United States of America. Tennessee, Sevier County: near Keener House, Great Smoky Mountains National Park, under mixed beech and hemlock, 3 Aug 1939, A. H. Smith 9781 (MICH 11068, non vide).

Pileus 30–60 mm broad; conic to obtusely conic at first, becoming broadly conic to applanate, with a broad, low umbo; surface dry, radially appressed fibrillose, becoming radially rimose, finely squamulose over the disc, dull brown (664139) to medium orangish brown (784333), with a slightly darker center, bruising greenish blue (445253), negative with the application of 15% KOH; context up to 7 mm thick on the disc, whitish, unchanging or slowly turning greenish blue on exposure; odor strongly sweet and fragrant, with a spermatic component; taste not assessed. Lamellae adnate to uncinate; close; with several tiers of lamellulae; at first buff (ded1e2), becoming dull cinnamon brown (904e2e); occasionally staining greenish blue where bruised; edges concolorous, not conspicuously fimbriate. Stipe 50–70 × 3–4 mm; equal above a rimmed, bulbous base; surface dry, densely pruinose the entire length; cortina not observed and presumably absent; dull brown (664139) to pinkish brown (ad8796), bruising greenish blue to blackish blue (33363d); basal bulb 6–8 mm broad, subglobose, rimmed, whitish; basal mycelium whitish; context dull cinnamon brown (735c3a), unchanging or slowly turning greenish blue on exposure, becoming hollow.

Basidiospores 8–11 × 6–9 μm (including nodules); average Q = 1.23; subelliptic to subcruciate or irregular in outline; stellate or prominently nodulose, with 7–11 conic to subconic nodules visible; ochraceous to golden or brownish in KOH; orangish in Melzer’s reagent; not cyanophilic. Basidia 35–40 × 8–11 μm; clavate to subclavate; 4-sterigmate; hyaline in KOH; not reviving well after sporulation. Pleurocystidia 50–65 × 15–23 μm; widely lageniform or, less commonly, subcylindric, subutriform, or subglobose, with a short basal pedicel; thick-walled (walls 1–3 μm thick), hyaline in KOH; apices crystalliferous; frequent. Cheilocystidia similar to pleurocystidia, frequent; paracystidia 20–30 × 5–8 μm, clavate to abruptly clavate, thin-walled, hyaline in KOH, basally clamped. Lamellar trama parallel, hyphae 4–15 μm wide, hyaline in KOH; elements of subhymenium narrow and cylindric, 1–2 μm wide. Caulocystidia 55–75 × 12–20 μm, similar to hymenial cystidia, in fascicles with paracystidia, frequent near stipe apex, less frequent near stipe base; elements of stipe trama parallel, 3–10 μm wide, smooth, hyaline in KOH. Pileipellis a radially oriented cutis; hyphae cylindric, 2–7 μm wide, walls smooth or finely encrusted, hyaline to brown in KOH, often clamped; cells of pileal trama 5–15 μm wide, cylindric to somewhat inflated, smooth, hyaline to brownish in KOH, septate, clamped, walls 0.5–1 μm thick. Clamp connections present.

Eastern North America; central Illinois and east Tennessee.

Growing scattered and gregariously on mossy soil under Carya glabra (Mill.) Sweet, Acer saccharum Marsh., Quercus velutina Lam., Quercus imbricaria Michx., and Carya ovata (Mill.) K. Koch. July. Fruiting in June (type recorded in August).

None. Known only from two collections.

United States of America. Illinois: Shelby County, Hidden Springs State Forest, 39°18.59'N; 88°41.29'W, 10 Jun 2011, M. Kuo 07101101 (NY, TENN 066447).

The Illinois collection described agrees with the macroscopic description of Smith (1941) in all details except the “somewhat bulbous base” recorded by Smith (basidiomes in Kuo 07101101 feature prominent, rimmed basal bulbs) and the color of the bruising, recorded by Smith as “greenish,” “sordid green,” and “sordid greenish gray” (basidiomes in Kuo 07101101 bruised greenish blue to blackish blue). The microscopic features recorded by Smith are also in agreement with those of Kuo 07101101, except for a slight difference in basidiospore morphology; Smith recorded spores with dimensions of “(8) 9–12 × (6) 7–10 μ” and 9–13 nodules, somewhat larger than the Illinois collection. Aside from these subtle differences, we find our current collection in complete agreement with the description of the type, which consists of but a portion of a single basidiome. While it is not possible to draw conclusions about the demographics of this species (now known from only two collections), we did observe genetic heterogeneity in all three gene regions sequenced.

Based on a multi-gene phylogenetic analysis (Fig. 1), I. insignis is most closely related, and indeed the sister group to, a small consortium of species from southeast Asia and Australasia. Inocybe pileosulcata E. Horak, Matheny & Desjardin has been recorded from Thailand and Malaysia in association with a wide assortment of ectomycorrhizal plant associates (Dipterocarpus, Shorea, Castanopsis, Pinus) in tropical lowland and montane forests (Horak et al. 2015). We are also aware of this species from dipterocarp forests in tropical regions of India. The two provisionally named Australasian species, I. vagata Matheny & Bougher ined. (=I. asterospora sensu E. Horak) and I. nobilissima Matheny & Bougher ined., occur in Australia and/or New Zealand, and have been recorded in forests dominated by sclerophyll vegetation (Eucalyptus, Leptospermum, Allocasuarina, Acacia) and/or mixed with Nothofagus in New Zealand. Despite the wide geographical disjunction in this group, all of these species share the presence of stellate-shaped basidiospores, distinct marginate basal bulb, and conspicuous odor, typically spermatic. All three Australasian and Asian species discussed above lack the strong sweet smell (but with a spermatic component) and the greenish blue staining of basidiomes that characterize I. insignis.

Collections of Inocybe xanthomelas Boursier & Kühner could possibly be confused with I. insignis due to their discoloration to a fuscous or grayish black color especially after drying. However, I. xanthomelas does not discolor greenish blue or blackish blue, nor is it closely related phylogenetically to I. insignis. Other species from Europe have been documented with nodulose spores, a marginate stipe base, and flesh that changes color, particularly upon drying. The taxonomic status of these species has been addressed by Esteve-Raventós et al. (2015), but none of these have the stellate spores that characterize I. insignis, and all are phylogenetically remotely related to the I. asterospora group (Ryberg et al. 2010).

The biogeographical relationship exhibited here (Fig. 1) – a disjunct relationship between eastern North America and southeast Asia and Australasia – is one not often recorded in mushroom-forming fungi. Disjunct relationships between eastern North America and temperate east Asian species have been suggested in various groups of macrofungi (Wu and Mueller 1997; but see Mueller et al. 2001) and between eastern North America and the Caribbean (Baroni et al. 1997). In other groups some cohesion between Asia and western North American macrofungal species has been reported (Petersen and Hughes 2003). A group worthy of investigation that might share a similar southeast Asia/Australasia-eastern North American disjunct is Gloeocantharellus (Corner 1969, Giachini et al. 2010). However, northern South American species have also been ascribed to Gloeocantharellus, and this element is apparently missing from the I. asterospora group.