In this article we will discuss about the origin and evolution of ascomycetes.
The phylogeny of Ascomycetes is still uncertain. The supporters of polyphyletic origin of fungi from the various classes of algae derive the Ascomycetes from the red algae (presumably the Florideae).
The floridean hypothesis was first proposed by Sachs in 1875. The-adherents of this hypothesis such as Thaxter (1896); Bessey, (1914, 1925, 1942,1950); Dodge, (1914); and Jackson, (1900) base phylogenetic speculation on the striking similarities (between the sex organs and fructifications of these two groups) which include:
(1) Female gametangium with a trichogyne.
(2) Occurrence of non-motile male cells called spermatia.
(3) Origin of sporogenous filaments (ascogenous hypha) in Ascomycetes and gonimoblast filament in Florideae as outgrowths from the female gametangium.
(4) Similarity between the ascocarp and cystocarp.
In addition to the above, lack of motile cells in the life cycle, filamentous thallus with apical growth and a central pore in the transverse septa are other features which the Ascomycetes share with the red algae.
To some phylogeneticists, the above-mentioned common features are an indication of direct relationship between the two groups.
They hold that the Ascomycetes have arisen from the red algae by the loss of photosynthetic pigments coupled by a change in the mode of nutrition to parasitism and saprophytism.
The adoption of land habit by the Ascomycetes resulted in the development of ascus as the spore producing structure and ascospores as the type of spores replacing tetrasporangia and naked tetraspores, respectively.
On the basis of floridean hypothesis, the lower Ascomycetes with the simplest sexual apparatus are considered degenerated forms. The immediate descendants produced spermatia and ascogenous hyphae.
Their original progenitors in the red algae were forms in which gonimoblast filaments developed directly from the carpogonium.
The next in order, probably, were the Ascomycetes in which the male sex organ (Antheridium) developed near the female sex organ (ascogonium) bearing ascogenous hyphae.
From these evolved the lower Ascomycetes with the loss of ascogenous hyphae and zygote functioning directly as an ascus.
In the eyes of opponents, the floridean hypothesis of origin of Ascomycetes does not stand critical examination. The chemical nature of cell wall is different in the two groups.
The method of ascospore formation in the Ascomycetes is unique and has no parallel in the red algae. They consider these morphological resemblances illusory.
At the most they consider it a case of parallel evolution of similar structures between two fundamentally different groups occurring in similar habitats.
To many mycologists Phycomycetes particularly the advanced Zygomycetes appear to be the ancestral stock from which the Ascomycetes have evolved. De Bary (1887) proposed the theory of monophyletic origin of Fungi.
He advocated that the Ascomycetes (along with the Basidiomycetes) arose from the Phycomycetes. The ardent supporters of this hypothesis are Dangeard, (1907); Claussen, (1912); Atkinson, (1915); Guilliermond, (1928), Fitzpatrick, (1930), Gaumann, (1952) and Hawker, (1967).
This hypothesis is based on the homology between the germ sporangium of Zygomycetes (Mucorales) and the ascus of the Ascomycetes.
The supporters of this view regard the Ascus as a specialised sporangium derived from the germ sporangium of Mucorales by the following changes among others:
(i) The number of meiospores has been reduced till it has become fairly constant at eight.
(ii) The cleavage method of spore formation has been replaced by free cell formation. The affinities with the Zygomycetes seem reasonable in some other respects as well.
The tendency among the Zygomycetes towards the development of the conidium-a typical asexual spore of the Ascomycetes and a tendency towards greater protection of the zygospore suggests a relationship with the Ascomycetes.
The conidia of many Ascomycetes such as Erysiphales, Aspergillales and others are similar to those of the Mucorales. Further the mycehum in both the Ascomycetes and Zygomycetes is usually filamentous and highly developed.
The only difference is that it is septate in the Ascomycetes and aseptate in the Zygomycetes. But in the order Entomophthorales of class Zygomycetes the hyphae are normally regularly septate.
Sexual reproduction by conjugating gametangia in some of the Hemiascomycetidae (lower Ascomycetes) is very similar to the higher Zygomycetes. There is fusion of unequal cells (gametangia) in each of which there is a single functional sexual nucleus.
The female gametangium in Dipodascus (Hemiascomycetidae) elongates to form an ascus whereas in some of the higher Zygomycetes it grows out to form a thick-walled zygospore.
In fact the transition from the gametangiogamous Zygomycetes to the Hemiascomycetidae is so gradual that some mycologists interpret these facts as showing further development of evolutionary tendencies found in the Zygomycetes.
Further support for De Bary’s hypothesis is furnished by the following similarities between the members of these two groups:-
(i) Similar glycogen-like type of stored food
(ii) Cell walls constructed mainly of chitin, and
(iii) Sexual fusion of the nuclei tends to be delayed until sometime after mating, both in some of the advanced Zygomycetes and lower Ascomycetes.
From the above-mentioned similarities between the members of these two groups one may infer that the ancestor of the Ascomycetes may have been an ancient member of the Zygomycetes.
On the basis of this hypothesis the lower Ascomycetes which lack ascogenous hyphae and have the zygote directly functioning as an ascus, are considered the immediate descendants of the ancestral stock.
These gave rise to forms in which plasmogamy by gametangial contact was not immediately followed by karyogamy and asci developed indirectly from the ascogenous hyphae arising as outgrowths from the ascogonium. Next in order evolved the higher Ascomycetes with spermatia instead of antheridia.
Hawker (1967) emphasized that similarities in fine structure (sparse smooth endoplasmic reticulum) and mitochondria with groups of parallel plate-like cristae and other similarities between the Zygomycetes and Ascomycetes make it highly probable that the Ascomycetes may have evolved from some ancient member of the Zygomycetes not closely similar to any extant Zygomycete.
In an attempt to explain the origin of hooks which are characteristic structures of the Asomycetes some phylogeneticists have recently proposed a new theory about the origin of Ascomycetes (Euascomycetidae). They consider hooks as degenerate sex organs.
According to them, sexual reproduction in the ancestors of Ascomycetes proceeded by the formation of lateral conjugation bridges between the adjacent cells of the same hypha.
During the course of evolution this process was replaced by conjugation of gametangia (gametangiogamy). The original process, however, persisted in the living Ascomycetes in the form of hooks which no longer function as they once did.
On the basis of this theory the Hemiascomycetidae which lack hooks have ended blindly leading from the Phycomycetes. They have nothing to do with the higher Ascomycetes.
The latter originated directly from some very primitive extinct Phycomycete stock and developed later parallel to the living Phycomycetes.