In this article we will discuss about:- 1. Occurrence of Allomyces 2. Thallus Structure of Allomyces 3. Life Cycle.
Occurrence of Allomyces:
The genus Allomyces was first discovered in India by E.J. Butler (1911). Subsequently it was found to be worldwide in distribution. It has been reported chiefly in tropical or warm countries. The genus comprises 5 species. Of these, A. arbuscula, A. macrogynus and A. javanicus are important. All the species are saprophytic, soil dwellers and found in the wet soil.
Thallus Structure of Allomyces:
The thallus in all the species is hyphal and is called the mycelium. It is filamentous and attached to the substratum by a tuft of delicate but well formed, branched rhizoidal hyphae constituting the rhizoidal system.
From the latter arises a single stout but slender hypha forming the lower trunk-like portion which subsequently undergoes several successive dichotomous branching’s to form the main, more or less, dichotomously branched part of the mycelium on which the reproductive organs are borne terminally.
At the time of dichotomy, the growing multinucleate hyphal tip divides into two new hyphal tips. Each of the new hyphal tips produces one of the two branches which develop nearly synchronously in the young mycelia. Septa are formed in both the hyphal branches nearly synchronously just at or above the dichotomy.
In Allomyces they are not formed in conjunction with cell division. The hyphae are thus multinucleate and coenocytic. Meyer and Fuller (1985) reported that ultrastructurally the septum consists of three layers with the electron opaque layer between two outer electron translucent layers.
In Allomyces each septum arises as 5 or 6 separate ingrowths of the wall material at each dichotomy. These protrusions broaden and fuse laterally and then grow centripetally leaving a central pore.
The lateral fusion between the protrusions is not always complete. Small pores may be left here in the rim of the mature septum. The septa in Allomyces are often referred to as false septa because they do not completely separate the protoplasm of adjacent somatic segments.
Life Cycle of Allomyces (Fig. 4.14):
Allomyces in its life cycle exhibits distinct alternation of generations—a significant feature which sets these fungi (Blastocladiales) apart from all other fungi, there occur two distinct but identical individuals in a single life cycle of Allomyces. One of these is the haploid gametothallus and the other diploid sporothallus.
1. Gametothallus:
It is homothallic and at a certain stage of maturity bears gametangia which are distinguished as male and female. Both produce gametes which are motile, uniflagellate and uninucleate.
The planogametes are unequal in size. The male gamete is about half the size of the female. The gametothallus, male and female gametangia and gametes which are all haploid structures constitute the gametophyte generation or haplophase. Meiozoospores are the pioneer structures of this phase and the anisoplanogametes, the last. The anisoplanogametes fuse in pairs to form the diploid zygote.
2. Sporothallus:
The diploid zygote germinates immediately to produce an alternate plant in the life cycle. It is the diploid sporothallus. At maturity the sporothallus produces two types of sporangia and not the gametangia. These are thin-walled mitosporangia and thick-walled, pitted resting or resistant meiosporangia. The mitosporangia produce diploid zoosores.
Since the nuclear division involved in their differentiation is mitotic, it is appropriate to call them mitozoospores. The latter, on germination, give rise to the diploid sporothalli. The mitozoospores thus serve to reduplicate the diploid generation or diplophase in the life cycle and thus play no role in the phenomenon of alternation of generations.
The nuclei in the meiosporangium prior to its germination undergoes meiosis. The meiosporangial protoplast containing the haploid nuclei undergoes cleavage to form haploid zoospores known as the meiozoospores or gonozoopores in contradiction to the mitozoospores.
The meiozoospore, on germination, produces an alternate plant in the life cycle called the gametothallus. The zygote, sporothallus, mitosporangia with meiozoospores and meiosporangia, which are all diploid, constitute the sporophyte generation or diplophase.
The diploid zygote is the pioneer structure of this phase and meiosporangia the last. The meiozoospores produced in the meiosporangia are the pioneer structures of the future haplophase.
The gametothallus bearing the gametangia and the sporothallus bearing the sporangia occur regularly one after the other in the single sexual cycle of Allomyces. This phenomenon is called alternation of generations.
Since the two alternating individuals (gametothallus and sporothallus) in the life cycle are morphologically similar, the alternation of generations in Allomyces is termed isomorpliic. The life cycle of Allomyces in which there is alternation of two vegetative individuals, the gametophyte or sporophyte with sporogenic meiosis is called Haploid-diploid or diplobiontic.
On the basis of his extensive studies of the collections obtained from all over the world Emerson (1941) divided the genus Allomyces into their sub genera: Euallomyces, Cystogenes and Brachyallomyces. They differ from each other in exhibiting alternation of generation.
While Brachyallomyces shows no alternation of generation because of the absence of gametophytic stage, Euallomyces shows isomorphic alternation of generation and Cystogenes shows heteromorphic alternation of generation having the gametophyte reduced to a small tiny cyst.
