Lactifluus volemus

''Lactifluus volemus'', formerly known as ''Lactarius volemus'', is a species of fungus in the family Russulaceae. It is widely distributed in the northern hemisphere, in temperate regions of Europe, North America and Asia as well as some subtropical and tropical regions of Central America and Asia. A mycorrhizal fungus, its fruit bodies grow on the ground at the base of various species of trees from summer to autumn, either individually or in groups. It is valued as an edible mushroom, and is sold in markets in Asia. Several other ''Lactifluus'' mushrooms resemble ''L. volemus'', such as the closely related edible species '' L. corrugis'', but these can be distinguished by differences in distribution, visible morphology, and microscopic characteristics. ''L. volemus'' produces a white spore print and has roughly spherical spores about 7–8 micrometres in diameter.

The colour of the ''L. volemus'' mushroom varies from apricot to tawny, and the cap may be up to 11 cm ( in) wide. The pale golden yellow gills on the underside of the cap are closely spaced and sometimes forked. One of the mushroom's most distinctive features is the large amount of latex ("milk") that it exudes when the gills are damaged, leading to the common names weeping milk cap and voluminous-latex milky. It also has a distinctive fishy smell, which does not affect the taste. The fruit bodies have been chemically analysed and found to contain several sterols related to ergosterol, some of which are unique to this species. The mushroom also contains a natural rubber that has been chemically characterized. Phylogenetic analysis suggests that ''Lactifluus volemus'' represents several species or subspecies, rather than a single taxon.

Taxonomy and naming

The first mention of ''Lactifluus volemus'' in the scientific literature was in Carl Linnaeus's 1753 ''Species Plantarum'', under the name ''Agaricus lactifluus''. In 1821, Swedish mycologist Elias Magnus Fries called it ''Agaricus volemus'' in his ''Systema Mycologicum''. In this work he proposed a grouping of related species (called a ''tribus'', or tribe) within the genus ''Agaricus'', which he named ''Galorrheus''. Fries later recognised ''Lactarius'' as a distinct genus in his 1838 ''Epicrisis Systematis Mycologici'', citing ''Galorrheus'' as a synonym. Although Linnaeus had published the species before Fries, Fries's name is sanctioned and thus has nomenclatural priority. In 1871 Paul Kummer raised most of Fries's tribes to generic rank, and so renamed the species ''Galorrheus volemus''. The variety ''L. volemus'' var. ''subrugosus'' was identified by Charles Horton Peck in 1879, but is now classified as a separate species, '' L. corrugis''. In 1891, Otto Kuntze moved the species into ''Lactifluus'', which was afterwards long considered a synonym of ''Lactarius'' but confirmed as a separate genus through molecular phylogenetics in 2008 and subsequent taxonomical rearrangements within the family Russulaceae.

Another historical synonym is ''Lactarius lactifluus'', used by Lucien Quélet in 1886, a renaming based on Linnaeus's ''Agaricus lactifluus''. ''Lactarius wangii'', reported by Hua-An Wen and Jian-Zhe Ying to be a new species from China in 2005, was synonymised two years later with ''L. volemus''.

The specific epithet "''volemus''" is derived from the Latin ''vola'', meaning "the hollow of the hand", suggestive of Fries's reference to the large amount of latex "flowing enough to fill the hand". Common names for ''L. volemus'' include the weeping milk cap, the tawny milkcap, the orange-brown milky, the voluminous-latex milky, the lactarius orange, the fishy milkcap, and the apricot milk cap. In the West Virginian mountains of the United States, the mushroom is called a "leatherback" or a "bradley". The latter name may originate from its German name ''Brätling''.Bessette ''et al''. (2009), p. 5.

Phylogeny

''Lactifluus volemus'' was the type species of the section ''Dulces'' in subgenus ''Lactarius'', and is currently classified in ''Lactifluus'' section ''Lactifluus''. The group around this ''L. volemus'' includes species with a dry cap, abundant latex, and a white or pale cream spore print. Because the closely related ''L. corrugis'' has overlapping morphological characters, including similar colouration in the cap and stem, it has been difficult to reliably distinguish between the two species. The difficulty in discerning the two is exacerbated by the fact that both species have several colour forms: Japanese specimens of ''L. volemus'' may have a red cap, a yellow cap with a long stem, or a velvet-like surface texture; the caps of ''L. corrugis'' may be either red, commonly rust-coloured. In 2005, Japanese researchers clarified the relationships between these two species and others in section ''Dulces'' using molecular phylogenetics, and by comparing differences in fatty acid composition, morphology, and taste. The colour variants group phylogenetically into different subclades, suggesting that they might better be considered as "different species, subspecies, or varieties". A 2010 molecular study of ''L. volemus'' of northern Thailand found that 79 tested specimens could be divided into 18 distinct phylogenetic species; six of these were described as new species: '' Lactifluus acicularis'', '' L. crocatus'', '' L. distantifolius'', '' L. longipilus'', '' L. pinguis'' and '' L. vitellinus''.

Description

Macromorphology

The fruit body of ''Lactifluus volemus'' has a fleshy and firm cap with a velvety or smooth surface and a shape that changes with maturity: it starts off convex, with edges curved inwards, then later grows flat with a depression in the middle. With a typical diameter of 5 to 11 cm (2– in), its colour ranges from apricot to tawny. The cap colouration, however, is somewhat variable, as has been noted in Asian, European, and North American specimens. The stem, whose height varies between 4 and 12 cm ( and 5 in), and which is typically between thick, has a slightly lighter colouration than the cap. It is firm, with a velvety or smooth surface that sometimes has depressions running longitudinally up and down its length. The gills are adnate to slightly decurrent, brittle, narrow, quite closely spaced and sometimes forked. Normally a pale golden yellow colour, the gills turn brown when bruised. Interspersed between the gills are lamellulae, short gills that do not extend to the stem. The flesh is whitish and firm. The mushroom smells somewhat fishy; one source suggests the odour is "like a dead shad

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