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Pythium recalcitrans sp. nov. revealed by multigene phylogenetic analysis

     Instituto Mediterráneo de Estudios Avanzados, IMEDEA (CSIC-UIB), Miquel Marquès 21, 07190, Esporles, Balearic Islands, Spain

Lassaad Belbahri
Gautier Calmin
François Lefort

     Laboratory of Applied Genetics, School of Engineering of Lullier, University of Applied Sciences of Western Switzerland, 150 route de Presinge, 1254 Jussy, Switzerland

Chris F.J. Spies
Adele McLeod

     University of Stellenbosch, Department of Plant Pathology, Private Bag X1, Matieland 7602, South Africa

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS TAXONOMY RESULTS DISCUSSION LITERATURE CITED

 

A new species of Pythium collected from grapevine roots (Vitis vinifera) in South Africa and roots of common beet (Beta vulgaris) in Majorca, Spain, is described. The phylogenetic position of the new species was investigated by multigene sequence analyses of the internal transcribed spacers (ITS1 and ITS2) of the rDNA region, as well as three other nuclear and three mitochondrial coding genes. Maximum likelihood phylogenetic analyses based on ITS rDNA and concatenated β-tubulin and cytrochrome c oxidase II alignment place Pythium recalcitrans together with P. sylvaticum and P. intermedium. Pythium recalcitrans sp. nov. is morphologically almost indistinguishable from other Pythium species that only form hyphal swellings in culture. However its species status is justified by the distinctiveness of the DNA sequences in all the genes examined. In culture P. recalcitrans exhibits fast radial growth, abundant spherical to subglobose hyphal swellings but produces no zoosporangia. Sexual structures are not seen in agar media but form in autoclaved grass blades floated on water. Multiple antheridia (1–7) are encountered with most of them diclinous and crook-necked. Oospores are thin-walled and either aplerotic or plerotic. P. recalcitrans was pathogenic to seedlings of Beta vulgaris and Solanum lycopersicum.

Key words: actin, -tubulin, β-tubulin, cox I and II, ITS rDNA, nadh I, oomycetes, plant pathogen, Pythium phylogeny

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS TAXONOMY RESULTS DISCUSSION LITERATURE CITED

 
Pythium is a large genus comprising more than 120 described species including many plant pathogens (Dick 1990, Lévesque and de Cock 2004). The genus is classified in the Pythiaceae within the oomycetes, a taxonomic group of predominantly filamentous diploid organisms unrelated to the true fungi (Eumycota) and currently assigned with other heterokont, biflagellate organisms to kingdom Straminipila (Dick 2001). Pythium species mainly cause seedling damping-off, root infections and in general rotting of soft tissues in soil and aquatic habitats (van der Plaats-Nikerink 1981). A few species have been reported as mycoparasites (van der Plaats-Nikerink 1981, Lifshitz et al 1984) and animal pathogens (de Cock et al 1987). Most Pythium species are homothallic with meiosis occurring in the gametangia before fertilization (Sansome 1961, Dick 1990). Only a few species have been demonstrated to be heterothallic (Campbell and Hendrix 1967, Papa et al 1967, Martin 1995) requiring the presence of an opposite mating to sexually reproduce. However low levels of homothallic behavior (selfings) have been occasionally observed in some isolates of heterothallic species (van der Plaats-Niterink 1981, Martin 1995).

Until the modern application of molecular biology, the taxonomy of Pythium mainly relied on morphological and physiological aspects. The delimitation of Pythium taxa has been hindered by the few distinctive morphological structures (e.g. sporangia, hyphal swellings and gametangia) and often their great variability in range of dimensions that are increasingly overlapping as the number of species grows. Furthermore only a few ecological aspects are available for species discrimination becausee most species are cosmopolitan, nonhost-specific pathogens (i.e. generalists), which mostly lack any known niche specialization (Hendrix and Campbell 1973). Altogether these problems have resulted in many misidentifications and superfluous species epithets (Lévesque and de Cock 2004).

