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A new isidiate species of Arthonia (Ascomycota: Arthoniaceae) from Costa Rica

     Institute of Botany, Karl-Franzens-University Graz, 8010 Graz, Austria

Robert Lücking ¹

     Department of Botany, The Field Museum, 1400 S. Lake Shore Drive, Chicago, Illinois 60605-2496

Loengrin Umaña-Tenorio

     Laboratorio de Hongos, Instituto Nacional de Biodiversidad (INBio), Apdo. 22-3100, Santo Domingo de Heredia, Costa Rica

	ABSTRACT

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The new corticolous species Arthonia isidiata is described from the Pacific lowlands of Costa Rica. A. isidiata is characterized by minute, cylindrical to coralloid isidia produced on the thallus surface. The species currently is known only from the type locality in Corcovado National Park, where it occurs abundantly in the coastal rainforest around Sirena Biological Station.

Key words: Arthoniales, Corcovado National Park, Costa Rica, lichens, lowland rain forest, Ticolichen

	INTRODUCTION

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The Costa Rican Lichen Biodiversity Inventory (TICOLICHEN) is a multinational project supported by the National Science Foundation concerning Costa Rica species (Umaña-Tenorio et al 2002). In the framework of this project, experts and students of diverse taxonomic groups of lichens meet once a year for field work in Costa Rica. During the second foray to Costa Rica in March–April 2003, the group visited Corcovado National Park in SW Costa Rica and collected in the vicinity of Los Patos and Sirena field stations near the Pacific Coast. There, the authors came across a peculiar species of Arthonia, which grows abundantly on the bark of trees. The species is recognized easily by its dark, flat ascomata and its greenish thallus bearing tiny isidia.

Vegetative dispersal is found in a number of Arthoniales, especially in species with tropical preference, and may involve soredia, isidia or goniocysts (Sérusiaux 1985, Thor 1997). Some species rely entirely on vegetative propagation, namely Cryptothecia rubrocincta, one of the most common and conspicuous sterile crusts in neotropical rainforests (Thor 1991). It reproduces exclusively by isidia that are formed on the upper surface of the thalli. Isidia are also common in Dichosporidium nigrocinctum (Thor 1990).

The new species develops black ascomata typical of Arthonia sensu lato. Some Arthonia species are known to develop more or less distinct soredia on the thalli or form thalli that are entirely powdery or granular (e.g., Arthonia endlicheri or A. insulata). However, isidia have not been described thus far in Arthonia and therefore we consider our taxon to represent a new species.

	MATERIAL AND METHODS

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The material was studied using a stereomicroscope (Wild M3B) and a compound microscope (Zeiss Axioskop) equipped for epifluorescence microscopy. Measurements given in millimeters were taken under the stereomicroscope; those in micrometers were taken with water mounts under the compound microscope. Lugol’s solution (I) was applied as 1% aqueous solution (Merck No. 92611). Potassium hydroxide (K) was used at a concentration of 10%. The color shift in K was observed with thin transversal sections of ascomata under the compound microscope. HP-TLC was carried out according to Arup et al (1993).

Voucher specimens of all collections made during the field trip are kept in CR (Costa Rican National Herbarium), INBio (Costa Rican National Biodiversity Institute), USJ (University of Costa Rica Herbarium), B (Berlin), GZU (Graz), and F (Field Museum).

	RESULTS

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Arthonia isidiata Grube, Lücking & Umaña-Tenorio sp. nov. FIGS. 1–2

View larger version (165K): [in this window] [in a new window]   FIG. 1. Arthonia isidiata. A. Thallus with ascomata (Lücking s.n.); the commonly observed mite might feed on lichens or lichen-feeding psocids. B. Lateral view of clusters of isidia (Grube 11534). C. Cluster of branched isidia in transmission light (same collection). D. Transversal section of thallus (indicated by arrows) and supporting leaf (dark cells) in transmission light Grube 11531). E. Transversal section of ascoma in transmission light (same collection). Scale bar in A–B = 1 mm; in C = 100 µm; in D–E = 50 µm.

