A. Taxonomy and Phylogeny
Pleurotus djamor was first described by botanist Georg Eberhard Rumphius in Herbarium Amboinense under the name Boletus secundus arboreus (Rumph 1750). Herbarium Amboinense is a catalog of the flora of modern-day Ambon Island, part of the Maluku Islands of Indonesia. Elias Magnus Fries sanctioned P. djamor under the name of Agaricus djamor in 1821 (Fries 1821). Karel Bernard Boedijn transferred P. djamor to the genus Pleurotus in 1959 (Wit 1959). Common names of Pleurotus djamor include the Pink Oyster Mushroom, the Salmon Oyster Mushroom, the Strawberry Oyster, and the Flamingo Mushroom (Stamets 2000).
Because of the Pink Oyster mushroom's wide pan-tropical distribution and considerable morphological variation it has been unclear whether worldwide populations represent one species or several genetically differing populations (Liou 2000). Synonyms in the literature include P. flabellatus Saccardo, P. ostreato-roseus Singer, and P. salmoneo-stramineus Vasil (Stamets 2000). Corner (1981) proposed six varieties based on color morphology, micromorphology and habit of Malaysian P. djamor populations: var. cyathiformis, var. djamor, var. fuscopruinosus, var. fuscoroseus, var. roseus, and var. terricola (Nicholl 2000). Phenetic analysis of 35 collections of color morphs from 9 different geographic locations showed no clear and consistent differences based on macro- and microscopic characters (Nicholl 1996).
Multiple studies have been done to determine intersterility, defined by the ability of haploid spore isolates to form dikaryons, of morphological and geographical populations of P. djamor (Liou 2000). Nicholl (1996) found no reproductive isolation of the 35 collections studied. Liou (2000) showed through pairings of specimens from 14 geographic locations that no matings barriers had evolved among allopatric populations of P. djamor worldwide. One generation of intercrossing showed no observable hybrid breakdown in the progeny of several interpopulation dikaryons (Liou 2000). This evidence strongly suggests that the P. djamor complex constitutes a single biological species.
LSU rDNA sequence analysis shows that Pleurotus djamor represents an early diverging, monophyletic lineage in the molecular phylogeny of Pleurotus (Vilgalys and Sun 1994). Analysis of ribosomal ITS sequences of 31 specimens from 14 geographic locations identified three major clades that agree with the three primary geographic areas of distribution: the neotropics, tropical Australia-Asia, and the Pacific region (Vilgalys and Sun 1994). ITS molecular phylogeny also classified Pleurotus calyptratus as a sister clade of P. djamor (Vilgalys et al. 1996, Huerta et al. 2010). P. calyptratus is a rare species, having only been only found in temperate central Europe, and is thought to be a weak pathogen of hardwoods in addition to a saprobic ecology (Vilgalys et al. 1996). Liou (2000) examined interspecific hybrids of P. djamor and P. calyptratus for reproductive isolation, finding frequent dikaryon formation with no fitness loss, but reduced spore load of fruiting bodies and frequent abnormal mycelial morphology of germinated hybrid spores, confirming species delineation.
The Pink Oyster shares the general shape and morphology of P. ostreatus with the exception of a wide range of color morphology. (Stamets 2000). The pileus is 2.0 – 7.0 cm wide, laterally attached to the substrate, spatulate to bell-shaped, with an off-center stipe if present, typically extremely short (Kimbrough 2000). Gills are deeply decurrent and somewhat crowded. P. djamor often has smaller fruit bodies than other Pleurotus species, with thin, lobed pilea (Liou 2000). Pileus is convex, expanding with age to broadly convex to plane (Stamets 2000). The pileus margin inrolled at first, then incurved, and eventually flattening and upturning at maturity (Stamets 2000). Primordia appear white to bright reddish pink, developing a dull to light pinkish cinnamon as the mushroom develops (Stamets 2000). Gills are strongly pigmented pinkish when young and fade to creamy white-beige at maturity (Stamets 2000). When overmature all parts of the mushroom lose pigmentation and fade to beige-white to straw colored (Stamets 2000). Often the mushroom appears in clusters (Kimbrough 2000).
