Mycologists have made numerous attempts to categorise fungi according to their vegetative and reproductive characteristics. Fungi are divided into four classes according to conventional classifications: Ascomycetes, Phycomycetes, Basidiomycetes, and Deuteromycetes. The Phycomycetes comprise fungi like Oomycetes, Chytridiomycetes, and Zygomycetes, lower fungi with an algal origin.
A fungus (PL: fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from the other eukaryotic kingdoms, which by one traditional classification include Plantae, Animalia, Protozoa, and Chromista.
The fungus kingdom encompasses an enormous diversity of taxa with varied ecologies, life cycle strategies, and morphologies ranging from unicellular aquatic chytrids to large mushrooms. However, little is known of the true biodiversity of the fungus kingdom, which has been estimated at 2.2 million to 3.8 million species. Of these, only about 148,000 have been described, with over 8,000 species known to be detrimental to plants and at least 300 that can be pathogenic to humans. Ever since the pioneering 18th and 19th century taxonomical works of Carl Linnaeus, Christiaan Hendrik Persoon, and Elias Magnus Fries, fungi have been classified according to their morphology (e.g., characteristics such as spore color or microscopic features) or physiology. Advances in molecular genetics have opened the way for DNA analysis to be incorporated into taxonomy, which has sometimes challenged the historical groupings based on morphology and other traits. Phylogenetic studies published in the first decade of the 21st century have helped reshape the classification within the fungi kingdom, which is divided into one subkingdom, seven phyla, and ten subphyla.
Before the introduction of molecular methods for phylogenetic analysis, taxonomists considered fungi to be members of the plant kingdom because of similarities in lifestyle: both fungi and plants are mainly immobile, and have similarities in general morphology and growth habitat. Although inaccurate, the common misconception that fungi are plants persists among the general public due to their historical classification, as well as several similarities. Like plants, fungi often grow in soil and, in the case of mushrooms, form conspicuous fruit bodies, which sometimes resemble plants such as mosses. The fungi are now considered a separate kingdom, distinct from both plants and animals, from which they appear to have diverged around one billion years ago (around the start of the Neoproterozoic Era). Some morphological, biochemical, and genetic features are shared with other organisms, while others are unique to the fungi, clearly separating them from the other kingdoms:
Although commonly included in botany curricula and textbooks, fungi are more closely related to animals than to plants and are placed with the animals in the monophyletic group of opisthokonts. Analyses using molecular phylogenetics support a monophyletic origin of fungi. The taxonomy of fungi is in a state of constant flux, especially due to research based on DNA comparisons. These current phylogenetic analyses often overturn classifications based on older and sometimes less discriminative methods based on morphological features and biological species concepts obtained from experimental matings.
The Eccrinales and Amoebidiales are opisthokont protists, previously thought to be zygomycete fungi. Other groups now in Opisthokonta (e.g., Corallochytrium, Ichthyosporea) were also at given time classified as fungi. The genus Blastocystis, now in Stramenopiles, was originally classified as a yeast. Ellobiopsis, now in Alveolata, was considered a chytrid. The bacteria were also included in fungi in some classifications, as the group Schizomycetes.
