(L.) Roussel (1796)
|pores on hymenium|
|cap is convex|
hymenium is adnateor subdecurrent
|stipe has a ring|
|spore print is brown|
|ecology is mycorrhizal|
Suillus luteus is a basidiomycete fungus, and the type species of the genus Suillus. It is a common fungus indigenous to coniferous forests of Eurasia, from the British Isles to Korea, and North America, and introduced to southern Australia and New Zealand. Commonly referred to as slippery jack or sticky bun in English-speaking countries, its names refer to the brown cap, which is characteristically slimy in wet conditions. It was initially described as Boletus luteus by Carl Linnaeus, though is now classified in a different family as well as genus. Suillus luteus is edible, though not as highly regarded as other bolete mushrooms, and is commonly prepared and eaten in soups and stews. The slime coating, however, may cause indigestion if not removed before eating.
The fungus grows in deciduous and coniferous forests and tree plantations, forming symbiotic ectomycorrhizal associations with living trees by enveloping the tree's underground roots with sheaths of fungal tissue. The fungus produces spore-bearing fruit bodies, often in large numbers, above ground in summer and autumn. The fruit body cap often has a distinctive conical shape before flattening with age, reaching 13 cm (5 in) in diameter. Like other boletes, it has tubes extending downward from the underside of the cap, rather than gills; spores escape at maturity through the tube openings, or pores. The pore surface is yellow, and covered by a membranous partial veil when young. The pale stipe, or stem, bears a distinctive ring, unlike most other boletes, and is up to 10 cm (4 in) tall and 3 cm (1.2 in) thick.
Taxonomy and naming
The slippery jack was one of the many species first described by the "father of taxonomy" Carl Linnaeus in volume two of his Species Plantarum in 1753, giving it the name Boletus luteus. The specific epithet is the Latin adjective lūtěus "yellow". It was reclassified in (and became the type species of) the genus Suillus by French naturalist Henri François Anne de Roussel in 1796. Suillus is an ancient term for fungi, and is derived from the word "swine". In addition to the British Mycological Society approved name "slippery jack", other common names that have been given to the fungus include "pine boletus" and "sticky bun".
August Batsch described Boletus volvatus (the specific epithet derived from the Latin volva "sheath", "covering" or "womb") alongside B. luteus in his 1783 work Elenchus Fungorum, placing both (along with B. bovinus in subordo Suilli). Several authors have placed it in other genera: Petter Karsten classified it as Cricunopus luteus in 1881—the genus Cricinopus defined by yellow adnate tubes, Lucien Quélet classified it as Viscipellis luteus in 1886, and Ixocomus luteus in 1888, and Paul Christoph Hennings placed it in the section Cricinopus of the genus Boletopsis in 1900.
In works published before 1987, the slippery jack was written fully as Suillus luteus (L.:Fr.) Gray, as the description by Linnaeus had been name sanctioned in 1821 by the "father of mycology", Swedish naturalist Elias Magnus Fries. The starting date for all the mycota had been set by general agreement as January 1, 1821, the date of Fries's work. Furthermore, as Roussel's description of Suillus predated this as well, the authority for the genus was assigned to British botanist Samuel Frederick Gray in the first volume of his 1821 work A Natural Arrangement of British Plants. The 1987 edition of the International Code of Botanical Nomenclature changed the rules on the starting date and primary work for names of fungi, and names can now be considered valid as far back as May 1, 1753, the date of publication of Linnaeus's work.
In their 1964 monograph on North American Suillus species, Alexander H. Smith and Harry Delbert Thiers classified S. luteus in the series Suilli of the section Suillus in genus Suillus. This is a grouping of related species characterized by the presence of either a ring on the stipe, a partial veil adhering to the cap margin, or a "false veil" not attached to the stipe but initially covering the tube cavity. Molecular phylogenetic analyses of ribosomal DNA sequences show that the most closely related species to Suillus luteus include S. pseudobrevipes, which is a sister species, and S. brevipes and S. weaverae (formerly Fuscoboletinus weaverae).
Chemical analysis of pigments and chromagens showed that Suillus was more closely related to Gomphidius and Rhizopogon than to other boletes, and hence Suillus luteus and its allies were transferred to the family Suillaceae from Boletaceae in 1997. Molecular studies have reinforced how distantly related these fungi are from Boletus edulis and its allies.
The cap is chestnut-, rusty- olive- or dark brown and generally 4-10 cm (rarely to 13 cm) in diameter at maturity. The cap has a distinctive conical shape, later flattening out. It is slimy to the touch, bare, smooth, and glossy even when dry and the cuticle is easily peeled off. The tiny, circular pores of the tubes are at first light yellow but turn olive to dark yellow with maturity. Like the skin of the cap, they can be readily peeled away from the flesh.
