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Caeoma cassiopae sp. nov., a rust on Cassiope tetragona in the Canadian Rocky Mountains

     Northern Forestry Centre, Canadian Forest Service, Natural Resources Canada, 5320-122 Street, Edmonton, AB, T6H 3S5 Canada

	ABSTRACT

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Caeoma cassiopae sp. nov. (Uredinales) is described on the arctic–alpine shrub Cassiope tetragona. It was found in three locations in the Rocky Mountains of west-central Alberta, and is the first rust reported on the genus Cassiope. The sori resemble the uredinia of species in the genus Chrysomyxa. The host in the Ericaceae also suggests affinity with that genus. However, the spore morphology, studied by light and scanning electron microscopy, does not resemble known species of Chrysomyxa.

Key words: arctic–alpine plant, Chrysomyxa, Ericaceae, mountain heather, Uredinales, uredinia

	INTRODUCTION

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Cassiope tetragona (L.) D. Don (Ericaceae) is an arctic–alpine shrub with a circumboreal distribution. It is a mat-forming, heath-like plant with evergreen, scale-like leaves. During a study in the Rocky Mountains of west-central Alberta, Canada, of the root endophytes of alpine ericaceous plants (Hambleton and Currah 1997), a rust fungus was discovered on the leaves of C. tetragona at Outpost Lake, Jasper National Park. It was later found in two other locations in the Rocky Mountains. In both cases, just one spore state was present, believed to be uredinia. No previous records of rust fungi (Uredinales) on this plant genus could be found. The rust has likely been overlooked previously because of its inconspicuous location inside the groove on the abaxial leaf surface and on the adaxial surface of the leaves, which are closely adpressed to the stem. It is therefore described as a new anamorphic rust species, Caeoma cassiopae.

	MATERIALS AND METHODS

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Spores and hand sections of leaves bearing sori were mounted in lactophenol or lactophenol–cotton blue for light microscopic examination. Spore size was measured under brightfield microscopy at 400x, and wall thickness and wart height were measured at 1000x. For scanning electron microscopy, spores were dusted onto aluminum stubs coated with adhesive; they were then coated with gold and examined with an Hitachi S-510 scanning electron microscope operated at 15 kV. Images were captured with Quartz PCI software, version 4.0, and the plate was composed using Adobe Photoshop 5.0.

	DESCRIPTION

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Caeoma cassiopae P.E. Crane, sp. nov. Figs. 1–6

View larger version (179K): [in this window] [in a new window]    FIG. 1–6. Caeoma cassiopae on Cassiope tetragona. 1. Shoot showing position of uredinia (arrows) in the abaxial groove of leaves of the previous year. Lower leaf has been pulled away from the stem. 2. Cross section of leaf with uredinium (arrow) in groove of the abaxial leaf surface. 3. Scanning electron micrograph of urediniospores, showing variable size and shape. 4. Light micrograph of urediniospores. 5. Scanning electron micrograph of two spores, showing crowded warts, occasionally joined laterally. 6. Closer view of cylindrical, evenly distributed, annulate warts. Scale bars: 1 = 1 mm, 2 = 0.3 mm, 3 = 50 µm, 4 = 25 µm, 5 = 10 µm, 6 = 5 µm

In Cassiopea tetragona, folia infecta saepe rubra vel flava, vel extremis rubra. Uredinia amphigenosa in foliis anni superioris, plerumque in fossa superficiei abaxialis, 0.3–0.5 mm lata, subepidermalia, erumpentia, cum peridio inconspicuo prope basim sori. Urediniosporae valde aurantiacae, catenulatae, cum cellulis intercalaribus solum conspicuis prope basim sori; forma et magnitudine variae, globosae, subglobosae, aliquando ellipsoideae vel ovoideae vel fusiformes, 20–40 x 14–22 ( = 25.4 ± 4.6 x 18.8 ± 2.2) µm, verrucosae; verrucae pariter sparsae per superficiem totam sporae, cylindricae, congregatae, plerumque discretae, aliquando in lateribus coniunctae, annulatae, cum summis asperis, coniunctiones basales intra verrucas tenuissimae vel absunt; tunica sine colore, tenuissima, tunica cum verrucis 1.5–2.5 µm crassa; foramina germinum non videntur.

