In this article we will discuss about:- 1. Introduction to Agaricus 2. Vegetative Body of Agaricus 3. Reproduction.
Introduction to Agaricus:
Agaricus is a saprophytic fungus, commonly grows on damp wood, decomposing organic matters like humus, horse dung etc. During summer with rainy weather, it is fairly common in the grassy lands. Butler and Bisbay (1958) recorded 25 species of Agaricus from India.
Two important edible species of Agaricus:
A. campestris, the field mushroom and A brunescens (syn. A bisporus), the cultivated mushroom, are cultivated commercially in various parts of India.
Another two species, A sylvaticus and A. placomyces are highly poisonous, those may cause gastrointestinal disturbances. A. xanthodermus is also found to be a poisonous species.
Vegetative Body of Agaricus:
Vegetative body is mycelial and consists of septate much branched hyphae. Spores on germination develop into monokaryotic or primary mycelium, either + or – type (Fig. 4.70A, B). The primary mycelium is short-lived and it soon transforms into dikaryotic or secondary mycelium (Fig. 4.70C) by the fusion of two cells of different monokaryotic mycelium (+ and -) following clamp connection (Fig. 4.71).
The hyphae of the dikaryotic mycelia interlace and twist together to form thick white hyphal cord, called rhizomorph which bear the fruit bodies.
Reproduction in Agaricus:
Agaricus reproduces by all the three means vegetative, asexual and sexual.
1. Vegetative Reproduction:
It is mostly propagated by vegetative means where dikaryotic mycelium develops spawn, the mushroom seed. The mass of spawn divides artificially into small blocks that are grown in soil supplemented with organic manure to obtain fruit bodies.
2. Asexual Reproduction:
It takes place by chlamydospores, that are formed rarely during unfavourable condition. Terminal or intercalary chlamydospores are developed on dikaryotic mycelium, which on germination during favourable condition produce dikaryotic mycelium.
3. Sexual Reproduction:
Sex organs are absent in Agaricus and sexual reproduction takes place by somatogamy. Most of the species including A. campestris are heterothallic, but A. brunescens is homothallic. Somatogamy includes plasmogamy, karyogamy and meiosis.
Karyogamy does not take place immediately after plasmogamy, but meiosis follows soon after karyogamy:
Two cells of monokaryotic hyphae of opposite strains (+ and -) come in contact with each other (Fig. 4.71 A).
The cell wall dissolves at the point of contact and a dikaryon (n + n) is formed (Fig. 4.71 B, C). This dikaryotic cell develops into dikaryotic mycelium by regular cell divisions through clamp connection (Fig. 4.71 D-G). The dikaryotic mycelia are subterranean and after aggregation at some points they form button which remains dormant before the rain comes during late summer. After rain, the soil becomes soft and the button develops into fruit body.
It takes place in the young basidium which develops on gills of the fruit body. Both the nuclei fuse together and form diploid nucleus.
It takes place soon after Karyogamy and forms four haploid nuclei. The basidiospores, thus formed on the sterigma of basidium are haploid and either of + or – type.
Development of Basidiocarp in Agaricus:
The underground dikaryotic mycelia aggregate at some points and form a knob-like structure, called button. The button does not grow in dry season and remains hidden one or two inch (2.5-5 cm) below the soil surface. In the late summer with heavy rain, when the soil becomes moist and soft, the button grows rapidly and develops the basidiocarp (Fig. 4.72).
During development, the button is differentiated into a basal bulbous part and an apical hemispherical region. The bulbous part gradually differentiates into elongated, solid, cylindrical structure, the stipe and the hemispherical region differentiates into a round, convex region, looks like the top of an open umbrella, the pileus (Fig. 4.72E).
Towards the bottom of the hemispherical region some hyphae are drawn apart and form a ring-like cavity, the prelameliar chamber (Fig. 4.72B, C). The upper surface of prelameliar chamber becomes deeply concave and lined with alternating radial bands of slow and rapidly dividing cells.
The region with rapid division forms gill-primordia, which develops into gills, that hang downwardly into the prelameliar chamber (Fig. 4.72D).
The top of the hemispherical region (pileus surface) expands resulting in the increase in radial interspaces between the gills. The edge of the pileus of young basidiocarp connects with the stipe by a membranous tissue called the velum, partial veil or inner veil (Fig. 4.72E).