Tubes comprising the hymenophore on the underside of the cap are 3–7 mm (0.1–0.3 in) deep, with an attachment to the stipe ranging from adnate to somewhat decurrent. The pores are tiny, numbering 3 per mm in young specimens, and 1–2 per mm in maturity. The stipe is 5–10 cm (2.0–3.9 in) tall and 2–3 cm (0.8–1.2 in) wide. It is pale yellow and more or less cylindrical but may bear a swollen base. A membranous partial veil initially links the stipe with the edge of the cap. When it ruptures, it forms a membranous, hanging ring. The top side of the ring is whitish, while the underside is characteristically dark brown to violet. This species is one of the few members of the genus Suillus that sport such a ring. Above the ring, the stipe features glandular dots—minute clumps of pigmented cells. Below the ring, the stipe is dingy white, sometimes streaked with brownish slime. In humid conditions, the ring has a gelatinous texture. The white flesh of the entire fungus does not discolour when damaged, and is soft—particularly in mature specimens. It has a "pleasant" taste and lacks any distinctive odour.
The spore print is ochre or clay-coloured, the elongated elliptical spores measuring 7–10 by 3–3.5 μm. Basidia (spore-producing cells) are four-spored, with dimensions of 14–18 μm. Cystidia are present on both the tube faces (pleurocystidia) and edges (cheilocystidia) either scattered or, more rarely, as bundles. They measure 20–35 by 5–7 μm and have a narrow club-shape. Clamp connections are not present in the hyphae of S. luteus.
Suillus luteus is often confused with Suillus granulatus, which is another common, widely-distributed and edible species occurring in the same habitat. Suillus granulatus is yellow-fleshed and exudes latex droplets when young but most conspicuously, it bears neither partial veil nor ring. Other than that, Suillus luteus is unlikely to be confused with other mushrooms, especially if its preferred home under pine trees and the whitish partial veil are taken into consideration. In Europe, the related Suillus grevillei is found under larch and has a yellow ring, while immature fruit bodies of Gomphidius glutinosus may look similar from above but have gills rather than pores underneath. In North America, the similar Suillus borealis and S. pseudobrevipes are similar with veils but lack the distinctive ring of S. luteus. In some specimens of S. luteus, the partial veil separates from the stipe (rather than the cap margin), leaving cottony patches of veil hanging from the cap margin. In this state, fruit bodies can be confused with those of S. albidipes. Unlike S. luteus, however, S. albidipes does not have glandular dots on its stipe.
Distribution, habitat, and ecology
Suillus luteus can be found all over the Northern Hemisphere. It is widespread across the British Isles. It is found as far east as South Korea. In North America it is found in the northeast, the Pacific Northwest and the southwestern United States. It is found in coastal and montane pine forests and exhibits a tolerance of the northern latitudes. It has also been widely introduced to Southern Hemisphere locales with plantation pines, including South America, Africa, Australia, and New Zealand. It is particularly common in plantations in Patagonia. In southwestern Australia, it is limited to areas of greater than 1000 mm (40 in) annual rainfall. The fungus fruits in spring, summer and fairly prolifically in autumn, following periods of wet weather. Mushrooms can appear in large troops or fairy rings.
Suillus luteus forms mycorrhizal associations with various species of pine, which include Pinus sylvestris, Pinus nigra, and Pinus peuce in Europe, and Pinus resinosa and Pinus strobus in North America. It does not require a specific soil but seems to prefer acidic and nutrient-deficient soil. The fungus has been shown to provide a protective effect against heavy metal toxicity when associated with the host Pinus sylvestris, preventing copper accumulation in the needles, and protecting seedlings against cadmium toxicity. The fungus produces hydroxamic acid-based siderophores, which are compounds that can chelate iron and extract it from the soil in nutrient-poor conditions.
Suillus luteus is an edible mushroom. Although some authors regard it as one of low quality, and generally inferior to co-occurring species such as Boletus pinophilus, the species is considered a delicacy in Slavic cultures (known as maslyata in Russian or maślaki in Polish, derived from the word for buttery). It was highly regarded in Calabria, even more than Boletus edulis, until the 1940s when increased interest in the latter species eclipsed the former.
Mushrooms conforming to Suillus luteus are imported to Italy from Chile.
Slippery jacks are frequently marinated, fried, or stewed (used both fresh or dried), and are known for their tendency to maintain very light flesh color during cooking if the skin is peeled beforehand. S. luteus and other Suillus species may cause allergic reactions in some people or digestive problems. The fungus is better cooked before eating, and some authors recommend discarding the glutinous cuticle and tubes before cooking. However, some people may find slippery jacks excessively slippery. Gastrointestinal symptoms could be due to high levels of arabitol.
Suillus luteus fruit bodies are sometimes infested with larvae, though not nearly as often as S. granulatus or Boletus edulis. Damage from maggots is much more common in warmer months, and rare late in the season with cooler weather. In a Finnish study, researchers found that 70–95% of fruit bodies in an intermediate stage of growth were infested with larvae. In contrast, fruit bodies collected from pine plantations were be relatively free of larvae.
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