In this article we will discuss about the life cycle of spharotheca with the help of a suitable diagram.
Mycelium of Spharotheca:
The mycelium is superficial i.e., ectophytic and is made up of branched septate hyphae, the cells being uninucleate. The mycelium forms a white cottony mass on the surface of the host. The parasite draws its nourishment from the host by means of globular or lobed haustoria which are formed by the hypha at the point of contact with the host.
Reproduction in Spharotheca:
Asexual Reproduction:
Asexual Reproduction takes place by means of conidia produced on conidiophores. As in Erysiphe, the short erect conidiophores arise from the mycelium. The mother cell cuts off conidia in a vertical chain in basipetal succession. The conidium is ellipsoid to barrel shaped, uninucleate, hyaline with distinct fibrosin bodies which are conical, disc or rod shaped structures.
The fibrosin bodies appear when the conidia are mounted in 3 percent KOH solution. At maturity, the conidia detach very easily and are disseminated by wind. After reaching a citable host the conidium germinates by producing a forked germ tube.
Sexual Reproduction:
Sexual reproduction takes place in later summers when conidia formation slows down. It takes place by means of antheridia and oogonia which are formed at the tips of closely situated hyphal branches.
Antheridium:
The terminal cell of the male hypha divides to form a slender anthenridium and a basal stalk cell. The antheridium is uninucleate and more slender than the ascogonium.
Ascogonium:
The terminal cell of the female hypha divides to form a club-shaped ascogonium and a basal stalk cell. The ascogonium is uninucleate.
Plasmogamy:
Both the sex organs grow side by side in close contact. At maturity the walls at the point of contact, dissolve and a pore is formed. The male gamete passes through this pore to reach ascogonium where plasmogamy occurs. After this the antheridium collapses.
Development of Cleistothecium:
The development of cleistothecium, the asci and ascospores as in after the plasmogamy, the pair of male and female nuclei settle down in the ascogonium. The ascogonium now produces a row of cells by successive division.
The penultimate cell of this row of cells must be dikaryotic, whereas the remaining cells may be uninucleate. This dikaryotic cell acts as ascus mother cell in which karyogamy occurs resulting in a diploid nucleus. This diploid nucleus undergoes meiosis and subsequent mitosis forming eight nuclei.
The ascus mother cell also elongates and forms the ascus. Each of the eight nuclei is enveloped by cytoplasm and eight ascospores are formed. Each ascus is ovoid broadly elliptic to subglobose and contains eight ascospores. A hyphal sheath develops and protects the developing asci which are enclosed in an ascocarp—the cleistothecium.
Cleistothecium:
The mature cleistothecium is globose and dark brown in colour. The wall is two layered thick, each several cells thick. The outer layer is made up of darker, melanised, polygonal and uninucleate cells whereas the inner one is made up of thin walled binucleate cells’ The outer layer is protective while the inner one provides nourishment to developing ascus.
The cleistothecium also bears a variable number of simple myceloid appendages. These are as long as the diameter of cleistothecium and they are usually interwoven with the mucelium.
The clehtothecium usually has a single ascus with eight ascospores. Each ascospore is ellipsoid to nearly spherical, uninucleate and haploid.
Dehiscence:
The ascus and ascospores are released only after the disintegration of cleistothecium. The ascospore germinates on a suitable host and forms new mycelium.
Gordon (1966) is of the view that although Sphaerotheca follows a more or less similar pattern in sexual cycle as in Erysiphe, generally, fertilised ascogonium might not be playing any functional role in the development of asci and ascospores. Thus, the ascogonium may be designated as pseudoascogonium and antheridium as pseudoantheridium.
Pyrenomycetes with flask-shaped perithecium:
This group includes Ascomycetes in which the ascocarp is a flask-shaped perithecium. The members of this group are often called the flask-fungi. The club-shaped or cylindrical asci are usually borne in a parallel series forming a fertile layer called the hymenium. The latter lines the base and generally the sides of the inner wall of the penthecium. Occasionally the asci are borne in basal tufts.
The ascospores in the majority of the species are liberated through a circular pore at the top of the fruit. It is termed the ostiole. The penthecia may be free or embedded in a stroma formed from the hyphae of the fungus. In either case the perithecium has a definite wall of its own.
Lining the ostiole are short hair-like hyphal growths arising from the inner wall of perithecial neck. These are the periphyses. There is considerable difference of opinion among the mycologists as to the number of orders included in thus series.
Most of the mycologists enlist Chaetomiales, Sphaeriales, Diaporthales Hypocreales, Clavicipitales, Coronophorales and Coryneliales. The genus Claviceps is usually selected as an example of the Pyrenomycetes with perithecium type of fructification as it received considerable attention of the mycologists.