Agaricus flavidodiscus L. A. Parra, Angelini & Callac 2018, sp. nov.
Creators: name: Parra, Luis A.
name: Angelini, Claudio
name: Ortiz-Santana, Beatriz
name: Mata, Gerardo
name: Billette, Christophe
name: Rojo, Carlos
name: Chen, Jie
name: Callac, Philippe
Publisher: name: Zenodo
Year: 2018
Subjects: Biodiversity
Taxonomy
Fungi
Basidiomycota
Agaricomycetes
Agaricales
Agaricaceae
Agaricus
Agaricus flavidodiscus

description: Agaricus flavidodiscus L.A. Parra, Angelini & Callac, sp. nov. (Fig. 20)   
 MycoBank: MB823283.   
 Etymology: because of the yellowing (in Latin “ flavidus ”) color at the center of the pileus (in Latin “ discus ”) in this species.   
 Original description, Macroscopic description: Pileus (1.5–)2.0–5.0 cm diam., at first hemispherical, sometimes trapezoid, then convex with the margin incurved for a long time, finally plano-convex, at the center rarely somewhat elevated or umbilicate-depressed, whitish with the center always typically colored yellow. In very mature basidiomata or those soaked by rain becomes completely pinkish brown, but always with a yellow center. Surface smooth, sericeous, not or slightly squamulose, sometimes and only in some basidiomata slightly fibrillose at the center, dull and dry. Margin thin, not exceeding the lamellae. Lamellae free, straight to slightly ventricose, intercalated with numerous lamellulae, at first pinkish white, then bright rose for a long time, finally pinkish brown with a paler and strongly eroded edge. Stipe 3–6 × 0.3–0.5 cm, cylindrical to slightly enlarged towards the base, curved and flexuose, stuffed, with an annulus in its upper third, sericeous above and below annulus, with the surface fibrillose-floccose, white, whitish cream or grayish in mature basidiomata, unchanging or slightly yellow on handling, base covered with a felty and white mycelial pad and with fine white mycelial strands. Annulus superous, double, white, thin but persistent, with the upper membrane perpendicular to stipe, up to 0.4 cm wide, and the lower membrane more narrow, appressed to the stipe. Context somewhat dense in pileus and stipe, when cut, white at first, then unchanging or slightly yellowing, odorless or odor slightly fungoid.   
 Microscopic description: Spores 4.33–4.82–5.17 × 3.10–3.30–3.51(–3.61) μm, Q=1.32–1.46–1.61, ellipsoid, smooth, brown, without apical pore. Basidia 13–26 × 5.5–8.0 μm, tetrasporic, clavate or slightly truncate at the apex, sterigmata up to 3 μm long. In LAPAM17 the basidia, basidioles and subhymenial hyphae have abundant internal granular brown pigment. Cheilocystidia absent in the three collections studied. Pleurocystidia not observed. Lower surface of the annulus consisting of cylindrical hyphae, hyaline, 2–6 μm wide, the wider the more constricted at septa, the latter easy to disarticulate into elements with rounded ends. Inflated elements not observed. Pileipellis a cutis, hyphae 2–5 μm, not or slightly narrowed at septa, terminal elements very abundant with rounded ends, with internal granular pigment yellow or light brown in water. Clamp connections not observed.   
 Macrochemical reactions: Schäffer’s reaction negative. KOH reaction positive, color orange yellow.   
 Habit, habitat, occurrence and distribution: Gregarious, sometimes in groups of numerous basidiomata, in moist lowland broadleaf forests. Uncommon. Recorded only from the Dominican Republic.   
  Species-specific ITS markers in A. sect. Xanthodermatei:  ccacgAaatct@194, acatgAgcttc@264, tagagAggagc@482   
 Note: This species is characterized by its small size, the whitish pileus with a tipically yellow center, double and thin annulus, not or only slightly yellowing on handling and for being a species that is practically odorless.   
 Material examined: DOMINICAN REPUBLIC, Puerto Plata, Sosúa, Puerto Chiquito, 5 December 2013 JBSD126487 (LAPAM39); Puerto Plata, Sosúa, Puerto Chiquito, 19 November 2011, JBSD126488 (LAPAM17); Puerto Plata, Sosúa, Puerto Chiquito, 5 December 2014, JBSD126489 Holotypus (LAPAM 60 Isotypus).   
 Taxonomic comments: Macroscopically the most similar species are A. ochraceidiscus Murrill (1918: 80) and A. ochraceosquamulosus Heinem., but both species are larger and their pilei are covered by ochraceous scales. Furthermore, the Schäffer’s reaction is positive of orange color in A. ochraceosquamulosus (Heinemann 1961; Pegler 1983). Microscopically, the spore size of the original description of A. ochraceidiscus (4.5–5.0 × 3.0–3.5 μm) agrees with our collections, but Murrill (1918) did not note cheilocystidia. However, the holotype of A. ochraceidiscus has been studied by Freeman (1979) and Pegler (1987) and both indicated larger spore sizes (Freeman: 4.5–6.0 × 3.8– 4.5 μm; Pegler: 5.0–5.7–6.0 × 3.5–4.0–4.2 μm) than those provided by Murrill, and are practically out of range of our measurements for A. flavidodiscus (4.33–4.82–5.17 × 3.10–3.30–3.51(–3.61) μm). In addition, both authors also mentioned the presence of cheilocystidia, “scattered” according to Freeman, but “crowded” clavate often subcapitate to Pegler. Similarly, the spore size of A. ochraceosquamulosus are also larger both in the original description by Heinemann (5.1–5.7 × 3.6–4.0 μm) and in collections studied later by Pegler (4.5–5.4–6.5 × 3.2–3.7–4.2 μm); and both authors also indicated the presence of clavate or pyriform cheilocystidia (described erroneously as lanceolate in Heinemann; see figure 8 on page 243) even if they were described as inconspicuous.   
 Additional comments: In A. flavidodiscus, we noted a high intraspecific variability with heteromorphisms at eight, four and zero positions in LAPAM 60, LAPAM 17, and LAPAM 39, respectively.
description: Published as part of Parra, Luis A., Angelini, Claudio, Ortiz-Santana, Beatriz, Mata, Gerardo, Billette, Christophe, Rojo, Carlos, Chen, Jie & Callac, Philippe, 2018, The genus Agaricus in the Caribbean. Nine new taxa mostly based on collections from the Dominican Republic, pp. 219-271 in Phytotaxa 345 (3) on pages 262-264, DOI: 10.11646/phytotaxa.345.3.2, http://zenodo.org/record/13708429