View larger version (14K): [in this window] [in a new window]   FIG. 2. Arthonia isidiata. Ascospores (Grube 11531). Scale bar = 20 µm.

Typus. – Costa Rica; Puntarenas; Parque Nacional Corcovado (AC Osa); Estación Biológica Sirena; 160 km SSE of San José and 50 km WSW of Golfito; Sirena and Guanacaste round trail NW of station, 8°29'N, 83°35'W, sea level, lowland and coastal rain-forest zone, old-growth coastal secondary forest and primary forest remnants on sandy soil, on bark (stem), April 7, 2003, M. Grube 11531 (INB 0003722807, holotype; GZU, isotype).

Paratypes. – Costa Rica. Puntarenas: Parque Nacional Corcovado (AC Osa), Estación Los Patos, 100–300 m, lowland rainforest zone, closed primary forest, on bark (stem), 6 Apr 2003, R. Lücking 16303a (F, GZU). Estación Biológica Sirena, sea level, coastal rainforest zone, disturbed coastal forest and exposed trees along beach, on bark (trunk), 7 Apr 2003, M. Grube 11521 (GZU), 11544 (INB 0003722809), 11545 (GZU), S. Will-Wolf, 12565, (CR, WIS); ibid., on bark (stem), 7 Apr 2003, M. Grube 11539 (GZU, INB 0003722808), 11560 (CR, GZU). Beach south of station, sea level, coastal rainforest zone, disturbed coastal forest and exposed trees along beach, on bark (stem), 7 Apr 2003, M. Grube 11655 (GZU). Sirena and Guanacaste round trail NW of station, sea level, lowland and coastal rainforest zone, old-growth coastal secondary forest and primary forest remnants on sandy soil, on bark (trunk), 7 Apr 2003, M. Grube 11534 (GZU), 11574 (CR, GZU); ibid., on bark (stem), 7 Apr 2003, M. Grube 11587 (CR, INB 0003722810); ibid., on bark (trunk), 7 Apr 2003, R. Lücking 16221 (F).

Thallus pale greenish to green, continuous, smooth and shiny, with isidia emerging from the surface, often delimited against other thalli by a brown prothalline line, 10–20 µm thick. Upper cortex ca 5 µm thick, composed of irregularly periclinally arranged hyphae. Algal layer continuous, 10–15 µm thick, sometimes subtended by a more or less hydrophobic layer, with yellow fluorescence in blue filter. Photobiont trentepohlioid, cells in chains, 5 µm thick. Isidia cylindrical to knotted, simple to coralloid, up to 0.4 mm tall and 20–30 µm diam. Ascomata black, irregularly rounded, up to 2.5 mm diam., hardly raised over thallus level, hydrophilic. Epithecium pale brownish, ca 20 µm thick, hyphal structure not different from hymenium. Hymenium more or less hyaline, ca 60–70 µm thick. Interascal hyphae in epithecium and hymenium 1–3 µm thick. Hypothecium brown, 20–30 µm thick, pigments on the cell wall or flake-like in intercellular spaces with interspersed ascogenous hyphae (FIG. 1). Asci often poorly developed, broadly clavate, 55–65 x 28–35 µm, with thickened lateral walls, thinner toward the indistinct stipe (Arthothelium-type), 8-spored; asci often degenerated and with aborted, brownish spores. Ascospores narrowly ovoid, 28–37 x 10–13 µm, hyaline to brownish when old, (3)5–6(8) septate, without perispore.

Chemistry. – Unknown crystallized pigment (TLC-run length classes A: 6/B': 6/C: 7) present below the algal layer and below the ascomata.

Substrate. – Smooth bark of smaller, often young trees.

