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Ghent University, Department of Biology, Group Mycology, K.L. Ledeganckstraat 35, B-9000 Ghent, Belgium
Andrea C. Rinaldi
Cattedra di Chimica Biologica, Università di Cagliari, I-09042 Monserrato Cagliari, Italy
Marco Leonardi
Giovanni Pacioni
Ornella Comandini 1
Dipartimento di Scienze Ambientali, Università dell’Aquila, Via Vetoio Loc. Coppito, I-67100 L’Aquila, Italy
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ABSTRACT |
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Lactarius is one of the larger genera of ectomycorrhizal Basidiomycota, with about 400 species recognized worldwide. The ectomycorrhizae formed by Lactarius tesquorum on Cistus sp., one of the most common and ecologically relevant shrubs in the semi-arid regions in the Mediterranean basin, are described here in terms of morphological, anatomical and molecular features. An ITS rDNA sequence-based phylogenetic analysis was performed on the related European Lactarius taxa (L. mairei, L. pubescens, L. scoticus, L. spinosulus, L. torminosulus and L. torminosus) currently classified together with L. tesquorum in the subgenus Piperites section Piperites. Piperites s.s. could be divided into two main clusters; L. mairei and especially L. spinosulus were related less closely to the other taxa. This study is part of a broader effort to extend our knowledge of the distribution, phylogeny and ectomycorrhizal biology of Lactarius species in selected ecosystems.
Key words: ectomycorrhizal fungi, ITS sequences, molecular phylogeny, Piperites, Russulales
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Members of the genus have been reported in ectomycorrhizal association with numerous trees and shrubs, and their important ecological role as late-stage root colonizers in a range of ectotrophic plant communities is largely appreciated (Hutchison 1999).
Despite a longstanding tradition of morphological taxonomy studies (focusing primarily on European and North American species), significant differences exist among the various classification systems, especially concerning the delimitation of different infra-generic groupings and the placement of certain taxa (Hesler and Smith 1979, Bon 1980, 1983, Heilmann-Clausen et al 1998, Basso 1999). A comprehensive natural classification of Lactarius, supported by molecular phylogenetic analysis, still is lacking. However, several investigations recently have been carried out that provided us with useful insight into the phylogeny of some species groups within Lactarius and other members of the Russulales (Eberhardt et al 2000, Henkel et al 2000, Miller et al 2001, Peter et al 2001, Eberhardt 2002, Miller and Buyck 2002).
In the past few years, we have initiated a long-term study to extend our knowledge on the distribution, ecology, phylogeny and ectomycorrhizal biology of Lactarius species occurring in selected ecosystems in Europe. The main idea is to explore the Lactarius ectomycorrhizal diversity of host trees or shrubs that characterize a given ecologically valuable ecosystem. Relevant Lactarius ectomycorrhizal types are fully characterized from a morpho-anatomical and molecular point of view, and the anatomical characters of the ectomycorrhiza are compared to those known from related taxa. At the same time, morphological and molecular data obtained for closely related Lactarius taxa, living either in the same habitat or linked to other hosts, are used to clarify the systematic position of the concerned species. The combination of morphological characters of sporocarp and mycorrhiza and their molecular features thus contributes to a reliable taxonomy in the genus.
This method has been applied successfully to the Lactarius symbionts of silver fir (Abies alba), mainly in central Italy, and related Lactarius species (Comandini et al 1998, Eberhardt et al 2002). Here, we focus on the Lactarius associations of Cistus spp., a group of about 20 woody, evergreen or semideciduous shrub species found in wide semi-arid areas from the Canary Islands throughout the Mediterranean region to the Caucasus. Cistus species are obligate seeding, early colonizers that follow disturbance, particularly fire, in low maquis-type Mediterranean ecosystems, rendering their ectomycorrhizal ecology particularly interesting. In the frame of this research, we identified the ectomycorrhizae formed by Lactarius tesquorum Malençon on Cistus sp. In addition, an ITS rDNA sequence-based phylogenetic analysis of the related European Lactarius taxa currently classified together with L. tesquorum in the subgenus Piperites (Fr. ex J. Kickx f.) sect. Piperites (Fr. ex J. Kickx f.) Burl. was carried out to clarify relationships at species level and infrageneric classification. The section Piperites is characterized by a sticky cap with tomentose, hairy or bearded margin, white and unchanging milk (in contrast to the yellowing milk in the Scrobiculati, which are placed in the same section by Hesler & Smith [1979] and Basso [1999]), scattered to numerous macrocystidia, ixocutis-structure in the pileipellis and mostly subreticulate spores ornamented with rounded ridges. Described on the basis of material originating from Morocco (Malençon 1979), L. tesquorum subsequently was recognized as a commonly occurring species in maquis-type habitats of many Mediterranean countries (Contu 1986, Lalli and Pacioni 1988).