Color morphology of P. djamor has been shown to be highly variable, even under controlled fruiting conditions, complicating species deliniation based on basidiome color (Nicholl 2000). Corner (1981) proposed six unique color morphs from studies of Malaysian P. djamor populations: var. cyathiformis (basidiome white to off-white, pileus eccentric), var. djamor (basidiome white to pallid ochraceous, pileus laterally attached), var. fuscopruinosus (pileus fuscous, lamella edge pruinose, umber), var. fuscoroseus (pileus livid fuscous, lamellae pink), var. roseus (basidiome pink), and var. terricola (basidiome white to off-white, pileus merismatoid) (Nicholl 2000). Nicholl (2000) observed that “...changes in basidiome color routinely occurred during basidiome development.” Guzman et al. (1995) note that “...the color of fruit bodies is often affected by microenvironment, and older specimens often fade.” In a review of the literature, Liou (2000) found that “according to field collection information, the color morphs do not appear to be correlated with geographic distribution (Corner 1981, Guzman et al. 1995).” Murakami and Takemaru (1990) report that an albino progeny from a pink parental specimens was due to a Mendelian recessive mutant allele, but overall the genetics of fruit body color are poorly understood. It has been hypothesized that the pink pigment is present in the cytoplasm of the fungal hyphae and spores, rather than the cell wall, thus attributing to the widely observed color morphologies (Stamets 2000).
Pleurotus djamor exhibits a wide range of color morphology for spores as well as fruit bodies. Stamets (2000) noted that “...the color of the fruit body directly influences the color of the spores. Pink mushrooms give pink spores. White to beige mushrooms, from the same dikaryon that produced the pink mushrooms, give off-white to light gray-beige spores. As the pink mushrooms fade with maturity, the spore color also changes.” The spores are cylindric, 6.0-0.9 x 3.0-3.7 micrometers, smooth, and do not turn blue in iodine (Kimbrough 2000). In reviewing the literature, Liou (2000) found that “...morphological studies have classified P. djamor varieties into two groups based on spore size (Corner 1981), although other studies have frequently documented specimens with intermediate spore size (Pegler 1986).” The basidia are clavate, 14.0-20.0 x 5.0-6.0 micrometers, and bear four sterigmata (Kimbrough 2000). The mycelium is dimitic, composed of generative and skeletal hyphae, although because of the late development of skeletal hyphae young fruit bodies often appear monomitic (Guzman 1993). Cheilocystidia (sterile gill structures) are present, as are clamp connections (Stamets 2000).
P. djamor occurs wild in pan-tropical climactic zones, and appears in the wild on all continents except Antarctica (Stamets 2000). It's three primary geographic areas of distribution are the neotropics, tropical Australia-Asia, and the Pacific region (Vilgalys and Sun 1994). The pink oyster mushroom is the most common species of wild Pleurotus in pan-tropical climates (Kimbrough 2000, Stamets 2000). It is saprotrophic, gaining its energy from the decomposition of wood, preferring tropical and subtropical hardwoods, including palms, deciduous trees, rubber trees and Opuntiae cactus (Stamets 2000, Corner 1981, Pegler 1986). It is a white-rot wood-decay fungus.
D. Overall Biology and Relevance for Humans
The pink oyster is a choice edible, grown commercially on cellulosic compost (Kimbrough 2000). Members of the Pleurotus genus can efficiently degrade lignin by excreting four ligninolytic enzymes, allowing cultivation on various raw lignocellulosic materials (sawdust, paper, agricultural wastes) (Stajić 2005). The Pink Oyster complex has some of the most aggressively growing mushrooms in the Pleurotus genus, so much so that it is known to completely colonize unpasteurized bulk substrates, outcompeting typical contaminants (Stamets 2000). Fruiting of the mushroom under cultivated conditions can occur as quickly as 2 weeks after grain spawn inoculation into pasteurized bulk substrates (Stamets 2000). P. djamor is thermophilic, and may be grown in many areas otherwise unfavorable to mushroom cultivation due to high temperature (Stamets 2000). The mushroom spoils rapidly however, with a marketable shelf life of no more than 4 to 5 days after harvest (Stamets 2000). One study found that P. djamor (as “P. flabellatus”) and Stropharia rugosoannulata were the best at rendering strawn, after fruiting, into a nutritious feed staple for ruminants, e.g. cattle. (Zadrazil 1979, Stamets 2000). Pleurotus djamor represents an invaluable resource for many cultivators in underdeveloped countries, with its thermophilic nature and aggressive growth rate allowing cultivation with minimal capital or labor input. It's ability to grow on untreated, raw cellulosic waste and produce both marketable edibles and amended ruminant feed allows it to contribute in multiple ways to food and economic security in developing nations.
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