	DISCUSSION

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The vegetation present at the type locality of Arthonia isidiata is classified as lowland tropical coastal rain forest (Holdridge 1967, Tosi 1969, Gomez 1986). The forest behind the littoral woodlands near Sirena Field Station thrives on a mixture of sandy and alluvial soils; the understory is sparse, and light conditions range from moderately exposed to rather shaded. A. isidiata grows frequently on trees with smooth bark in this area, while it was not found in the apparently drier forests with secondary character south of the field station.

The thallus of Arthonia isidiata is thin and shiny and distinctly epiphloedic, which is in contrast to many other corticolous Arthonia species but agrees with the foliicolous taxa of Arthonia and Eremothecella (Lücking 1995). Furthermore, the upper surface of the thallus consists of a thin layer of conglutinated hyphae, which resembles a thin upper cortex. Such a cortical layer was not observed so far in other corticolous Arthonia species, but a thin hyphal layer may be present in foliicolous taxa (Grube and Lücking 2001), and a thin corticiform layer is known from Amazonomyces. The isidia of Arthonia isidiata emerge from the algal layer and remain coated by the cortical layer and thus fit the definition for isidia given in the Dictionary of Fungi (Kirk et al 2001) in being photobiont-containing protuberances of the cortex. However, the term isidia also is applied in those species of Arthoniales that lack a distinct cortex, such as Cryptothecia rubrocinta and Dichosporidium nigrocinctum (Thor 1990, 1997). On the other hand, the isidia of Arthonia isidiata resemble the isidiate protuberances of certain Porina species common in the same habitat, such as P. tetracerae, P. conspersa and P. exasperatula. In the case of Porina, it has been questioned whether these structures are genuine isidia or adaptations to enhance gas exchange in a permanently humid environment (Harris 1995, Lücking and Vezda 1998). However, both functions are not mutually exclusive and the observation that the protuberances of Arthonia isidiata detach easily when the thalli are dry supports their interpretation as isidia.

Although most collected thalli of Arthonia isidiata had ascomata, ascospores were often poorly developed. Transversal sections of large ascomata (i.e., >1 mm diam) frequently contained asci with aborted, brownish ascospores, while sound ascopores were more common in young ascomata, indicating that mature thalli rely on the propagation by isidia.

Recent phylogenetic studies indicate that Arthonia in its traditional sense is a paraphyletic or polyphyletic entity, and some distinctive groups already have been recognized and in part formally segregated (Sérusiaux 1992, Grube et al 1995, Grube and Matzer 1997, Sundin and Tehler 1998, Lücking et al 1998, Grube 1998, 2001, Sundin 2000). Our new species does not belong to Arthonia in the strict sense (comprising A. radiata and A. punctiformis groups) but seems to be related to the bulk of chiefly foliicolous taxa presently assigned to Arthonia sensu lato on one hand and Eremothecella on the other. Corticolous taxa with flat, reddish brown ascomata, provisionally recognized as Coniangium (Sundin and Tehler 1998), also seem to belong in this group in a wide sense. Typical species of the latter group, however, have much tinier ascomata and asci of the Arthonia-type (Grube 1998). Molecular phylogenetic studies are required to elucidate natural relationships in this highly complex group of chiefly tropical taxa.

	ACKNOWLEDGMENTS

 
We like to thank the participants of the second Ticolichen foray for the convivial atmosphere in Costa Rica: José Luis Chaves, Eida Fletes, Daniela Lizano, Harrie Sipman, William Buck, Susan Will-Wolf, Marie Trest and Matthew Nelsen. We are grateful to Ulrike Grube for assistance with the chemical analyses and for providing a photograph of the isidia. The National Science Foundation, Biotic Surveys and Inventories, is acknowledged for financial support to the TICOLICHEN project (Grant DEB-0206125 to The Field Museum, PI Robert Lücking).

	FOOTNOTES

 
Accepted for publication April 26, 2004.

¹ Corresponding author. E-mail: rlucking{at}fieldmuseum.org

	LITERATURE CITED

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