In addition to L. tesquorum, the section includes six more, generally accepted European taxa, comprising two well-known, common birch-associated species that are widely distributed in Europe: L. torminosus (Schaeff. : Fr.) Pers. and L. pubescens Fr. L. scoticus Berk. & Broome is a small morphological mimic of L. pubescens, growing in arctic-alpine birch (for a detailed comparison between L. pubescens and L. scoticus as L. favrei, see Jahn 1982). Knudsen and Borgen (1994) described L. torminosulus, a dwarf version of L. torminosus, an arctic species associated with Betula nana or B. glandulosa. L. mairei Malençon differs from the other representatives by the association with Quercus. L. spinosulus Quél. by most modern authors placed in the section Colorati because of the resemblance to L. lilacinus has been placed in the Piperites by Neuhoff (1956), Heilmann-Clausen et al (1998) and Basso (1999). It shares most characteristics with L. mairei (spore ornamentation and squamulose, zonated cap) and shares the host tree, Betula, with most other Piperites. Pearson (1950) described a variety of L. mairei, L. mairei var. zonatus, differing from the type by a brighter color, a distinctly zonated cap and smaller spores. Another variety, L. mairei var. ilicis, was proposed by Lalli and Pacioni (1988) and differs in the smaller size and the more gracile habitus, a shorter stipe and shorter hairs on the pileus. The same authors described L. tesquorum var. splendidus characterized by a vividly orange, zonated pileus and growing with Quercus ilex.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, 1987–1998, 1991). Munsell Soil Color Charts (1975) were used as reference for the descriptions of ectomycorrhizae. Voucher specimens of mycorrhizae were deposited in AQUI as dried and fixed/ preserved material (4% glutaraldehyde or EtOH 50%), respectively, together with slides.
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Molecular analysis. –
DNA preparation..
DNA was prepared by the CTAB procedure (Doyle and Doyle 1990), modified as follows. Dried material was ground in liquid N2 (about 50 mg). One mL of extraction buffer (0.1 M Tris-HCl, pH8, 0.5 M NaCl, 0.05 M EDTA and 0.01 M ß-mercapto-ethanol) and 50 µL of 10% SDS were added and the mixture was incubated 30 min at 65 C. This extract was centrifuged 10 min at 14 000 rpm (Eppendorf centifuge). An equal volume of isopropanol was added to the supernatant and, after mixing the solution, it was centrifuged again as above. The pellet was washed with 70% ethanol, air dried and dissolved in 400 µL of dH2O. Then 400 µL of CTAB buffer (2% w/v CTAB, 0.2 M Tris-HCl, pH7.5, 2 M NaCl and 0.05 M EDTA) was added and, after incubation for 15 min at 65 C, the mixture was extracted twice with chloroform/iso-amylalcohol (24:1). Two volumes of 96% ethanol were added, and the mixture was centrifuged again as above to pellet the DNA. The pellet was washed with 70% ethanol and dissolved in 100 µL of T0.1E (10 mM Tris-HCl, pH8, and 0.1 mM EDTA).
PCR and direct sequencing of the ITS region..
For the PCR reaction, primers ITS1-F and ITS4-B (Gardes and Bruns 1993) were used. This touchdown PCR profile was used: 95 C during 2 min, then 10 cycles of denaturation at 95 C (15 s), primer annealing at 63 C (20 s) with the temperature decreasing 1 C at each cycle, polymerization for 1 min at 72 C; followed by 35 cycles with the same profile for denaturation and polymerization but with a constant annealing temperature of 53 C; polymerization was completed by an incubation of 2 min at 72 C. The PCR products were purified with ExoSAP (USB, U.S.A.). DNA sequencing reactions were performed with the ABI PRISM® BigDyeTM Terminators verson 3.0 Cycle Sequencing Kit using primers ITS1-F, ITS2, ITS3 and ITS4-B (Gardes and Bruns 1993) on an ABI PRISM® 377 DNA Sequencer. Phred and Phrap Software (Ewing et al 1998, Ewing and Green 1998) was used to process raw data.