On Cassiope tetragona, infected leaves often discolored red or yellow, or red-tipped. Uredinia amphigenous on leaves of previous year, most common in groove of abaxial surface, 0.3–0.5 mm wide, subepidermal, erumpent, with rudimentary peridium near base of sorus. Urediniospores deep orange, catenulate, with intercalary cells visible only at base of sorus; variable in shape and size, globose, subglobose, occasionally ellipsoidal, ovoid, or fusiform, 20–40 x 14–22 ( = 25.4 ± 4.6 x 18.8 ± 2.2) µm, verrucose; warts evenly distributed over entire spore surface, cylindrical, crowded, mostly discrete, occasionally joined laterally, annulate, with bumpy tops, basal connections between warts very fine or lacking; wall colorless, very thin, wall including warts 1.5–2.5 µm thick; germ pores not seen.

Specimens examined. CANADA. ALBERTA: On slope below Cadomin Caves, Cadomin, 9 Jul 2001, P.E. Crane and A. Crane, DAOM 230243 (HOLOTYPE), CFB 22213, PUR N2535 (ISOTYPES); near Outpost Lake, elev. 2010 m, Jasper National Park, AB, 30 Jun 1996, S. Hambleton, CFB 22042; Peyto Lake, Banff National Park, 51°42' N, 116°28' W, 6 Sep 2001, P.E. Crane and A. Crane, CFB 22214.

	DISCUSSION

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This previously undescribed rust was placed in Caeoma based on the morphology of the sori. The genus Caeoma Link is traditionally used for sori that lack obvious bounding structures and that produce catenulate spores with intercalary cells (Cummins and Hiratsuka, in press). Similar sori are found as the aecial state of Melampsora and the uredinia of Chrysomyxa, Coleosporium, and other genera. Although the function of the spores in the life cycle of Caeoma cassiopae is uncertain, the sori are assumed to be uredinia rather than aecia because spermogonia are lacking. Furthermore, the rust occurs on a host in the Ericaceae, the plant family most commonly infected by members of Chrysomyxa, and the sori and spores resemble the uredinia and urediniospores of that genus (Crane 2000). This rust might also be endocyclic, producing basidia upon germination of the spores; spore germination was not done to examine this possibility. Until telia are found, its affinity cannot be confirmed. Two of the collections were made in late June and early July, and at this time snow was still present at Outpost Lake. If Caeoma cassiopae behaves as most species of Chrysomyxa, one would expect telia at this time of the growing season. It is probable, therefore, that the rust has a reduced life cycle, existing only in the asexual state because of the short growing season, or telia may be rare, as with Chrysomyxa empetri (Faull 1937). The rust likely overwinters as mycelium in the evergreen leaves of the host.

Caeoma cassiopae differs in spore morphology from other rusts found on ericaceous plants in North America and Europe (Crane 2000). The spores are evenly warted and lack the vertical smooth area or groove found in urediniospores of most species of Chrysomyxa, including C. empetri (Crane 2000). Spores of Chrysomyxa neoglandulosi P.E. Crane, found on Ledum glandulosum, also lack this surface feature, but the warts are much finer, spores are typically globose to subglobose, and they have a much more restricted size range than the spores of Caeoma cassiopae. Recent studies (Crane 2000, 2001, and unpubl) of the genus Chrysomyxa show that some species have very consistent spore size and shape, whereas others vary considerably in these characters within a sample. This regularity, or lack of it, therefore, may itself be considered a character. Although Chrysomyxa nagodhii P.E. Crane on Ledum groenlandicum Oeder has a large range in urediniospore size, similar to that of Caeoma cassiopae, the urediniospores of the former species have a smooth appearance and lack the discrete, evenly distributed surface warts found in Caeoma cassiopae (Crane 2001). It is also unlikely that Caeoma cassiopae is conspecific with any of the Chrysomyxa species occurring exclusively in Asia. Although life cycles are incompletely known for many of these, most lack uredinia, being either microcyclic on their conifer host or heteroecious and demicyclic. The two species confirmed to have uredinia, Chrysomyxa dietelii Syd. and C. succinea (Sacc.) Tranz., both occur on rhododendrons, hosts that are usually placed in a different subfamily from Cassiope in the Ericaceae (Anderberg 1993). Chrysomyxa dietelii has narrower, more elongated urediniospores with coarser ornamentation than C. cassiopae (Sydow and Butler 1907, P.E. Crane unpubl). Distinctive, stalked telia are characteristic of C. succinea, and uredinia occur only on specific rhododendron hosts (Hiratsuka and Sato 1969). For these reasons it is unlikely that they are conspecific with the rust on Cassiope.