Phylogenetic analyses of sequence data have transformed fungal taxonomy. These analyses have been aided by the availability of large sequence databases such as GenBank. Unlike morphological characters, DNA sequence analysis is not subject to environmental interactions and hence is stable and fully reproducible. Lévesque and de Cock (2004) constructed a phylogenetic tree of 116 species of Pythium based on parsimony analysis of the internal transcribed spacers (ITS) sequences of the rDNA region. Their study revealed 11 clades distributed into two main clusters corresponding to Pythium species with filamentous (Clades A–D) and globose (Clades E–J) sporangia and a small clade including species with ovoid-papillate (Clade K) sporangia, which is out-group to the filamentous and globose clades of Pythium. Villa et al (2006) incorporated the cytrochrome c oxidase subunit II (cox II) and β-tubulin genes for investigating the phylogenetic relationships of 58 isolates of Pythium and 17 isolates of Phytophthora. Their study, which included 39 Pythium species, identified four clades of which one clade is composed mainly of Pythium species with subglobose-ovoid papillate sporangia. This clade could be an evolutionary bridge between Pythium and Phytophthora.

In this paper a multigene sequence analysis approach is followed to discern the taxonomic status and phylogenetic position of a new Pythium species that shows morphological characters similar to those of other species, such as P. sylvaticum Campbell and Hendrix. A case of nonhost specificity and cosmopolitanism within the genus Pythium is documented.

	MATERIALS AND METHODS

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Isolation procedure.— – Details of the origin of all the Pythium isolates used in the study are provided (TABLE I). A semiselective medium PARP (Jeffers and Martin 1986) was used for isolations made from necrotic roots of Beta vulgaris L. and Vitis vinifera L. Roots were washed in tap water and blotted dry on paper towels. V. vinifera roots were further surface sterilized by immersions in 70% ethanol for 2 s. Pieces of necrotic rootlets were plated onto PARP medium and incubated at 20 C for 2 d. Hyphal tips were transferred to carrot agar (CA, Brasier 1967) for B. vulgaris isolates or cornmeal agar for V. vinifera isolates. Stock cultures were maintained at 20 C on CA slants covered with liquid paraffin, and subcultures have been deposited at the CBS (Centraalbureau voor Schimmelcultures, Utrecht, The Netherlands).

The ability of the isolates to produce sporangia was investigated under different experimental conditions. Three blocks ca. 1 x 1 cm surface area taken from a 2 d old colony grown on CA, were transferred to 90 mm diam plates flooded with ca. 10 mL sterile distilled water or soil-water extract (Moralejo et al 2004) or with sterile distilled water plus a few autoclaved seeds of B. vulgaris and Phaseolus sp. (5–10/species). Petri dishes were incubated under continuous fluorescent white light at 20 and 25 C. The formation of sporangia and sexual structures also were investigated with either autoclaved or freshly harvested grass blades (van der Plaats-Niterink 1981) or thin autoclaved carrot slices floated on sterile distilled water (Matthews 1931). Filamentous or globose structures suspected of being capable of zoospore release were examined repeatedly with a Wild M5A binocular dissecting microscope (Leica Microsystems, Heerbrugg, Switzerland) at 50x magnification over 3 d at room temperature. The ability of these structures to release zoospores also were checked after exposing the liquid culture plates to 7 C for 45 min, followed by incubation at room temperature.

The formation of sexual structures on CA and other media in darkness was investigated for 1 mo. Because no oogonia were observed we further investigated the possibility of the new Pythium being heterothallic. All possible combinations of "intraspecific" pairings were tested. For this purpose mycelial plugs were placed ca. 2 cm apart on CA in 90 mm Petri dishes and incubated at 20 C in the dark for 2 wk. Interspecific pairings with both mating types of P. intermedium de Bary (+/– compatible strains) also were carried out as described above. Agar blocks 1 x 1 cm in surface area from the contact line were mounted on glass slides and examined under an Olympus BX-50 compound microscope at 400x magnification. Approximately 25 sexual and asexual structures chosen at random were measured with a calibrated eyepiece at 400x magnification on CA. Digital camera images were captured with Olympus DP-12 software.

The keys to Pythium by Waterhouse (1967), van der Plaats-Niterink (1981) and Dick (1990) were used for identification. The morphology of the isolates was compared with the original descriptions of similar species provided by Matthews (1931), Middleton (1943) Waterhouse (1968) and van der Plaats-Niterink (1981).