Data analysis..
ITS sequences were aligned in ClustalX (Thompson et al 1997), and the alignment was corrected manually where necessary. The alignment was deposited in TreeBASE (ID NUMBER SM1521-4590). Phylogenetic analysis of the ITS sequences was performed with PAUP*4b10 (Swofford 2002). Maximum-parsimony analysis was performed with all sites treated as unordered and unweighted and with gaps treated as missing. For tree searching, the branch and bound algorithm was used. Support for the internodes of the most-parsimonious trees was estimated by 10 000 bootstrap replicates with a heuristic search with 10 random-addition sequences for each replicate. Sequences were deposited in GenBank (TABLE I).
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RESULTS |
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Anatomical characters of mantle in plan views.. Outer mantle layers are plectenchymatous (FIGS. 1a, 2a), formed by a loose net of hyphae, 2–2.5(3) µm diam, hyaline, clampless, frequently branched, sometimes straight, others wavy, frequently septate; elbow-like protrusions are present. Middle mantle layers are densely plectenchymatous, to nearly pseudoparenchymatous (FIGS. 1b, 2b); hyphae are without special pattern, 4–5.5 µm diam; abundant laticifers, 5–8 µm diam, thin walled, frequently branched, and septate are present. Inner mantle layers are densely plectenchymatous, to nearly pseudoparenchymatous (FIGS. 1c, 2c); in some parts are present hyphal cells 8–10 µm long and 5–7 µm diam, whereas in other areas are present densely arranged hyphae 3–4 µm diam. Tips are organized like other parts of the mantle.
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). Vessel-like hyphae are 5–7 µm diam, with cell walls less than 1 µm thick, very long, septate, with distance between septa (17)22–40(45) µm, frequently filled with abundant whitish content (like laticifers) (FIG. 1d, left side). Peripheral hyphae are 2–2.5 µm diam, thin walled, with the same characters of those of the very outer mantle, rather loosely woven in the outer surface of the thicker rhizomorphs (FIG. 1d, right side). Thinner rhizomorphs are slightly differentiated, with compactly arranged hyphae. Anatomical characters, longitudinal section.. Mantle is 10–15 µm thick, densely plectenchymatous, with very long laticifers (5–8 µm diam) in the middle part (FIG. 2d); no mantle differentiation can be observed; hyphal cells are 4–10 µm long and (1)2–5 µm diam. Tannin cells are mostly thin, collapsed, 3–4 µm diam. Cortical cells form 2(3) rows; cells are usually radially rectangular or with irregular shape, measuring 35–47 µm radially and 13–22 µm tangentially. Hartig net surrounds 1(2) rows of cortical cells, forming palmetti-like lobes 2–2.5 µm diam in plan view, and 1 row of rectangular cells 1.5–3 µm in section.
DNA analysis. – The ITS sequences of the sporocarps of L. tesquorum from Sardinia and of the ectomycorrhizae found beneath them were identical, thus confirming the identity of the collected mycorrhizal structures.
The sequence alignment contained 728 characters of which 146 variable basepairs were parsimony-uninformative and 72 were parsimony informative. Russula brevipes (AF349714) was chosen as outgroup. L. hepaticus (AF096989) and L. quietus (AJ272247) from subgenus Russularia sect. Russularia were included in the alignment, and two Lactarius species belonging to the section Zonarii subsect. Scrobiculati, namely L. scrobiculatus AF140262 and L. intermedius AF140256, were included to assess the relationships of this group with the Piperites. An ITS sequence from L. tesquorum was available from GenBank (AF096986) and added to the phylogenetic analysis. Maximum-parsimony analysis of the ITS sequences resulted in three shortest trees (FIG. 3) with a length of 301 steps, CI 0.82, RI 0.73 and RC 0.60. These trees differ only in the arrangement of the species within the L. tesquorum, L. scoticus and L. pubescens clade. Main observations from this phylogenetic analysis are: (i) the section Piperites is not monophyletic (bootstrap <50%); (ii) the majority of species fall into two well-supported groups (bootstraps 83 and 97%): on one side L. torminosus and L. torminosulus and on the other side L. tesquorum, L. scoticus and L. pubescens. They group together with a bootstrap support of 83%; (iii) L. mairei and L. spinosulus seem more distantly related to this core group of Piperites; (iv) Scrobiculati included in the analysis clearly form a distinct group (supported by a bootstrap value of 89%) separated from the Piperites.
