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The following points highlight the seven main evolutionary tendencies of Ascomycetes.
(i) The gradual transition from the aquatic forms to purely terrestrial species noticeable in the Phycomycetes culminates in the Ascomycetes which are fungi of drier, terrestrial habitats completely lacking motile cells in their life cycle.
(ii) The tendency towards increased septation of hyphae in the advanced Zygomycetes becomes firmly established so that the extensively developed septate mycelium becomes the characteristic feature of the somatic phase of the Ascomycetes.
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(iii) The septate hyphae of the Ascomycete mycelium show a further tendency to come together and intertwine compactly to form fungal tissue characteristic of compact bodies such as the stroma and sclerotia.
(iv) From the primitive to the more advanced Zygomycetes there is a tendency for the sporangia to be transformed into sporangioles and these into conidia which are the typical asexual spores of Ascomycetes. The conidia play an important role in the spread of Ascomycetes and thus form a striking feature in the life cycle.
In the lower Ascomycetes the conidia are formed in exceptional abundance and throughout the period of mycelial growth. In fact in some species of Ascomycetes conidia alone appear to be sufficient for survival. These fungi have thus lost the capacity to produce sexual spores. Such Ascomycete species are allotted to the class Imperfect Fungi.
(v) The conidia bearing hyphae (conidiophores) in many Ascomycetes become organised into definite, asexual fruiting bodies such as acervuli, sporodochia, pycnidia and synnemata.
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(vi) Reduction of Sexuality. Among the lower Ascomycetes -the sexual apparatus is well.
(vi) Reduction of Sexuality: Among the lower Ascomycetes -the sexual apparatus is well developed. Both the sex organs, antheridia and oogonia or ascogonia are well marked and functional. They are usually developed on the same mycelium.
In few cases as in the family Laboulbeniaceae, they occur on distinct individuals. There is however, a gradual and progressive degeneration of the sex organs as we proceed from the lower to the higher Ascomycetes.
In the most advanced Ascomycetes there is complete suppression of the sex organs. In them the sexual apparatus is wholly lacking. This progressive reduction in sexuality can be well illustrated by a comparison of species.
They can be arranged in a series illustrating the gradual reduction and gradual elimination of the sexual apparatus.
Firstly, the antheridium ceases to function. In some species of Aspergillus both the sex organs are developed. Sexual fusion takes place. Same is the case in Lachnea stercorea.
There is, however, no migration of the contents of the antheridium into the oogonium in this species. It show that the antheridium has become non-functional. There is another species of the same genus, which goes a step further. It is Lachnea cretea.
In this species the antheridium is absent. The ascogonium is well developed and functional. Coprobia (Humaria) granulatus illustrates another step towards degeneration of sexuality.
It is characterised by the absence of trichogyne from the ascogonium. The antheridium is also lacking. Coprobia (Humaria) rutilans illustrates the final stage in the elimination of sex organs.
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The antheridia are absent and so are the ascogonia or the oogonia. The sexual process in these advanced forms becomes extremely simplified. It takes place by the copulation between the cells of two vegetative hyphae.
Finally even this impulse towards sexuality disappears. The two vegetative nuclei of adjacent cells of the same homothallic mycelium come together to establish a dikaryon.
The two nuclei of the dikaryon fuse in the ascus mother cell to form a synkaryon. The latter by meiosis followed by mitosis gives rise to 8 haploid ascospores within the ascus.
Hence in the final stage the development of the sexual apparatus m the life cycle is totally abandoned.
(vii) Gradual and Progressive Differentiation (complexity) of Ascocarp:
The degeneration of the sexual apparatusin the Ascomycetes is accompanied by a corresponding morphological differentiation of the ascocarp. In other words the ascocorp becomes more complex as we proceed from the lower to the higher Ascomycetes.
This evolutionary series starts with the primitive Ascomycetes such as the yeasts in which the ascospores are produced in typical vegetative cells which are diploid. In Eremascus the zygote itself enlarges to form a globose ascus.
Taphrina goes a step further. Special asci are formed from the binucleate ascogenous cells. But both in Eremascus and Taphrina the asci are not grouped in ascocarps. The next step is illustrated by Aspergillus and Talaromyces (Penicillium).
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In both these genera the asci are grouped in a small cleistothecium. The asci in the cleistothecium are distributed at random. They do not form a definite hymenial layer.
The series then passes through the sub-series Pyrenomycetes and Discomycetes. The former is characterised by the presence of perithecium type of ascocarp and the latter with a shield type called the apothecium.
In both asci are organised into a definite fertile layer called the hymenium. The series finally ends in the complex fructification of Morchella. The ascocarp of Morchella is very complex.
It is differentiated into a stalk-like portion, the stipe surmounted by a fertile cap-like, conical portion called the pileus. The Morchella ascocarp with its pileus and stipe can be morphologically compared with the stipe and pileus of the basidiocarp of the Basidiomycetes.
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(viii) Another evolutionary trend is the origin of dikaryophase in the life cycle which in the advanced Ascomycetes increases in duration and importance but never becomes independent.
It remains dependent on the haploid maycelium for its nutrition and occurs only inside the fruit body. The diplophase correspondingly becomes transitory and insignificant represented only by the fusion nucleus of the young ascus.