Pathogenicity tests.— – A soil infestation assay was performed to test the pathogenicity of isolates Py26 and STEU-6208. Inoculum was produced by first soaking 200 g of rye (Secale cereale) seeds overnight in an equal volume of water, followed by draining of the water and autoclaving in a 500 mL flask. The medium was inoculated with two 12 mm diam colonized agar plugs and incubated at 20 C for 1 wk in the dark.

Two-week-old seedlings of common beet (Beta vulgaris), tomato (Solanum lycopersicum L.) and cauliflower (Brassica oleracea L.) were transplanted into individual pots (50 x 50 x 50 mm) containing sterile peat soil. The seedlings were left to root at 20 ± 5 C for 2 wk in a plant growth chamber. Thereafter 200 g of the inoculum substrate were spread evenly in a tray, on top of which 12 plantlets removed from the pots were placed, and the set flooded with water for 24 h. Controls consisted of four plantlets of each species that were placed into another tray without inoculum that also was flooded with water for 24 h. The trays were incubated in a greenhouse at 20 ± 5 C and flooded 24 h every week for 1 mo. In total 10 seedlings of each species were inoculated with each of the isolates separately. Pathogenicity on seedlings was evaluated by recording the percentage of seedlings affected by damping-off 4 wk after inoculation.

DNA isolation, amplification and sequencing.— – Mycelial DNA was obtained from pure cultures of four P. recalcitrans isolates (TABLE II) grown in pea broth (Kroon et al 2004). DNA was extracted from frozen mycelia with a DNA-Easy Plant Minikit (QIAGEN, Basel, Switzerland) according to the manufacturer’s specifications. Genomic DNA quality was checked with a NanoDrop NT-100 UV spectrophotometer (Witec AG, Switzerland). DNA amplifications were performed for three nuclear and three mitochondrial loci. Ribosomal DNA ITS amplifications were carried out using universal primers ITS4 and ITS5 that target conserved regions in the 18S and 28S rDNA genes (White et al 1990). Amplifications for the β-tubulin (β-tub) gene, the NADH dehydrogenase subunit I gene (nadhI) and the cytochrome oxidase subunit I gene (cox I) were performed according to Kroon et al (2004) with primers TUBUF2 and TUBUR1 for the β-tub, NADHF1 and NADHR1 for nadh1 and COXF4N and COXR4N for cox I. The cytochrome oxidase subunit II gene (cox II) was amplified with primers FM75 and FM78 and PCR conditions as described by Martin and Tooley (2003). Actin and tubulin (-tub) genes were amplified with primers and PCR conditions described by Harper et al (2005). PCR products were purified with a Minelute PCR Purification Kit (QIAGEN, Switzerland) and quantity and quality were checked as described above. Amplicons were sequenced directly in both senses, and the consensus sequences were submitted to GenBank (TABLE II).

Nucleotide sequences were aligned manually with Sea-view (Galtier et al 1996). The AICc criterion as implemented in Modeltest suggested GTR+I+G as an appropriate model. Metropolis-coupled Markov chain Monte Carlo analyses (MCMC; Larget and Simon 1999, Mau et al 1999) were performed with MrBayes (version 2.0; Huelsenbeck and Ronquist 2001) with the general time reversible (GTR) model allowing all rates to be different (Lanave et al 1984, Rodríguez et al 1990). To correct the among-site rate variations, the proportion of invariable sites (I) and the alpha parameter of gamma distribution (G), with eight rates categories, were estimated by the program and taken into account in all analyses. The program was run for 4 000 000 generations and sampled every 100 generations, with four simultaneous chains. A 50% majority rule consensus of the remaining trees was computed to obtain estimates for the probabilities that groups are monophyletic given the sequence data (posterior probabilities). Branch lengths were computed as mean values over the sampled trees. The trees sampled before the chains achieved stationarity were discarded. NJplot (Perrière and Gouy 1996) and Treeview (Page 1996) were used to view Bayesian trees. To confirm that the posterior probability distribution of the MCMC processes is stationary (Huelsenbeck and Ronquist 2001), the Bayesian analysis was repeated three times on a personal computer, always starting with random trees and default parameter values of the program. All these analyses were run for 4 000 000 generations.