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described a variety of L. tesquorum, L. tesquorum var. splendidus, from central Italy and reported its association with Quercus ilex. The same authors also recognized three varieties of L. mairei, one of which (L. mairei var. ilicis) associated with Q. ilex and occurring sympatrically with L. tesquorum var. splendidus (Lalli and Pacioni 1988). The type material of both L. tesquorum var. splendidus and L. mairei var. ilicis, collected in 1979 and 1980, respectively, is conserved at AQUI. An attempt to amplify the ITS region of the DNA extracted from these samples failed; this confirms the difficulties encountered by other researchers (Eberhardt pers comm) trying to analyze the DNA of relatively old Lactarius herbarium samples. More recent collections of L. mairei var. ilicis (from AQUI and SIENA, see TABLE I), described morphologically by Antonini and Antonini (2002), also were studied by ITS sequence analysis. We combined morphological and molecular data to get insights on the status of these taxa. The most important spore characters are illustrated in FIG. 4. In addition to the size and the shape of the spores, important features can be observed in the amyloidity of the plage and in the type of ornamentation: height of the ornamentation, composing elements (warts/ridges/spines), degree of reticulation. FIGURE 4c shows spores that are distinctly more elongate and narrower than FIG. 4a, b and d. The ornamentation in FIG. 4c is almost completely reticulate and composed of rather low and regular ridges, while the ornamentation in FIG. 4b and d consists of abundant, isolated, short ridges and warts and shows a spiny aspect because of the irregular height. FIG URE 4a also shows a spiny aspect, but the ridges are much more connected and thus form a more reticulate ornamentation than in FIG. 4b and d. In all spores, the plage is inamyloid or at most very slightly distally amyloid.
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DISCUSSION |
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, Malloch and Thorn 1985) to be linked to this host genus (e.g., Pisolithus tinctorius, Russula cistoadelpha, R. monspeliensis, Hygrophorus pseudodiscoideus var. cistophilus, Amanita muscaria, A. cistetorum, Hebeloma erumpens, H. cistophilum, H. cylindrosporum, H. crustuliniforme, Leccinum corsicum), only very few accounts of Cistus ectomycorrhizal types exist in the literature. To date, only the full characterization of the ectomycorrhizae formed by Tuber spp. on Cistus incanus (Fontana and Giovannetti 1979, Giovannetti and Fontana 1982) and by Laccaria laccata and Boletus rhodoxanthus on Cistus ladanifer (Torres et al 1995, Hahn 2001) are available. A number of other contributions have focused on the association of hypogeous ascomycetes such as Tuber and Terfezia with Cistus spp., both under natural conditions and in culture, but they usually resulted in preliminary and/or not exhaustive descriptions of morpho-anatomical details of relevant mycorrhizae (Chevalier et al 1975, Fusconi 1983, Leduc et al 1986). It is of interest to note that Cistus spp., in analogy with other host plants, such as Eucalyptus spp., inhabiting fire-susceptible ecosystems, also may form vesicular arbuscular mycorrhizae. This plasticity is considered an adaptive trait to the cyclical pattern of accumulation and loss of organic resources due to fire (Smith and Read 1997). Common host-dependent features of all ectomycorrhizal types described so far on Cistus spp., also present in our L. tesquorum samples, are: simple or monopodial-pinnate ramification systems; small diameter of ectomycorrhizal tips (0.26–0.35 mm in L. tesquorum, up to 0.3 mm in B. rhodoxanthus); thin mantle (10–20 µm in L. laccata, 10–15 µm in L. tesquorum, up to 18–25 µm in B. rhodoxanthus); cortical cells generally present in two rows, tangentially rectangular, mainly radially rectangular orientated in our samples, more rarely square to radially rectangular orientated (e.g., in B. rhodoxanthus longitudinal sections); Hartig net generally uniseriate, surrounding 1(2) row of cortical cells and rarely reaching the endodermis ("cortical Hartig net").
