Lesson 2: Biological Classification- Fungi

Introduction

Tthe term fungus (plural- fungi) is a latin word meaning mushroom. The term fungi was coined by Gaspard Baubin (1560-1624) The science of study of fungi is known as mycology (Gk. Mykes: mushroom; logos; study) or mycetology. It  is a member of a large group of eukaryotic   organisms that includes microorganisms such as  yeasts  and molds, as well as the more familiar mushrooms. The Fungi  are classified as a kingdom that is separate from  plants, animals and bacteria. One major difference is that fungal cells have  cell walls that contain chitin,  unlike the cell walls of plants, which contain cellulose. These and other differences show that the fungi form a single group of related organisms, named the Eumycota (true fungi or Eumycetes), that share a common ancestor (a monophylectic group). This fungal group is distinct from the structurally similar  slime molds (myxomycetes) and  water molds (oomycetes).  It is regarded as a branch of botany, even though genetic studies have shown that fungi are more closely related to animals than to plants.

Abundant worldwide, most fungi are inconspicuous because of the small size of their structures, and their cryptic lifestyles in soil, on dead matter, and as symbionts of plants, animals, or other fungi. They may become noticeable when fruiting, either as mushrooms or molds. Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange. They have long been used as a direct source of food, such as mushrooms and  truffles, as a leavening agent for bread, and in fermentation of various food products, such as wine, beer and soya sauce. Since the 1940s, fungi have been used for the production of antibiotics, and, more recently, various enzymes produced by fungi are used industrially and in detergents. Fungi are also used as biological agents to control weeds and pests. Many species produce bioactive compounds called mycotoxins, such as alkaloides and polyketide,, that are toxic to animals including humans. The fruiting structures of a few species contain psychotropic compounds and are consumed recreationally or in traditional spiritual ceremonies. Fungi can break down manufactured materials and buildings, and become significant pathogens of humans and other animals. Losses of crops due to fungal diseases (e.g. Rice blast disease or food spoilage can have a large impact on human food supplies and local economies.

The fungus kingdom encompasses an enormous diversity of taxa with varied ecologies, life cycle strategies, and morphologies ranging from single-celled aquatic chytrids to large mushrooms. However, little is known of the true biodiversity of Kingdom Fungi, which has been estimated at around 1.5 million species, with about 5% of these having been formally classified. Ever since the pioneering 18th and 19th century taxonomical works of Carl Linnaeus,  Christian 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 last decade have helped reshape the classification of Kingdom Fungi, which is divided into one subkingdom, seven phyla, and ten subphyla.

Pictures of Fungi

Mycena chlophanos

  

Father of mycology : Pier Antonio Micheli (1679-1737)

Father of systematic mycology: E.M. Fries (1794-1878)

Father of modern mycology and plant pathology : H.A.de Bary

Father of Indian mycology and plant pathology: E.J. Butler

K.R. Kirtiker was the first indian scientist who collected and identified fungi
Whittaker (1969) placed this group of achlorophylloous organisms in a separate kingdom 'Fungi'

Shared features:

With other  eukaryotes: As other eukaryotes, fungal cells contain membrane bound with chromosomes that contain DNA with non-coding regions called introns and coding regions called exons. In addition, fungi possess membrane-bound cytoplasmic organelles such as mitochondria, sterol-containing membranes, and  ribosomes of the 80S type. They have a characteristic range of soluble carbohydrates and storage compounds, including sugar alcohols (eg.,mannotol), disaccharides, (e.g., trehalose), and polysaccharides (e.g., glycogen, which is also found in animals.

With animals: Fungi lack chloroplast and are heterotrophic organisms, requiring preformed organic compounds as energy sources.
With plants: Fungi possess a cell wall and vacuoles. They reproduce by both sexual and asexual means, and like basal plant groups (such as ferns and mosses) produce  spores. Similar to mosses and algae, fungi typically have haploid nuclei.
With euglenoides and bacteria: Higher fungi, euglenoids, and some bacteria produce the aminoacid L-lysine in specific biosynthesis steps, called the α- aminoadipate pathway

The cells of most fungi grow as tubular, elongated, and thread-like (filamentous) structures and are called hyphae, which may contain multiple nuclei and extend at their tips. Each tip contains a set of aggregated vesicles—cellular structures consisting of proteins, lipids, and other organic molecules—called Spitzenkorper. Both fungi and oomycetes grow as filamentous hyphal cells.] In contrast, similar-looking organisms, such as filamentous green algae, grow by repeated cell division within a chain of cells.
In common with some plant and animal species, more than 60 fungal species display the phenomenon of  bioluminiscence.