	TAXONOMY

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Pythium recalcitrans L. Belbahri et E. Moralejo, sp. nov. (FIGS. 1A–H)

View larger version (163K): [in this window] [in a new window]   FIG. 1. Morphology of Pythium recalcitrans grown on carrot agar (CA) and grass blades floated in water. Clavate appresorium (A) formed on the plate floor. Spherical hyphal swellings produced on a colonized grass blade (B) and on CA (C). Doliiform hyphal swelling (D) formed on CA. Sexual structures developed on grass blades (E–I); many antheridia surrounding an oogonium (E–G); plerotic (H) and aplerotic (I), thin-walled oospores.

Hyphae principales usque ad 10 µm latae, hyalinae. Appressoria magna, simplicia, clavata, falcata vel forma irregularia formata. Temperaturae cardinales in CA: minima 4 C, optima 15–25 C et maxima 34 C. Coloniae in CA indescriptae vel modice radiantes, mycelium aerium gossypinum prope marginem. Intumescentiae hyphales in CA abundantes post 7 dies, intercalares vel in ramis secundariis terminales, globosae vel subglobosae, 25.6 µm diam, raro doliiformes vel citriformes, vel etiam cylindraceae in hyphis principalibus. Zoosporae haud visae. Oogonia solum formata abundantia ab omnibus isolatis in laminis herbarum aqua distillata submersa, levia, globosa vel subglobosa, 26.1 ± 2.9 µm diam. Antheridia diclina, hamata, vel apice angustata, multiplicia, usque ad 7 in quoque oogonio. Oosporae globosae, leves, 22.7 ± 2.8 µm diam, tenuitunicatae (c. 0.6 µm), pleroticae vel apleroticae.

All measurements for each morphological structure are the average of three isolates of P. recalcitrans (TABLE I). Main hyphae up to 10 µm wide, hyaline, smooth, straight or slightly undulate. Hyphal branch system short and dendroid. Hyphal contents evacuate after 1 wk; many cross-septa then remain visible. Appressoria abundant on the plate floor, large, simple, clavate, falcate or sometimes irregularly shaped (FIG. 1A). Cardinal temperatures on CA, 4 minimum, 15–25 optimum and 34 C maximum (FIG. 2). Exposures to 35 C for 2 d were lethal for all isolates. Colonies on CA unpatterned to slightly radiate with a smooth outline, aerial mycelium near the margins cottony; on CMA mycelium submerged, diffuse and slightly dendroid; on PDA unpatterned with a smooth outline, and on MEA hyphae tending to be slightly spiral and dextrogyrous from above. Hyphal swelling abundant within 1 wk on CA, terminal on secondary branches or mostly intercalary, spherical to subglobose 25.6 (15.5–30.3) µm diam (FIG. 1B–C), a few doliiform to limoniform (FIG. 1D), or cylindrical on the main hyphae. Zoospores not observed under standard conditions in distilled water, soil extract or on seeds. Sexual structures: (On CA) not observed in agar culture, either in intraspecific or interspecific pairings with P. intermedium and other Pythiums. Oogonia readily and abundantly formed by all isolates on grass blades incubated on distilled water; smooth-walled, spherical to subglobose, 26.1 ± 2.9 µm diam (19.6–30.1) (FIG. 1E–I). Antheridia diclinous when the stalk insertion is discernible, sometimes with a broadened attachment, mostly crook-necked and multiple, with up to seven seen per oogonium (FIG. 1E–G). Oospores spherical, smooth, 22.7 ± 2.8 µm diam, thin-walled (0.6 µm), both plerotic and aplerotic (FIG. 1H). Many aborted oospores present in colonized grass blades.

View larger version (12K): [in this window] [in a new window]   FIG. 2. Temperature-growth relationships of three isolates of Pythium recalcitrans grown on carrot agar in the darkness. RGR = radial growth rates.

	RESULTS

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Sequences and phylogenetic analysis.— – The products of ITS amplification of the four sequenced isolates were all 821 bp long. All three South African isolates shared identical sequences and differed at a single nucleotide position in the ITS2 region from the sequence of the Spanish isolate. BLAST run through the GenBank database revealed a unique sequence pattern, distinct from those of any described species. The closest relatives were P. irregulare isolate 13–29 (AY907915 [GenBank] ), P. sylvaticum (AY907915 [GenBank] ), P. paroecandrum (DQ091297 [GenBank] ) and P. spinosum (AF492017 [GenBank] ) with minimum scores of 72–80% identity over all the ITS sequence positions. The maximum identity was 100% with P. paroecandrum over 79% (E value = 1e-153), 98% with P. irregulare over 72% (4e-154), 98% with P. spinosum over 71% (6e-153) and 97%, with P. sylvaticum over 80% (2e-152), of the rDNA ITS sequence. Phylogenetic analyses of the ITS sequence data revealed that P. recalcitrans falls in Clade F sensu Lévesque and de Cock (2004) (FIG. 3).