Further expansion of the pileus causes discontinuation with velum and the pileus fully expands out like the top of an open umbrella, with numerous gills attached on its lower side (Fig. 4.72E). The velum remains attached with the upper part of the stipe in the form of a ring, the annular ring or annulus.
Due to centrifugal growth (i.e., grows outward from the centre) of the dikaryotic mycelium, fruit bodies are formed in a circular ring around the spot where the fruit body formed in the last year.
These fruit bodies are arranged in a ring, imagine that these circular ring marks the path of dancing fairies, called fairy ring (Fig. 4.73). The perfect ring of Agaricus is usually less than 5 metres in diametre, but Shantz (1917) reported a perfect ring of about 50 metres in diameter.
Structure of Basidiocarp in Agaricus:
A. External Structure:
The mature basidiocarp (Fig. 4.74, 4.75) is an open umbrella shaped structure with a broad expanded pileus on a long massive stalk-like stipe. The pileus is 5-12 cm (2-5 inch) in diametre with a convex upper surface, may be of white, yellow or light brown in colour.
The lower surface of the pileus bears about 300-600 radially arranged gills of 3 different lengths normal, half and quarter; the last one lying between half and normal length gills. The young gills are light pink in colour, but becomes purple or brown at maturity.
The gills never touch the stipe. The stipe is elongated, thick, solid, soft and cylindrical structure and light-pink or white in colour. The fruit body remains attached with the substratum by rhizoidal mycelium.
B. Internal Structure:
It is differentiated into central medulla, composed of loosely interwoven hyphae and an outer cortex, made up of densely compacted hyphae (Fig. 4.76A).
Like stipe, it is also differentiated into outer compact and inner loose hyphae.
It is differentiated into three regions trama, sub-hymenium and hymenium (Fig. 4.76D):
It is the central sterile region of the gill, consists of many layers of loosely arranged interwoven hyphae.
(ii) Sub-Hymenium (Hypothecium):
It is also a sterile zone, situated on both
sides of trama, formed by the lateral branches of hyphae develops from trama.
This layer is present on the outer-side of sub-hymenium, composed of fertile basidium and club-shaped sterile paraphyses.
Development of Basidium in Agaricus:
Young basidium is aseptate, fertile dikaryotic cell present in the hymenial zone (Fig. 4.76E-J). As the basidium matures, the nuclei (+ and -) of dikaryon fuse together and form diploid nucleus (2n). This diploid stage is ephemeral. The diploid nucleus undergoes meiosis and forms four haploid nuclei (n), of which two are of “+” strain and the other two of “-” strain.
At the apex of the basidium, four peg-like or horn-shaped outgrowths are developed, known as sterigmata. The sterigma swells at the tip and after collecting one nucleus with cytoplasm, it develops into a single basidiospore. In between sterigma and basidiospore a small projection is present, called hilar appendage.
Basidiospores are oval, thin- walled and uninucleate (Fig. 4.76K).
Dispersal of Basidiospore:
The mature basidiospores are discharged from the basidium by water-drop mechanism. A drop of liquid appears at the hilar appendage. The drop gradually increases in size and creates a pressure which helps the spore to detach from sterigma (Fig. 4.76J). The spores of a basidium disperse in rapid succession.
Germination of Basidiospore:
Falling on suitable substratum, the basidiospore germinates by initiating germ tube which develops into primary mycelium, either + or – strain depends on the strain of spore (+ or -) (Fig. 4.76)).
Agaricus campestris, A. latipes, A. exaltatus, A. pratensis, A. squalidus.
The diagrammatic and graphic life cycles of Agaricus are presented in Fig. 4.77 and 4.78 respectively.
It is a technique to study the arrangement of gills in different way. The stipe is removed from the basidiocarp close to the pileus. The pileus is then kept for a few hours on moist cotton. Then the pileus is placed, gills-side down, on a black paper and kept for overnight.
Total set up is then covered with bell jar to avoid any foreign disturbance. The pileus is then carefully lifted and the spores fallen on the black paper become clearly visible. The distribution of spores in a radiating manner indicates a duplicate arrangement of gills. This is called spore print (Fig. 4.79).