Agaricus – Structure, Reproduction and Life Cycle

Agaricus

Introduction

The order Agaricales includes 197 genera and 4,000 species. Most of the species are saprophytic, growing on dead and decaying wood and leaves and on organic rich soil.

Species like Armilleriella mellia grow as parasite on woody trees. Some agaricales form mycorrhiza with forest trees. Many fleshy agaricales are edible, whereas some (e.g., Amanita phalloides, A. muscaria) are highly poisonous. Species of Psilocybe (sacred mushroom) produce hallucination.

General Features

The primary mycelium which develops from homokaryotic basidiospore is short lived. The dikaryotic secondary mycelium forms the major part of the life-cycle.

It transforms into complex tissues of tertiary mycelium in the form of fruiting bodies. In some species, hyphae become aggregated into rhizomorphs.

Asexual reproduction is very rare, but a few species produce oidia, chlamydospores or sclerotia. Sexual reproduction is usually by somatogamy. The members produce well-developed and distinct fruiting bodies called basidiocarps.

The basidiocarps are fleshy with ‘monomitic type’ of construction. The non-septate basidia develop in the form of hymenium on gills or lamellae. The gills develop on the under surface of the fruiting body like spokes of a cycle wheel. Hence, these are also known as gill fungi.

Classification of Agaricus

The classification of Agaricales posses a major problem. In the older systems, members of the order were included in either one (Agaricaceae) or two (Agaricaceae and Boletaceae) families. With the increase of our knowledge, the order is divided into several more natural families.

On the basis of the presence or absence of annulus, spore colour and ornamentation, mode of anachment of gills to the stipe and composition of trama.

Smith (1973) recognised 16 families (Boletaceae. Hygrophoraceae, Tricholomataceae, Entolomataceae, Amanitaceae. Pluteaceae, Lepiotaceae, Agaricaceae, Bolbifiaceue, Suophariaceae, Coprinaceae. Cortinariaceae, Paxillaceae, Gompridiaceae, Russulaceae. and Cantharellaceae) within the order.

However. Singer (1975, 83) recognised 18 families within the order Agaricales.

Family Agaricaceae

Distinctive feature of family Agaricaceae are:

  1. The fruiting body is well-differentiated into a stipe and a pileus.
  2. The annulus is typically present on the stipe.
  3. The lamellae are free.

Agaricus campestris

KingdomMycota
DivisionEumycota
Sub-DivisionBasidiomycotina
ClassHymenomycetes
Sub-ClassHolobasidiomycetidae
OrderAgaricales
FamilyAgaricaeae
Table 1. Classification of Agaricus campestris

Agaricus (=Psalliota) is a saprophytic fungus which occurs on dead and decaying leaves, wood logs, manure piles and other similar surroundings in rainy season. These fungi are commonly known as mushrooms.

There are about 17 species of Agaricus in India which are locally known as kukarmutta, khumb or dhingri. A. campestris (the field mushroom) and A. bisporus ( =A. brunnescens; the cultivated mushroom) are common edible mushrooms, commercially cultivated in various parts of India. Species like A. silvaticus and A. xanthodermus are highly poisonous.

Thallus Structure

The structure of Agaricus can be studied under the following two heads:

  1. Vegetative mycelium
  2. Fruiting body (basidiocarp)

Vegetative Structure of Agaricus campestris

The vegetative mycelium remains underground and is of two types. The primary mycelium formed by the germination of basidiospore, is hyaline, septate and monokaryotic. Depending on the strain of the basidiospore, it may of (+) or (-) strain.

The primary mycelium is short-lived and soon gets transformed into the secondary mycelium. The secondary mycelium, formed by the fusion of two primary mycelia of opposite strains, is perennial, branched and septate. It contains dolipore septa and its cells are binucleate. The hyphae of the secondary mycelium twist together to form white hyphal cords, called rhizomorphs.