View larger version (25K): [in this window] [in a new window]   FIG. 3. Bayesian inference tree using rDNA ITS sequences showing phylogenetic relationships between Pythium species. The numbers on the nodes represent the posterior probability/percentage bootstrap support based on Bayesian analysis of the dataset. Pythium ITS sequences are indicated by their GenBank accession in the figure. The phylogeny was rooted with Clade K, as suggested by Villa et al (2006), who used Phytophthora ITS sequences as outgroups. Bar = 0.1 substitution positions.

Because the well known "total evidence" approach (Kluge 1989) was followed, separate analyses of the different sequenced loci were not performed. Instead analysis was conducted on a β-tub and cox II concatenated dataset. In the combined phylogenetic analysis with both genes, P. recalcitrans falls in Clade 2 sensu Villa et al (2006) together with species bearing globose to spherical zoosporangia, such as P. irregulare Buisman, P. paroecandrum Drechsler, P. paddicum Hirane, or those with spherical hyphal swellings (i.e. P. sylvaticum, P. spinosum, P. intermedium, P. ultimum Trow and P. violae Chesters and Hickman) (FIG. 4).

View larger version (27K): [in this window] [in a new window]   FIG. 4. Phylogenetic relationships between Pythium and Phytophthora spp. derived from a matrix of concatenated alignments of cox II and β-Tub gene sequences. The numbers on the nodes represent the posterior probability/percentage bootstrap support based on Bayesian analysis of the dataset, as described in Materials and Methods. The sequences of β-Tub and cox II of Pythium and Phytophthora isolates used in this analysis are the published sequences of Villa et al (2006). The phylogeny was rooted with sequences from Phytophthora. Bar = 0.1 substitution positions.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS TAXONOMY RESULTS DISCUSSION LITERATURE CITED

 
The morphological examination of the new Pythium species barely allows discrimination from other species in the keys by Waterhouse (1967), van der Plaats-Niterink (1981) and Dick (1990). However phylogenetic analyses based on each of the sequences of the rDNA ITS region and the concatenated β-tub and cox II alignments clearly demonstrated that Pythium recalcitrans is a new species. The same phylogenetic analyses consistently indicate that P. recalcitrans shares a common ancestor with all the Pythium species within Clade F of Lévesque and de Cock (2004) forming a monophyletic group supported by high values of posterior probabilities. On a short sequence run of the ITS region, P. recalcitrans displayed the highest sequence homology with P. sylvaticum and was positioned in the same subclade as P. mamillatum, P. spinosum, P. paroecandrum, P. irregulare and P. intermedium (FIG. 3).

P. recalcitrans is almost indistinguishable morphologically from other Pythium spp. enclosed as group HS (hyphal swellings) in the monograph of van der Plaats-Niterink (1981), except that it does produce sexual structures. The HS group is a nontaxonomic group traditionally suited for unidentified species that do not usually form zoosporangia and sexual structures under standard cultural conditions but do form hyphal swellings. As most species within this group P. recalcitrans is also a soil inhabitant associated with root infections of terrestrial plants. P. recalcitrans has been detected over a short time on two hosts having different life-forms (herbaceous plants and deciduous shrubs) in the northern and southern hemispheres and in Mediterranean climates (Balearic Islands and South Africa), always associated with horticulture. In addition P. recalcitrans also most likely is associated with Rhododendron because DNA extraction from Rhododendron in Poland, followed by an oomycete specific ITS-PCR, cloning and sequencing, has revealed the presence of the ITS sequences of this species (Tomasz Oszako, pers comm). These ITS sequences (UASWS0288, UASWS0209 and UASWS0291) were 100% identical to ITS sequences of the South African isolates. Therefore it seems that P. recalcitrans has a wide host-range, geographic distribution and ecological niche. As expected for a population that is likely not to be genetically structured by its hosts, geographical isolation and niche occupation, the isolates of P. recalcitrans were phenotypically similar and showed little or no polymorphism in the sequences of the six loci examined for each isolate.

