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Page number:239 
Remarks (internal):It is relatively difficult to find fresh basidiomata of R. myrmecobroma that have not been extensively damaged by ants. While ants will disarticulate and carry off pieces of a variety of fungal fruiting structures, they seem to be especially attracted to R. myrmecobroma, and in Guyana only R. batistae Singer seems to be more attractive. Microscopically both fungi appear to have relatively primitive traits such as an abundant gloeoplerous system with embedded dermatocystidia in the pileipellis and abundant macrocystidia of two types deeply rooted in the hymenium, all strongly positive in sulfovanillin. Both also have an acrid to strongly acrid taste. It is unknown whether any of these characteristics serve as an attractant to the ants.
From an ecological standpoint, R. myrmecobroma may be a quite important mycorrhizal symbiont in Guyana’s Dicymbe forests. In a recent study by Smith et al. (2011) in the Upper Potaro Basin, R. myrmecobroma was the most frequently ECM fungus out of 118 taxa recovered by molecular methods on ECM roots of 19 trees each of three sympatric host species, D. corymbosa, D. altsonii, and the papilionoid Aldina insignis. In a second study (ME Smith & TW Henkel, unpublished) investigating mycorrhizal symbionts of the ECM dipterocarp, Pakaraimaea dipterocarpacea, and the sympatric Dicymbe jenmanii in the Upper Mazaruni Basin, R. myrmecobroma was the sixth most common mycobiont recovered among about 50 species of EM fungi associated with 20 trees of each host species, indicating a broad host and distribution range for the fungus in Guyana. It is interesting to speculate that movement of tissue pieces by the ants might contribute to the frequency of R. myrmecobroma in the forest.
Molecular analysis (SL Miller, unpublished) indicates a close relationship between R. myrmecobroma and R. batistae, despite the fact that macroscopically they are very different. Russula batistae has irregular, broad, subdistant to distant lamellae, no lamellulae, and a deeply sulcate pileus margin, while R. myrmecobroma has regular, rather narrow, crowded lamellae, abundant lamellulae, and a regularly short-striate margin. Singer erected R. subsect. Batistinae in the large section Pelliculariae to accommodate R. batistae. This subsection likely will not suffice as no velar tissue has been observed in R. myrmecobroma, suggesting a need to refine the description of the subsection. Buyck (1990a) has shown that R. sect. Pelliculariae is highly heterogeneous and possibly not monophyletic, indicating the possible need for a new infrageneric group at the sectional level to accommodate R. batistae and R. myrmecobroma.
Description type:Original description 
Description:Russula myrmecobroma S.L. Mill., Aime & T.W. Henkel, sp. nov. Plates 3-4
MycoBank MB 564264
Pileus 3.5−4.5 cm broad, broadly convex at first, then plane and finally depressed; margin entire when young, then undulating, obscurely striate at first, then with regularly spaced and sized striations; striations 2 mm long; pellis dull, dry, smooth at disk, frequently pruinose at mid-radius and at margin, yellowish brown (5E−F−8 to 5D−E 5) overall at first, becoming hygrophanous with irregular reticulate pattern especially halfway the radius and with color fading to dark blond (5C−4, 5C−D−3) to orange grey (5B−2) with irregular areas of 5B−3−5; disk and margin not hygrophanous and usually remaining darker. Lamellae 1−2 mm broad at mid-radius, adnate, sinuate or slightly subdecurrent, crowded, forked at stipe and occasionally elsewhere, arising at different positions relative to the stipe giving uneven appearance, pallid cream (near 4A−2), becoming spotted with yellow or reddish brown where injured; lamellulae present, 4−5 ranked; edge entire. Stipe 3−5.7 x 0.8−1.1 cm, equal to subclavate or tapering to the base, centrally attached, moderately distinct longitudinal ridges beneath felted subtomentose covering especially below, tomentum white to pale grey (5A−B−1), pale orange grey (5B 2−3) beneath, grey brown where handled and where longitudinal ridges become exposed. Context in pileus 2−2.5 mm at mid-radius, stuffed pallid grey cream; context in stipe cylinder whitish marbled with pale grey, turgid to pliant, pallid cream to pale grey brown, discoloring pale orange or orange brown where injured; odor sharp acrid heading toward latex paint; taste strongly acrid; FeSO4 quickly salmon on stipe surface and trama.
Basidiospores 6.4−7.2(-8) x 5.6−6.8 µm (Q = 1.05−1.14−1.17), subglobose to broadly ellipsoidal; ornamentation reticulate, consisting of low blunt spines, 1−1.3 µm high, connected by fine lines or verrucae, strongly but often partially amyloid; suprahilar plage not distinct to finely reticulate. Basidia 55−62 x 12−15 µm, subclavate to nearly cylindrical, 4−spored; sterigmata stout 7−10 x 2−3 µm. Cystidia 80−95 x 15−20 µm, subclavate to subfusiform, thin or thick walled, emergent for ca. 20−30 µm, numerous, arising from gloeopleurous elements, with refringent to crystalline contents, SV+, thick walled lamprocystidia present but not numerous. Marginal cells 55−105 x 8−15 µm, narrowly subclavate, tortuous, thin-walled, optically empty, abundant. Subhymenium distinct, a gelatinous layer composed of interwoven cylindrical flattened and variously swollen hyphae of 2−5 µm diam. Lamellar trama with large sphaerocytes and scattered hyphae, with gloeopleurous fragments and embedded dermatocystidia near the pileus. Pileipellis orthochromatic in Cresyl Blue, two-layered; subpellis gelatinized, forming a dense mat close to the underlying trama, of tightly interwoven hyphae; hyphae 2−5 µm diam, thin-walled, frequently septate, with scattered strongly refringent gloeopleurous elements of 5 µm diam, frequently terminating with cylindrical to swollen or mucronate embedded dermatocystidia; suprapellis composed of 2−5 strongly inflated, spherical cells, often gradually smaller towards the terminal cell, the latter cylindrical to narrowly subclavate, ampullaceous, or mucronate, resembling an epithelium; pileocystidia dispersed, terminal, more or less the same diam. as other terminal elements, 50−90 x 10−13 µm, contents granular-refringent in KOH. Stipitipellis a turf of variously shaped caulocystidia, 100−170 x 9−14 µm, these narrowly cylindrical, mucronate, capitate or tortuous.
Habit, habitat, and distribution − Solitary or fruiting in small numbers on root mats of the ECM trees Dicymbe corymbosa, D. altsonii, D. jenmanii, Aldina insignis and Pakaraimaea dipterocarpacea, widespread in the Pakaraima Mountains but not common; found May through early July during wet weather and also in December-January. Known from Mabura Hill and the Upper Potaro and Upper Mazaruni River Basins of Guyana.
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