Somatic Structures and Occurrence of Basidiomycetes

In this article we will discuss about the somatic structures and occurrence of basidiomycetes, explained with the help of suitable diagrams. 

The somatic mycelium is composed of septate branched hyphae a portion of which usually serves the function of absorption of nutriment from, the substratum and the rest performing different other functions. It may be white, bright yellow or orange, deep-brown to charcoal-black.

The mycelium usually grows in a wide range of substrata like, damp shady places containing decayed vegetative matters, rotten logs, grassy land, sometimes on animal dung, and similar other places.

In parasitic forms the mycelium lives in or along the host tissue receiving nutrition from the host protoplasm. In most Basidiomycetes there is profuse development of hyphae which may even become interwoven into macroscopic structures like, sclerotia, rhizomorphs, and basidiocarps.

The basidiocarps are usually of definite form showing considerable internal differentiation of tissues. Again there are fungi included under the Basidiomycetes which possess poorly developed hyphae. The somatic mycelium in course of development passes through three stages.

When young, the mycelium is multinucleate, a stage that follows immediately after the production of a germ tube during the germination of a basidiospore. This multinucleate condition of the mycelium is very short and is followed by the appearance of septa (dolipore septa) resulting in the formation of septate mycelium having uninucleate cells, monokaryotic, is known as the primary mycelium.

The hyphae of the primary mycelium are usually wide- angled. The primary mycelium gives rise to secondary mycelium in which the cells are binucleate, dikaryotic, the two nuclei are derived from different parent cells. But Buller used the term haploid mycelium to the mycelium possessing uninucleate cells and diploid mycelium to that with cells each containing a pair of conjugate nuclei.

Since nuclei in a primary mycelium (haploid mycelium) are derived from the original single nucleus of the basidiospore, they are all identical—Homokaryotic and the mycelium is homokaryotic mycelium. In the majority of the Basidio­mycetes the homokaryotic mycelium does riot produce basidiocarp.

The dikaryotic condition is attained from the monokaryotic condition in a variety of ways, a process known as dikaryotization or diploidization. The dikaryotic mycelium- is the secondary mycelium whose hyphae are narrow-angled.

The secondary myceliuim is responsible for the development of basidia and basidiospores. The basidiocarp of-the Basidiomycetes is developed from the dikaryotic mycelium. In a basidiocarp, the portion of the dikaryotic mycelium producing sterile tissue is the tertiary mycelium and that producing fertile layer is the secondary mycelium.

In majority of the Basidiomycetes the secondary mycelium is characterized by’ the presence of bridge-like hyplial connections known as clamp connections. The clamp connections are more often present at every septum parti­cularly in narrow hyphae.

These structures are usually absent in broader hyphae. Clamp connections were discovered by Hoffmann in 1856 and were observed by Bail (1856), de Bary (1859, 1866, 1884), Tulasne (1861), Brefeld (1877, 1888, 1889), and many others. But the details of nuclear behaviour associated with clamp connection were worked out by Kniep (1915) and Mile. Bensaude (1918).

The formation of clamp connection starts when the binucleate terminal cell of a dikaryotic hypha is ready to divide (Fig. 252A). A small pocket known as clamp is formed on the lateral wall usually in the middle of the cell and between the two nuclei (Fig. 252B).

The clamp grows backwards away from the apex of the cell. The two nuclei of the terminal cell then change their position. One of them moves into the newly formed clamp and the other remains behind its base (Fig. 252G). They then divide simultaneously, conjugate division (Fig. 252D).

During division the spindle axis of the nucleus in the clamp lies obliquely and that of the other remains more or less parallel to the main axis. Towards the end of the division of the nuclei, the four daughter nuclei which are about to be formed, exhibit remarkable change of position. One daughter nucleus from the clamp and one from the main axis pair toward the end of the hypha.

Of the other two daughter nuclei, one remains in the clamp and the other moves down in the main axis. In the meantime the clamp grows further and bends down touching the wall of the main axis. Now two septa appear. One at the base of the clamp along the main axis which cuts off a basal cell from the original apical hyphal cell.

Whereas, the other septum is formed directly under the base of the clamp being arranged obliquely with the first septum.

In the meantime one of the four daughter nuclei is imprisoned in the clamp until the clamp grows further and its end comes in contact with the lateral wall (Fig. 252E). The walls at the point of contact between the clamp and the lateral wall dissolve and a passage is formed. Through this passage the nucleus of the clamp migrates in the basal cell and meets its conjugate mate.

Thus a short semicircular hyphal outgrowth is developed just behind a septum during cell division in such a way as to connect the two cells together (Fig. 252F). This short semicircular hypha is known as a clamp connection. Functionally, however, it is what may be called a by-pass hypha through which the contents of the two adjacent cells are in communication with each other.

Each of the septa has a minute pore in its centre, and through the two pores of the two septa protoplasm flows. Clamp connections ensure separation of the sister nuclei of the dikaryon, at each division, into two daughter cells. Since they occur often on the dikaryotic mycelium, it was at one time believed that they are connected with the diploidizing process.

But it has now been shown that they merely help in the flow of protoplasm and food material from cell to cell through the pores in the septa. They are, however, a visible sign of the presence of conjugate nuclei.

But again, though clamp connections are always associated with conjugate nuclear division, the converse is not true, for such division may take place without their formation. Depending on the thickness of hypha the nuclei can pass from cell to cell through the central pores of the septa. In the opinion of some, the development of clamp connection and crozier formation of an ascogenous hypha are homologous processes.

Their contention is based on the fact that a clamp generally occurs at the base of a basidium which again arises from the terminal cell of a dikaryotic hypha, a condition very similar to the development of ascus from an ascogenous hypha whose terminal cell is also dikaryotic. Whereas, others express grave doubt and rather consider them as analogous processes.

In general, clamp connections arise singly (Fig. 252 G to I). But in some cases they may develop in two or more (Fig. 252 J). Again clamp connections are entirely lacking in certain Basidiomycetes and interestingly enough they have been reported in some Tuberales belonging to the Ascomycetes.

But in almost all Basidiomycetes clamp connection is present and this is a character of taxonomic importance of the group. Although clamp connections are connected with dikaryotic mycelium, they appear in monokaryotic mycelium also, e.g., Stereum hirsutum and in multinucleate primary mycelium, e.g., Coprinus narcoticus.

Normally the two nuclei of a conjugate pair attract one another and divide simul­taneously, thus providing pairs of conjugate nuclei for new cells. Yet, under certain conditions in some Basidiomycetes and Ascomycetes, the two nuclei of a conjugate pair may become separated in different cells and these cells may develop into mono­karyotic hyphae.

Thus a dikaryotic mycelium may produce monokaryotic cells or monokaryotic branches. This is a process which is just the reverse of diploidization and is known as de-diploidization. De-diploidization is thus the production of monokaryotic cells or hyphae by a dikaryotic mycelium.