P. recalcitrans conformed to the major behavioral trends described for its closer related species of Clade F in the phylogeny of Lévesque and de Cock (2004) because it exhibited fast colony growth, moderate cardinal temperatures and did not form zoospores under standard conditions tested including different temperatures. However we cannot rule out the possible release of zoospores from sporangia (= hyphal swellings) under other environmental conditions. The new species resembles P. sylvaticum (Campbell and Hendrix 1967) in the morphology of its asexual structures and colony pattern but it differed in being apparently only self-breeding (homothallic) without showing any enhancement of sexual fertility when intraspecific and interspecific matings were performed, as well as in forming both aplerotic and plerotic oospores with thinner walls. Nonetheless van der Plaats-Niterink (1981) reported that some isolates of P. sylvaticum did form a few sexual structures in pure culture after long storage. More isolates would need to be examined before discarding the possibility of the new species being heterothallic, as occurs with its close relatives P. intermedium and P. sylvaticum (FIG. 3). When oogonia were formed P. recalcitrans was readily distinguishable from the other species within Clade F, such as P. spinosum, P. irregulare, P. cylindrosporum Paul, P. mamillatum, P. kunmingense Yu, in having smooth walled oogonia. It differed from P. intermedium and P. attrantheridium (Allain-Boulé et al 2004) by not producing terminal chains of deciduous hyphal swellings (= "conidia"). Unlike P. paroecandrum oogonia of P. recalcitrans were not catenulate and no sessile monoclinous antheridia were observed. P. macrosporum differed in the formation of thicker-walled oospores, usually contorted antheridial stalks and the production of zoospores. Some of the members of Clade I, sensu Lévesque and de Cock (2004), such as P. ultimum, P. debaryanum, P. splendens and P. heterothallicum, resembled P. recalcitrans morphologically. In fact some isolates identified as P. debaryanum Hesse by Robertson (1980) looked similar to P. recalcitrans and were distinguished only because the former formed sexual structures in pure culture. P. recalcitrans differed from P. ultimum in having larger oogonia, thinner-walled oospores and lack of short monoclinous antheridia; P. splendens was heterothallic and formed larger sporangia (=hyphal swellings) mostly in a terminal position.

ITS rDNA has been used widely in phylogenetic studies, but it does not necessarily reflect the evolutionary patterns of the entire genome (Shen 2001). However, when tree topologies based on separate genes consistently coincide in grouping together some species, there are good reasons to think of close relationships within the species. The concatenated alignment approach of the two markers β-tub and cox II undertaken in this study circumvents problems associated with rDNA or single-gene phylogenies and selection of robust phylogenetic markers. Thus genes coding for metabolic and structural proteins, such as cox I and II, β-tub and NADH dehydrogenase subunit I, are attracting attention. Two other genes tested in the genus Pythium in this study (-tubulin and actin) proved to be easily sampled because their amplification was efficient both in Pythium and Phytophthora (Belbahri unpubl data) and could be used to broaden phylogenetic markers in both genera. These genes are conserved and the alignment of their sequences is less ambiguous compared to rDNA (Bruns et al 1991).

Our results suggest that it might be possible to piece together the Peronosporales and Pythiales with concatenated alignments of these two markers (Görker et al 2007). This is of interest because we expect the number of available phylogenetic marker sequences to increase substantially. However our study also showed that certain nodes of the tree were difficult to resolve even with concatenated alignment data. Thus using new markers such as -tubulin and actin genes could be of great value for constructing more congruent phylogenies of oomycetes.

The molecular phylogenies presented here for the genus Pythium support the recent studies correlating molecular clusters with subgroups defined on sporangial form (Briard et al 1995, Matsumoto et al 1999, Martin and Tooley 2000, Lévesque and de Cock 2004, Villa et al 2006). Based on these results and on our analysis it seems that the classification of oomycetes needs to be revised.

	ACKNOWLEDGMENTS

 
We are grateful for financial support from the Conselleria d’Agricultura del Govern Balear. We thank Walter Gams for proposing the epithet "recalcitrans" and for the Latin description.

	FOOTNOTES

 
Accepted for publication January 4, 2008.