Scientific classification of all forms of life (before 1970), was dominated by the concept of two kingdoms - Animalia and Plantae. Fungi have been assigned to the plant kingdom because of their mode of nutrition, non-motile nature, presence of cell-wall, and reproduction by spores. Similarly, bacteria and photosynthetic protists were considered as plants and the protozoa as animals. During 19th century, it was realized that the most basic feature of all green plants is that they are phototrophs and autotrophs. On the other hand, animals are chemotrophs and heterotrophs. From these criteria it can be observed, that fungi are nearer to animals than plants, although they are not mobile like animals. However, the two-kingdom system was under criticism as this system oversimplified the true nature of evolutionary history. Thus, in 1860 a third kingdom Protoctista, was proposed which included organisms like fungi, bacteria, algae and protozoans, by the German biologists J. Hogg and Ernst Haeckel. These organisms were characteristically having simple cellular organization and features of both plants as well as animals. But it was clear even then that they did not form any natural group being not closely related. Afterwards in 1938 a modification of this system was suggested by Herbert Copeland by removing bacteria from Protoctista and projecting a fourth kingdom Monera. Furthermore, Whittaker modified this system by removing fungi from Protoctista and placing them in their own kingdom Mycetae. The next major change came with the development of electron microscopy (1945- 60), when scientists understood the fundamental difference in the cellular organization of bacteria and other organisms. The concept changed from animals and plants to prokaryotes (Gr. pro = primitive; karyos = kernel, nucleus) and eukaryotes (Gr. eu = true). The prokaryotes are those where the cells do not have a true nucleus and eukaryotes have true nucleus. The lack of a well-developed nucleus and the simple cell structure of a bacterium led to its placement in prokaryotes, while all other organisms were termed eukaryotes. With the recognition of differences between prokaryotes and eukaryotes, bacteria
Until the 1980s, the major groups of fungi and the relationships between them were based on light microscopic examinations of comparative morphology and the developmental patterns of the sexual reproductive structures. Examples of traditional taxonomic features include the presence or absence of septa in hyphae, fine details of the type, formation and release mechanisms of spores, or aspects of biology or ecology of fungi. Before we put before the readers the modern systems of classifications, lets us first discuss the major criteria used in the classification of fungi.
i. Basic shape of the fungal thallus.ii. Form, colour and size of the asexual or sexual spore producing structures. The basics of the fungal life cycles and morphology were already established bythe year 1887, and were used for classification by Anton de Bary (1887). Gross morphological features helped to establish many taxa within Ascomycota,Basidiomycota and Lichens. The diversity of forms exhibited by many of thesewere readily recognizable by eye or at low magnification because of their largesize. For most other fungi, being microscopic, a light microscope is required afterproper staining of a squash mount for studying their morphological features. Recently, scanning electron microscopy has been employed to study patterns andornamentations on the spore surface. As there are large numbers of morphological specifications associated with sexualsporulation in fungi, therefore these are considered important in fungalclassifications even today. However, if there is no sexual sporulation then the smaller morphological detailsassociated with asexual sporulation is used. And if sporulation is absent altogether, other specific features such as, sclerotiumform in filamentous fungi and cell shape and budding pattern in yeasts are used.
British mycologist, GC Ainsworth (1905-1998) was former director, CommonwealthMycological Institute. His scheme of classification was considered as an ideal one thatreflected natural relationships. In this scheme, the fungi were divided into two Divisions.Fungi with plasmodium or pseudoplasmodium were classified in the DivisionMyxomycota, whilst the majority of fungi, which were filamentous, were classified in theDivision Eumycota.
Alexopoulos and Mims (1979) placed all fungi including slime moulds in Kingdom Myceteaeof Superkingdom Eukaryonta. Kingdom Myceteae included 3 divisions, 8 subdivisions, 11classes, 1 form-class, 3 subclasses, and 3 form-subclasses.
classification are: Recognition of the artificial nature of three or even five kingdom system ofclassification. Recognition that the organisms traditionally known as fungiare polyphyletic. Acceptance of the theory of phylogenetic systematics. Development and application of molecular techniques in mycology; and Additional discoveries of new taxa including fossils. In an attempt to recognize monophyletic groups, the organisms once classified asfungi now are considered in three different groups, the monophyletic Kingdom Fungi,Kingdom Stramenopila and Kingdom Protista. This classification recognizes the fact that, all organisms that have been called fungiare not closely related. For example the old concept of fungi including Oomycota andSlime moulds is that they are polyphyletic - these groups do not share acommon ancestor. On the other hand Fungi and Animals are sister groups - these are closelyrelated lineages that share a recent common ancestor with each other than withother lineages. The kingdom level classification of the major groups of organisms considered at onetime as fungi, given by Alexopoulos, Mims and Blackwell (1996) shows thehypothesized evolutionary relationships of the organisms included in it. The Kingdom Fungi includes four phyla, namely