The overall spore morphology of Caeoma cassiopae most closely resembles that of urediniospores of Chrysomyxa chiogenis Dietel on Gaultheria and Chrysomyxa vaccinii (Ziller) P.E. Crane on Vaccinium parvifolium Smith (Crane 2001). The range of spore size is also similar, but Caeoma cassiopae has a maximum spore length of 40 µm, whereas the other two species have maximum length of 32 µm. The new rust does not have warts joined into maze-like ridges, a characteristic of C. chiogenis. The surface morphology of the spores also differs from C. vaccinii, which has flat-topped, less-crowded warts that are variable in size and shape and have more complex basal connections.

Rust fungi are thought to have coevolved with their host plants and can sometimes be used to infer relationship of the hosts. Interestingly, the plant genera Cassiope, Gaultheria, and Vaccinium are considered by some authors to belong to the same subfamily, Vaccinioideae, of the Ericaceae (Stevens 1971, Cullings 1994, Kron 1997), but the position of Cassiope is controversial (Anderberg 1993, Judd and Kron 1993, Kron 1997). The presence of similar rust fungi on the three plant genera might support other evidence that they are closely related. However, because the observed spore state provides so few characters for comparison of these rust taxa, their proposed close relationship is at present speculative. A phylogenetic analysis of the rust fungi specific to the Ericaceae sensu lato might contribute to an understanding of the relationship among the host genera.

Although the rust flora of North America is thought to be fairly well documented, the discovery of this rust and recent clarification of the identity of related rusts on Ericaceae and Picea (Crane 2001) indicate that the diversity and basic biology of these pathogens are still incompletely known on this continent.

	ACKNOWLEDGMENTS

 
I thank Sarah Hambleton for bringing this rust to my attention (collected under Parks Canada research permit) and for helpful comments on the manuscript. I also thank Yasu Hiratsuka for helpful comments and discussions.

	FOOTNOTES

 
¹ Corresponding author, pacrane{at}nrcan.gc.ca

Accepted for publication June 27, 2002.

	LITERATURE CITED

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS DESCRIPTION DISCUSSION LITERATURE CITED

 
Anderberg AA., 1993 Cladistic interrelationships and major clades of the Ericales. Plant Syst Evol 184:207-231

Crane PE., 2000 Systematics and biology of the genus Chrysomyxa (Uredinales) [Ph.D. Thesis]. Edmonton, Alberta: University of Alberta. 249 p

———. 2001 Morphology, taxonomy, and nomenclature of the Chrysomyxa ledi complex and related rust fungi on spruce and Ericaceae in North America and Europe. Can J Bot 79:957-982

Cullings K., 1994 Molecular phylogeny of the Monotropoideae (Ericaceae) with a note on the placement of the Pyroloideae. J Evol Biol 7:501-516

Cummins GB, Hiratsuka Y., 2002 Illustrated genera of rust fungi. 3rd ed. Minneapolis, Minnesota: APS Press (In press)

Faull JH., 1937 Chrysomyxa empetri—a spruce-infecting rust. J Arnold Arbor 18:141-148

Hambleton S, Currah RS., 1997 Fungal endophytes from the roots of alpine and boreal Ericaceae. Can J Bot 75:1570-1581

Hiratsuka N, Sato S., 1969 Notes on Chrysomyxa on species of Rhododendron. Trans Mycol Soc Jpn 10:14-18

Judd WS, Kron KA., 1993 Circumscription of Ericaceae (Ericales) as determined by preliminary cladistic analyses based on morphological, anatomical, and embryological features. Brittonia 45:99-114

Kron KA., 1997 Phylogenetic relationships of Rhododendroideae (Ericaceae). Am J Bot 84:973-980[Abstract]

Stevens PF., 1971 A classification of the Ericaceae: subfamilies and tribes. Bot J Linnean Soc 64:1-53

Sydow HJ and P, Butler EJ., 1907 Fungi Indiae orientalis. Ann. Mycol Berlin 5:502-503

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