¹ Corresponding author. E-mail: vieaemr{at}uib.es

	LITERATURE CITED

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS TAXONOMY RESULTS DISCUSSION LITERATURE CITED

 
Allain-Boulé N, Tweddel R, Mazzola M, Belanger R, Lévesque CA. 2004. Pythium attrantheridium sp. nov. taxonomy and comparison with related species. Mycol Res 108:795–805.[CrossRef][Medline]

Belbahri L, Calmin G, Pawlowski J, Lefort F. 2005. Phylogenetic analysis and real time PCR detection of a presumably undescribed Peronospora species on sweet basil and sage. Mycol Res 109:1302–1312.[CrossRef][Medline]

———, ———, Sanchez-Hernandez E, Oszako T, Lefort F. 2006a. Pythium sterilum sp. nov. isolated from Poland, Spain and France, its morphology and molecular phylogenetic position. FEMS Microbiol Lett 255:209–214.[CrossRef][Medline]

———, Moralejo E, Calmin G, Oszako T, Garcia JA, Descals E, Lefort F. 2006b. Phytophthora polonica, a new species isolated from declining alder stands in Poland. FEMS Microbiol Lett 261:165–174.[CrossRef][Medline]

Brasier CM. 1967. Physiology of reproduction in Phytophthora [Doctoral dissertation]. Hull, UK: University of Hull.

Briard M, Dutertre M, Rouxel F, Brygoo Y. 1995. Ribosomal RNA sequence divergence within the Pythiaceae. Mycol Res 99:1119–1127.[CrossRef]

Bruns TD, White TJ, Taylor JW. 1991. Fungal molecular systematics. Ann Rev Ecol Sys 22:525–564.[CrossRef]

Campbell WA, Hendrix FF. 1967. A new heterothallic Pythium from southern United States. Mycologia 59:274–278.[CrossRef]

de Cock AWAM, Mendoza L, Padhye AA, Ajello L, Kaufman L. 1987. Pythium insidiosum sp. nov., the etiologic agent of pythiosis. J Clin Microbiol 25:344–349.[Abstract/Free Full Text]

Dick MW. 1990. Keys to Pythium. Reading, UK: University of Reading, School of Plant Science, Department of Botany. 64 p.

———. 2001. Straminipilous fungi: systematics of the Peronosporomycetes including accounts of the marine straminipilous protists, the plasmodiophorids and similar organisms. Dordrecht: Kluver Academic Publishers.

Galtier N, Gouy M, Gautier C. 1996. SEAVIEW and PHYLO_WIN, two graphic tools for sequence alignment and molecular phylogeny. Comp Appl Biosci 12: 543–548.[Medline]

Göker M, Voglmayr H, Riethmüller A, Oberwinkler F. 2007. How do obligate parasites evolve? A multi-gene phylogenetic analysis of downy mildews. Fung Genet Biol 44:105–122.[CrossRef]

Harper JT, Waanders E, Keeling PJ. 2005. On the monophyly of chromalveolates using a six-protein phylogeny of eukaryotes. Int J System Evol Microbiol 55:487–496.[Abstract/Free Full Text]

Hendrix FF, Campbell WA. 1973. Pythiums as plant pathogens. Ann Rev Phytop 11:77–98.[CrossRef]

Huelsenbeck JP, Ronquist F. 2001. MrBayes: Bayesian inference of phylogeny. Bioinformatics 17:754–755.[Abstract/Free Full Text]

Jeffers SN, Martin SB. 1986. Comparison of two media selective for Phytophthora and Pythium species. Plant Dis 70(11):1038–1043.[CrossRef]

Kluge AG. 1989. A concern for evidence and a phylogenetic hypothesis of relationships among Epicrates (Boidae, Serpentes). Syst Zool 38:7–25.[Abstract]

Kroon LPNM, Bakker FT, van den Bosch GBM, Bonants PJM, Flier WG. 2004. Phylogenetic analysis of Phytophthora species based on mitochondrial and nuclear DNA sequences. Fung Genet Biol 41:766–782.[CrossRef]

Lanave C, Preparata G, Saccone C, Serio G. 1984. A new method for calculating evolutionary substitution rates. J Mol Evol 20:86–93.[CrossRef][Medline]

Larget B, Simon DL. 1999. Markov chain Monte Carlo algorithms for the Bayesian analysis of phylogenetic trees. Mol Biol and Evol 16:750–759.