The dikaryotic mycelium shows centrifugal growth, i.e., hyphae grow outward from a centre in a circle. A circular colony of hyphae is thus formed in the soil. At maturity, hyphae develop fruiting bodies at their tips in a more or less circular ring.

As the outer edge of the colony grows, older parts in the centre degenerate. When fruiting bodies are formed next time, the ring is larger in diameter. Thus successive crops of fruiting bodies are produced in larger and larger rings. Based on the ancient belief that they marked the path of dancing fairies, these rings are called fairy rings.

Fruiting Body (Basidiocarp)

The basidiocarp is the aerial part of Agaricus and it develops from rhizomorph. This part is commonly known as mushroom. It is differentiated into a stipe and a pileus.

The stipe is about 6-9 cm in height and the pileus is 5-10 cm in diameter. There are about 300-600 gills in the under-surface of the pileus. arranged in radial fashion.

Reproduction in Agaricus campestris

Agaricus reproduces by vegetative, asexual and sexual means.

Vegetative Reproduction in Agaricus campestris

The edible mushrooms are propagated by vegetative means. Small pieces of dikaryotic mycelium are used as inoculum. The pieces are grown in soil rich in organic manure to obtain basidiocarps.

Asexual Reproduction in Agaricus campestris

It is not a common method of propagation in Agaricus. It takes place by chlamydospore formation. The chlamydospores develop in terminal or intercalary positions on the secondary mycelium. They germinate to produce dikaryotic mycelium.

In some species oidia are also formed, but these are involved mainly in dikaryotization rather than developing directly into new mycelia.

Sexual Reproduction in Agaricus campestris

Most species of Agaricus are heterothallic (A. bisporus is homothallic), but they do not have sex organs; in traditional sense. Primary mycelia formed by the germination of basidiospores of two different strains act as male and female sex organs.

There is somatogamy between the somatic hyphae of opposite strains and this results in diplotization and formation of the secondary mycelium. The secondary mycelium later develops fruiting bodies, known as basidiocarps. Somatogamy between two primary hypha of opposite strains takes place through plasmogamy, karyogamy and meiosis.

1. Plasmogamy

In this step, two primary monokaryotic hyphae of opposite strains come in contact with each other. At the point or their contact, the cell walls are dissolved and a dikaryon on is formed.

A dikaryolic mycelium develops by successive divisions of the dikaryotic cell. At the time of the division of dikaryotic cell, both nuclei of the dikaryon divide synchronously and form four haploid daughter nuclei, two of (-) and two of (+) strain.

Out of these, two nuclei (one of(+) and one of (-) strain) are transferred to the daughter cell by the formation of clamp connection. The dikaryotic mycelium is perennial and subterranean. It produces basidiocarps under favourable conditions.

2. Karyogamy

In this step, both nuclei of a dikaryon fuse to form a diploid nucleus. Karyogamy takes place in young basidium.

3. Meiosis

Soon after karyogamy, meiosis takes place in the basidium. The basidiospores, thus formed in the basidium, are haploid.

Development of Basidiocarp

The subterranean secondary mycelium takes nutrients from the soil and then forms the fruiting body or the basidiocarp. Basidiocarps develop as small ‘white-knot’ like structures at the tips of the underground hyphae. These hyphal knots enlarge gradually and give rise to button stage.

At button stage, the developing basidiocarp is differentiated into a basal bulbous part and an apical hemispherical region. The basal part forms the stipe, and the apical hemispherical part, the pileus.

Some hyphae at the junction of the stipe and pileus are drawn apart and form a ring-like chamber, called pre-lamellar chamber. The inner surface of the roof of the pre-lamellar chamber becomes deeply concave and it is lined with alternating radial bands of slow and rapidly dividing cells. The latter form gill primordia, which develop into gill lamellae that hang downward into the prelamellar chamber.

As pileus expands, there is an increase in radial interspaces between the gills. A membrane, called velum or inner veil, connects the margin of the pileus with the stipe. Due to the elongation of stalk, the buttons are raised on the soil surface.

