Lactarius bisporus

Lactarius bisporus Verbeken & F. Hampe

Mycobank number: MB 807217

Basidiocarps angiocarpous, semihypogeous, 10–35 mm diam., 10–15 mm high, subglobose, rather regular. Peridium with minutely velutinous surface, chamois-leather-like, ochraceous yellow to yellow-brown. Stipe absent. Columella absent. Gleba whitish, strongly labyrinthuloid, with small loci, with some, but very few, gelatinous veins between the loci. Latex scarce, whitish hyaline, unchanging on the gleba and unchanging on white paper, unchanging with KOH. Taste bitter to astringent, disagreeable but not burning acrid. Smell not remarkable. Basidiospores globose to subglobose, 9.8–11.3–13.0 × 9.8–11.2–12.5 µm, Q = 1.00–1.01–1.04, n = 20; ornamentation amyloid, consisting of isolated, up to 2(3) µm high spines and isolated warts; spines conical, subacute, with rounded to tapering, rarely forked apex; apiculus up to 2.5(3) µm long; plage not distinct, inamyloid. Basidia 2-spored, subcylindrical to clavate, sometimes irregular and somewhat tortuous, 45–55 × 12–18 µm, with 5–12 µm long sterigmata. Macrocystidia very variable in shape, clavate, subcapitate, ventricose or lageniform, 25–55 × 12–20 µm, sometimes slightly but distinctly thick-walled. Pseudocystidia present. Peridiopellis consisting of 2–3 µm broad hyphae which are rather periclinally arranged in the subpellis, but form a densely intricate and interwoven layer on top and are embedded in a gelatinous matrix.

Habitat: primary tropical forest with Dipterocarpus tuberculatus Roxb. Known only from the type collection.

Notes: The ITS analysis shows that this new species of truffle-like Russulales belongs to the genus Lactarius, as all other known angiocarpous milkcaps known so far. While it is obvious from the tree that the angiocarpous habit evolved several times in the three major clades of the genus, representing L. subg. Piperites, L. subg. Russularia and L. subg. Plinthogalus, we see that this new Thai species is most closely related with L. pomiolens, a species from Sri Lanka (Verbeken et al. 2014). Lactarius pomiolens, also an angiocarpous taxon, forms one of the earlier diverging lineages in the genus and has not been assigned to any of the existing subgenera. Both angiocarpous species L. bisporus and L. pomiolens form a separate isolated small clade as does the group around L. kabansus Pegler & Piearce, a tropical African species showing some superficial resemblances with L. subg. Plinthogalus and the morphologically very isolated dark-spored L. chromospermus Pegler. Morphologically, this new species clearly differs from L. pomiolens by the 2-spored basidia and the isolated spines and warts as spore ornamentation. The spores in L. pomiolens are extremely large for spores formed by 4- spored basidia (10.5–12.0–13.3 × 10.2–11.5–12.8 µm) and have very high wings. As already stated before (Verbeken et al. 2014) the spore ornamentation within truffle-like milkcaps shows large differences: isolated spines or warts on one hand, high ridges on the other hand, but this character does not seem to contain much phylogenetic signal. Furthermore, L. pomiolens has latex that turns immediately sulphur yellow while the latex in L. bisporus is white and unchanging. As the name indicates, L. pomiolens is characterized by a strong and sweet apple-like smell. Many species with normally 4-spored basidia show a low percentage of 2-spored ones, but exclusively 2-spored basidia are rare in the genus. It is remarkable that though exclusively 2-spored basidia are very exceptional in agaricoid milkcaps (such as e.g. L. acerrimus Britzelm.), they seem to be more common in angiocarpous representatives. Other Asian 2-spored angiocarpous milkcap species are L. echinus Stubbe & Verbeken, L. falcatus Verbeken & Van de Putte, Arcangeliella lactifera (B.C. Zhang & Y.N. Yu) J.M. Vidal and A. nanjingensis (B. Liu & K. Tao) J.M. Vidal. Both L. echinus and A. lactifera differ from the newly proposed species by the longer (up to 4 µm) and more regular spines. Lactarius falcatus has lower spines, at most 2 µm long, which are typically curved at the apex and also differs macroscopically by latex that is turning immediately bright pale yellow. Martellia nanjingensis has smaller spores (8– 11.5 × 7.5–10 µm) with spines up to 1.5 µm long and differs macroscopically by the brown to dark brown basidiocarps (Tao et al. 1993).