In addition to L. tesquorum, the only other Lactarius sect. Piperites for which descriptions of ectomycorrhizae have been published is L. pubescens, in association with Betula pendula and Populus tremuloides (Godbout and Fortin 1985, Ingleby et al 1990). Within the closely related Scrobiculati, the mycorrhizae of L. scrobiculatus on Picea abies and Tsuga heterophylla (Amiet and Egli 1991, Kernaghan and Berch 1997) and that of L. intermedius on Abies alba have been characterized (Eberhardt et al 2000). A comparison of mycorrhizal appearance and structure (TABLE II) clearly reveals that the mycorrhizae of L. tesquorum and L. pubescens share many structural features, whereas the L. scrobiculatus and L. intermedius mycorrhizae are rather dissimilar from those of both L. tesquorum and L. pubescens. This can be considered a confirmation of the infrageneric classification proposed by Heilmann-Clausen et al (1998), placing the groups Piperites and Scrobiculati in different sections within the subgenus Piperites, rather than the scheme of Basso (1999), placing the two groups together in section Piperites. The assignment of the Piperites and Scrobiculati to different sections also is fully supported by the topology of the ITS-based phylogram depicted in FIG. 3.
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An unexpected, and to some extent surprising, result of the molecular analysis is the distance of the morphologically similar L. tesquorum and L. mairei. Indeed, the two taxa are distinct within the Piperites, having a predominately Mediterranean distribution and different ecological requirements with respect to the other species of the group, which occur more frequently or exclusively in northern European countries. L. mairei is associated with Quercus spp. on calcareous soil, whereas all other species (except L. tesquorum) are linked to Betula spp. In this context, it is expected that the Mediterranean species, L. mairei and L. tesquorum, would form a separate cluster from the Nordic Betula taxa, but this is not supported by the ITS analysis.
Relationships of the intraspecific taxa L. tesquorum var. splendidus and L. mairei var. ilicis are inconclusive based on microscopic analysis because most indicated differences are macroscopic (see also Antonini and Antonini 2002). But on examination of the spores of L. tesquorum var. splendidus, it becomes clear that this taxon should be a variety of L. mairei. The spores of L. tesquorum are somewhat longer and distinctly narrower and the ornamentation is more regularly reticulate, while in L. mairei spore ornamentation is characterized by a spiny aspect and ridges that do not form a reticulum but often are arranged somewhat in parallel (FIG. 4). It is interesting to note that Lalli and Pacioni (1981) first regarded this taxon as a forma of L. mairei before describing it as L. tesquorum var. splendidus. Molecular analysis of L. mairei var. ilicis (FIG. 3) confirms its status as a variety of L. mairei.
In our analysis, L. spinosulus is related closely to the members of section Piperites. When a phylogenetic analysis is conducted with more taxa, the position of L. spinosulus is variable and a close relationship to the Piperites is uncertain (unpubl data, Eberhardt 2000). Neither the placement of the species in section Piperites nor the placement in section Colorati is confirmed by molecular analysis. The correct position of L. spinosulus in the genus consequently remains unclear.
In addition to L. tesquorum, only L. cistophilus (subgenus Piperites, sect. Uvidi) is known to be linked specifically to Cistus spp. (Basso 1999). Another species, L. cyanopus (subgenus Piperites, sect. Deliciosi), recently was described from northwestern Italy growing with the same host in nature (Basso 1998, 1999, 1999b). However, it is more probable that it grows under Pinus (also present at the type locality), which would match better the host specificity displayed by Deliciosi, almost exclusively associated with members of the Pinaceae. Also, the association of L. rugatus, a species usually growing with Quercus in Mediterranean-type habitats, with Cistus has been claimed by some authors (Brotzu 1998).
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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1 Corresponding author. E-mail: comandin{at}univaq.it
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LITERATURE CITED |
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