AGARICUS - CLASSIFICATION, VEGETATIVE STRUCTURE, REPRODUCTION, ECONOMIC IMPORTANCE

            A. CLASSIFICATION:

Division – Mycota

    Sub-division – Eumycotina

        Class – Basidiomycetes

             Sub-class – Homoobasidiomycetidae

                  Series – Hymenomycetes

                       Order – Agaricales

                             Family – Agaricaceae

                                   Genus – Agaricus

            Agaricus commonly known as “mushroom” is a saprophytic fungi which is cosmopolitan in distribution. Wild species of Agaricus (most commonly Agaricus compestris) occur in open fields, grass lands, manure heaps, wood logs, rotten straw heaps, etc., during rainy season. The cultivated mushroom has appeared under a number of names in literature. Agaricus bisporus and A. rodmani are the common cultivated mushrooms. All the above mentioned species of Agaricus are cultivated for commerce.

         B. STRUCTURE OF THE VEGETATIVE BODY:- The primary mycelium is short lived, septate and originates from the germination of a basidiospore. The mature haploid uninucleate (or bi-nucleate) basidiospore germinates to produce multicellular mycelia; each cell of which contains many nuclei within granular vacuolated protoplasm with oil drops and reserve food.

             Next secondary mycelium originates by the fusion of two primary mycelia. The secondary mycelium consists of branched septate hyphae with cells multinucleate. Sometimes, the secondary mycelium twists together producing compact mass of hyphal strands known as rhizomorphs.

            C. REPRODUCTION:

            1. Asexual reproduction:- Asexual reproduction in Agaricus is absent. But in few species, it takes place by the formation of smooth or verucose thick walled chlamydospores. Such chlamydospore germinates producing a new mycelium. In some species oidia formation takes place.

            2. Sexual reproduction:- Species of Agaricus are heterothallic. Sex organs are completely lacking in Agaricus. Sexual fusion is thus accomplished by two different somatic hyphae.

            Plasmogamy – Two vegetative hyphae with uni-nucleate haploid cells from mycelia with opposite strains or from the same mycelium come in contact. The intervening wall at the point of contact dissolves. The fusion cell now possesses two nuclei which move towards each other to lie in a pair, resulting in the formation of dikaryotization. The dikaryotized cell by successive divisions gives rise to dikaryotic mycelium.

            The dikaryotic mycelium is perennial. It forms the chief food absorbing phase of the fungus and at the appropriate time bears fructification called the basidiocarp. The basidiocarps are formed when the mycelium has absorbed and accumulated abundant food supply. They are developed in groups from the mycelium under suitable temperature and sufficient moisture.

            Karyogamy – It means the fusion of two nuclei of the dikaryon. It is considerably delayed and takes place in young basidium. Karyogamy is immediately followed by meiosis and takes place in the basidium, prior to basidiospore formation. The basidiospores are thus haploid.

            Structure of Basidiocarp – The mature basidiocarp is umbrella-shapedstructure and consists of long stalk-like structure called stipe and a broad cap-like pileus.

            Stipe – It is stalk-like, thick, fleshy, hollow structure and whitish-pink in colour. A membranous structure called annulus is present on the upper part of the stipe. The hollow central region is composed of loosely arranged hyphae, but the peripheral region is composed of compactly hyphae forming pseudoparenchymatous tissue.

            Pileus – It is umbrella-shaped extended upper convex surface whose upper surface is thin, dry, smooth and white or cream in colour. From the underside is present numerous gills, in which fertile hymenium is present on both the sides. A gill in section shows the following structures – (a) Trama – It is a central region of the gill and consists of loosely arranged interwoven hyphae running from the pileus. Cells of this region are multinucleate. (b) Sub-hymenium – It is the middle region and lies on both sides of the trama. The cells of this region are iso-diametric and multinucleate. (c) Hymenium – It is the fertile region of the gill. This region is composed of several club-shaped binucleate cells forming a palisade like layer. Some of the cells of this region develop into basidia and others into sterile and slender structure called paraphyses.

            Development of Basidium – The basidium is unicellular and binucleate (dikaryotic). The two nuclei fuse (karyogamy) with each other to form the diploid nucleus. Karyogamy is immediately followed by meiosis which results in the formation of four haploid nuclei. Of the four nuclei two are of (+) strain and the other two are of (–) strain. Now four peg-like outgrowths, called sterigmata develop at the tip of the basidium. Next, the tip of the sterigmata swells and the haploid nuclei move into the swellings. These uninucleate swellings at the tip of the sterigmata develop into basidiospores.

            The mature basidiospore after discharge, on falling to a suitable substratum germinates into primary mycelium of either (+) or (–) strain.

           D. ECONOMIC IMPORTANCE:- Most of the members of Agaricales are edible. A. bisporus is cultivated white mushroom which is specially cultivated for food in different states of India, Europe and N. America. A. compestris is the field mushroom, as this species is recognized widely as field mushroom, so it in the sense of edible. Edible mushroom has some value for its protein, mineral and vitamin contents. Based on the cultivation of edible mushroom, a great number of industries have been developed in the western world and in India. Solan district of Himachal Pradesh is the main cultivation centre of mushroom.

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