Lévesque CA, de Cock AWAM. 2004. Molecular phylogeny and taxonomy of the genus Pythium. Mycol Res 108: 1363–1383.[CrossRef][Medline]

Lifshitz R, Stanghenellini ME, Baker R. 1984. A new species of Pythium isolated from soil in Colorado. Mycotaxon 20:373–379.

Martin F. 1995. Meiotic instability of Pythium sylvaticum as demonstrated by inheritance of the nuclear markers and karyotype analysis. Genetics 139:1233–1246.[Abstract]

———, Tooley PW. 2000. Phylogenetic relationships of some Pythium species inferred from sequence analysis of the mitochondrially encoded cytochrome oxidase I and II genes. Mycologia 92:711–727.[CrossRef]

———, ———. 2003. Phylogenetic relationships among Phytophthora species inferred from sequence analysis of the mitochondrially encoded cytochrome oxidase I and II genes. Mycologia 95(2):269–284.[Abstract/Free Full Text]

Matthews VD. 1931. Studies on the genus Pythium. Chapel Hill: University of North Carolina Press. 136 p.

Matsumoto C, Kageyama K, Suga H, Hyakumachi M. 1999. Phylogenetic relationships of Pythium species based on ITS and 5.8S sequences of the ribosomal DNA. Mycoscience 40:321–331.[CrossRef]

Mau B, Newton MA, Larget B. 1999. Bayesian phylogenetic inference via Markov chain Monte Carlo methods. Biometrics 55:1–12.[Medline]

Middleton JT. 1943. The taxonomy, host range, and geographical distribution of the genus Pythium. Mem Torrey Bot Club 20:1–171.

Moralejo E, Puig M, Man in’t Veld WA. 2004. First report of Phytophthora tentaculata on Verbena sp. in Spain. Plant Pathol 53:806.[CrossRef]

Page RD. 1996. TreeView: an application to display phylogenetic trees on personal computers. Comput Appl Biosci 12:357–378.[Free Full Text]

Papa KE, Campbell WA, Hendrix FF Jr. 1967. Sexuality in Pythium sylvaticum: heterothallism. Mycologia 59:589–95.[CrossRef][Medline]

Perrière G, Gouy M. 1996. WWW-query: an on-line retrieval system for biological sequence banks. Biochimie 78: 364–369.[Medline]

Robertson GI. 1980. The genus Pythium in New Zealand. NZ J Bot 18:73–102.

Rodriguez F, Oliver JF, Martin A, Medina JR. 1990. The general stochastic model of nucleotide substitution. J Theor Biol 142:485–501.[Medline]

Sansome E. 1961. Meiosis in the oogonium and antheridium of Pythium debaryanum Hesse. Nature 191:827–828.[CrossRef]

Shen QS. 2001. Molecular phylogenetic analysis of Grifola frondosa (Maitake) and related species and the influence of selected nutrient supplements on mushroom yield [Doctoral dissertation]. Pennsylvania State University Graduate School. 141 p.

Sikes DS, Lewis PO. 2001. PAUPRat: PAUP* implementation of the parsimony ratchet. Beta software, version 1. (Distributed by the Authors, Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, USA.)

van der Plaats-Niterink AJ. 1981. Monograph of the genus Pythium. Studies in Mycology 21. Baarn, Netherlands: Centraalbureau Voor Schimmelcultures.

Villa NO, Kageyama K, Asano T, Suga H. 2006. Phylogenetic relationships of Pythium and Phytophthora species based on ITS rDNA, cytochrome oxidase II and beta-tubulin gene sequences. Mycologia 98(3):410–422.[Abstract/Free Full Text]

Waterhouse GM. 1967. Key to Pythium Pringsheim. Mycol Pap 109:1–15.

———. 1968. The genus Pythium: diagnoses (or descriptions) and figures from the original papers. Mycol Pap 110:1–50.

White TJ, Burns T, Lee S, Taylor J. 1990. Amplification and direct sequencing of fungal ribosomal RNA genes for phylogenetics. In: Innis MA, Gelfand DH, Sinsky JJ, White TJ, eds. PCR protocols: a guide to nethods and applications. San Diego: Academic Press. p 315–322.

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