and AJ309326
[GenBank]
) were reported by Liu et al (2001), which enabled comparison to sequence data determined here. These C. sobolifera ITS sequences aligned poorly with data from our Beauveria isolates nor did they nest within Beauveria when analyzed phylogenetically (Rehner and Buckley unpubl). Further examination of the form and development of conidiogenesis and a more detailed phylogenetic placement of C. sobolifera is needed to determine the status of the C. sobolifera anamorph.
Despite the long-held view that Beauveria is strictly mitosporic and presumably clonal (but see Paccola-Meirelles and Azevedo 1991, Bello and Paccola-Meirelles 1998, for discussion of parasexuality in B. bassiana), the phylogenetic connection between Cordyceps and Beauveria demonstrated here suggests that many, if not all, species of Beauveria are sexual. Why sexual reproduction by Beauveria has not previously been observed is curious in view of extensive history of research for both Beauveria and Cordyceps. Nonetheless, the present finding should stimulate efforts to integrate the collection, culturing and phylogenetic analysis of Cordyceps teleomorphs and Beauveria anamorphs wherever possible. The accumulating evidence that Beauveria is sexual suggests the potential for developing conventional approaches to genetic analysis and genetic improvement of this important genus of entomopathogens.
Conclusions.— – The molecular phylogeny inferred for Beauveria provides a perspective on the current taxonomic understanding of this genus and a foundation for future revisionary systematic studies. With only one exception (i.e. clades A and C), phylogenetic terminals resolved in the present analysis correspond to species previously described on the basis of morphology. Thus, the broad patterns of diversity in Beauveria have been accurately predicted by prior morphological studies. However, for groups scrutinized in some detail, e.g. B. bassiana s.l. (clades A and C), the inferred patterns of underlying phylogenetic diversity indicate a history of cryptic diversification, possibly signifying these lineages consist of multiple cryptic species. These results demonstrate that deep sampling of globally distributed species complexes, coupled with molecular phylogenetic analyses, is an expedient strategy for assessing species diversity, and a necessary first step to detailing their evolutionary history and historical ecology. The discover y of Cordyceps teleomorphs associated with Beauveria contradicts earlier assumptions that Beauveria is strictly asexual. Appreciation of this reproductive option will expand the scope and character of future investigations of this widespread and important group of entomopathogenic fungi.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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1 Corresponding author. E-mail: rehners{at}ba.ars.usda.gov
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LITERATURE CITED |
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