Suillus luteus

Suillus luteus is a species of bolete fungus commonly referred to as slippery jack or sticky bun. The fungus was originally assigned to Boletus by Carl Linnaeus in 1753, before being made the type species of Suillus in 1796. The slimy cap is typically up to 10 cm (4 in) wide and conical before flattening. A membranous partial veil connects the cap to the stem when young. Yellow tubes extend downward from the underside of the cap, which spores escape through. The pale stem is up to 10 cm (4 in) tall and 3 cm (1+1⁄4 in) thick, bearing small dots near the top. Unlike most other boletes, it bears a distinctive membranous ring, tinged brown to violet on the underside.

A common fungus native all across Eurasia from Ireland to Korea, it has been introduced widely elsewhere, including North and South America, southern Africa, Australia and New Zealand. The fungus grows in coniferous forests in its native range, and pine plantations in countries where it has become naturalized. It forms symbiotic ectomycorrhizal associations with living trees by enveloping the tree's underground roots with sheaths of fungal tissue. The fruit bodies are often produced in large numbers above ground in summer and autumn.

The mushroom is edible, though not as highly regarded as other boletes. It is commonly prepared and eaten in soups, stews or fried dishes. The slime coating, however, may cause indigestion if not removed before eating. It is often sold as a dried mushroom.

The species was one of the many first described in 1753 by the "father of taxonomy" Carl Linnaeus, who, in the second volume of his Species Plantarum, gave it the name Boletus luteus. The specific epithet is the Latin adjective lūtěus, meaning "yellow". The fungus was reclassified as (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 swine. In addition to the British Mycological Society approved name "slippery jack", other common names for this bolete include "pine boletus" and "sticky bun"—the latter referring to its resemblance to the pastry.

German naturalist August Batsch described Boletus volvatus (the specific epithet derived from the Latin volva, meaning "sheath", "covering" or "womb") alongside B. luteus in his 1783 work Elenchus Fungorum. Batsch placed both of these species, along with B. bovinus and the now obsolete names Boletus mutabilis and B. canus, in a grouping of similar boletes he called "subordo Suilli". Boletus volvatus is now considered a synonym of Suillus luteus. Several authors have placed the slippery jack in other genera: Finnish mycologist 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 1 January 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 1 May 1753, the date of publication of Linnaeus's work. In 1986, a collection of fruit bodies from Sweden was designated as the neotype of Suillus luteus.

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 group is characterized by the presence of either a ring on the stipe (or stem), a partial veil adhering to the cap margin, or a "false veil" not attached to the stipe but initially covering the tube cavity. Species closely related to Suillus luteus include S. pseudobrevipes (a sister species), S. brevipes and S. weaverae (formerly Fuscoboletinus weaverae). A genetic study of nucleotide DNA reinforced the species' monophyly and low genetic divergence, with material of S. luteus from the United Kingdom, Austria, Germany and North America forming a clade, in contrast with some other species, such as S. granulatus, which were shown to be polyphyletic.

Chemical analysis of pigments and chromogens showed that Suillus was more closely related to Gomphidius and Rhizopogon than to other boletes, and hence Suillus luteus and its allies were transferred from the Boletaceae to the newly circumscribed family Suillaceae in 1997. Molecular studies have reinforced how distantly related these fungi are from Boletus edulis and its allies.