The upper hemispherical region of the button grows more rapidly than the stalk. This causes rupture of the velum and the upper hemispherical region finally expands out as an open umbrella-like structure with numerous gills attached to its lower surface. The gills are exposed by the rupture of the velum.

At this stage, remnants of the velum are still attached to the stipe in the form of ring, called annulus. ln dry season, when soil is hard, buttons do not grow and remain underground. In rainy season, when soil is moist, they grow rapidly and come out of the soil surface. Hence, many basidiocarps can be seen in rainy season on the soil surface.

Mature Basidiocarp

The mature basidiocarp is an umbrella-shaped structure with a long massive stipe and a broad pileus. The stipe is a thick, fleshy and cylindrical structure, light pink or white in colour.

An umbrella-like pileus, 5-12 cm in diameter, is present at the distal end of the stipe. The upper convex surface of the pileus is white, light brown or yellow in colour. About 300-600 radially arranged gills hang down from the inner surface of the pileus.

All gills are not of the same length; they may be of full, half or quaner length. The surface of the gill is enveloped by a fertile layer, the hymenium. The gills are of light pink colour when young, but they tum brown or purplish black at maturity.

Internal Structure of Basidiocarp

The stipe is composed of numerous longitudinally intertwined hyphae. The hyphae are compactly arranged and form a pseudo-parenchymatous tissue in the peripheral region, whereas they are loosely arranged with large intercellular spaces in the central region. The peripheral region of the stipe is known as cortex and the central region as medulla.

The internal structure of the pileus is similar to that of the stipe. The pileus is also differentiated into an outer cortex and a central medulla. The hyphae of the stipe region extend in the distal end to form pileus.

Internal Structure of Gills

The internal structure of the gill is, however. complex and the following three regions can be distinguished.

  1. Trama. This is the central sterile region of the gill. It consists of many loosely arranged interwoven hyphae.
  2. Sub-hymenium (hypothecium). This region is situated on both sides of the trama. It is formed by the lateral branches of hyphae of trama region. The cells of these hyphal branches are isodiametric and 2-3 nucleate. This is also a sterile zone like trama.
  3. Hymenium. This is the outermost layer of the gill. It is fertile and composed of hyphae of the sub-hymenium region. In this region the cells are arranged in palisade-like layer. These aseptate fertile cells are known as basidium. Club-shaped sterile paraphysis occur in between the fertile cells.

The young basidium is a dikaryon and as the basidium matures the two nuclei fuse to form a diploid nucleus. This diplophase is ephemeral and after karyogamy the diploid nucleus divides meiotically to form four haploid nuclei.

Of these four nuclei, two are of (+) strain and two of (-) strain. At the distal end of the basidium, four peg-like outgrowths are formed. These outgrowths are known as sterigmata. The sterigmagata swells up at their tips and forms a single basidiospore. Thus, four monokaryotic basidiospores are formed in a basidium.

The young basidiospore is unpigmented but it develops brown or black pigments at maturity. At the junction of the mycelium basidiospore and the sterigma, there is a small projection, known as hilar appendics. Due to the presence of hilar appendics, the basidiospore is slightly oblique placed on the sterigma.

Dispersal and Germination of Basidiospores

As the basidiospore matures, a drop of liquid appears at the hilar appendics which remains surrounded by a limiting membrane. The basidiospore thus lies just above this drop. The drop increases in size gradually and attains a size of about one-fifth of the spore. Then the basidiospore is suddenly shot away from the sterigma.

The four basidiospores of a basidium are dispersed in rapid succession. When the basidiospore falls on a suitable substratum, it germinates by producing a germ tube that grows into a primary monokaryotic mycelium. Depending on the strain of the basidiospore, the mycelium may be of (+) or (-) strain. The primary monokaryotic mycelia form dikaryotic mycelium by somatogamy.