Unique features:

Some species grow as single-celled yeasts that reproduce by budding or binary fission  Dimorphic fungi can switch between a yeast phase and a hyphal phase in response to environmental conditions.
The fungal cell wall is composed of  glucans and  chitin; while the former compounds are also found in plants and the latter in the exoskeleton of arthropods fungi are the only organisms that combine these two structural molecules in their cell wall. In contrast to plants and the oomycetes, fungal cell walls do not contain cellulose.
   
Most fungi lack an efficient system for long-distance transport of water and nutrients, such as the xylem and phloem in many plants. To overcome these limitations, some fungi, such as Armillaria, form rhizomorphs, that resemble and perform functions similar to the roots of plants. Another characteristic shared with plants includes a biosynthetic pathway for producing terpenes that uses mevalonic acid and pyrophosphare as chemical building blocks. Plants have an additional terpene pathway in their chloroplasts, a structure fungi do not possess. Fungi produce several secondary metabolites that are similar or identical in structure to those made by plants.

Omphalotusnidiformis, a bioluminescent mushroom

Definition:Alexopoulos (1952) defined fungi as nucleated, achlorophyllous organisms, which typically reproduce sexually and asexually and whose ususally filamentous , branched somatic structures are surrounded by walls containing cellulose or chitin or both. It is cosmopolitan in distribution.

Habitat: The fungi are ubiquitous i.e foun in almost every habitat. They thrive well in moist , dark and wet conditions. The wet soil rich in humus is the most favourable condition for the fungi.  A few forms of aquatic forms of fungi are : Saprolegenia, Allomyces, Achyla etc.

Distribution: The fungi do not possess chlorophyll , hence are unable to produce their own food. The food is obtained from external source, i.e. By the process of extracellular digestion and absorption  of th digested material. This is the heterotrophic mode of nutrition.

Accordingly they are divided into two typesi.e. Parasites and saprophytes.

Parasites: They obtain their food from the living hosts. The parasites are of two types: Obligative parasites; Obligate parasites ( e.g.Puccinia) and facultative parasites (e.g. Ustilago)

Saprophytes: They derive their food from dea and decaying organic matter.  Saprophytes are of two types, i.e obligative  saprophytes.  (e.g.Mucor, Rhizopus), and Facultative saprophytes(  e.g. Eusarium).

Some fungi are symbiotically associated with algae to form lichens.  Afew also have symbiotic association with higher plants ( e.g. Pinus) called mycorrhiza.

Thallus Organization:

The plant body of  fungus is a thallus. Which may be either acellular or multicellular.the acellular thallus nmay be motile (e.g. Synchytrium) or non- motile (e.g. Saccharomyces). The mulicellular thallus is tubular, filamentous, branched and is called mycelium. The unit of mycelium is hypha.

The mycelium may be septate or aseptate. The aseptate mycelium lacks septa. It is multinucleate and is called coenocytic,  e.g the members of phycomycetes. In aseptate  mycellium , septa formation occurs during an injury and the separation of reproductive structures. Individual cells may be uninucleate, binucleate or multinucleate.

Each septum is perforated by a central pore. The pore may be simple or dolipore. The eptal pores may remain partially plugged by woronin bodies.

Mycelium may be eucarpic ( only a part forms the reproductive boduy) or holocarpic ( the whole mycelium is transformed intyo a reproductive body).The eucarpic forms may be monocentric ( having  a single sporangium or polycentric ( having many sporangia).

Fungal tissues

In some of the higher fungi , the mycelium gets organized into loosely or compactly woven structure, which looks like a tissue called Plectenchyma (  Gk: plekein- to weave, enchyma- tissue).It is of two types:

Prosenchyma: (Gk: Pros- toward, enchyma-tissue): It consists of loosely woven hyphae whci are almost parallel to each other and the cell and hyphae are easily distinguishablr from each other.

Pseudoparenchyma( Gk ;pseudo- false; enchyma-tissue) It consists of compact mass of parenchyma like tissue whre the hyphae are closely packed and interwoven so that they loose their individuality and canno be easily distinguished from each other.

The fungal mycelium also forms some specialized structures like rhizomorphs, sclerotia and stroma.

Cell structure

The fungi are eukaryotic cell type and possess a true nucleus. They are bound by  definite cell wall. The cell wall is made up of fungal cellulose / chitin ( nitrogen containing polysaccharided or heteropolymer of acetyl glucoseamine whci are common to the insects).  True cellulose is found in the cell wall of some phycomycetes (e.g. phytopthora)

The cell wall encloses protoplast , which is further differentiated into plasma membrane , cytoplam, nucleus, and vacuoles. The cytoplasm has all the eukaryotic cell organelles , with the exception of plastids.The dictyosomes  are also typical . Lomasome (Moore and McAlear,1961) is a convoluted complex of membranous outgrowth of plasmalemma.

Vacuoles are many but small . Near the hyphal tip; the cytoplasm contains small vesicles called chitosomes . They contain cell wall materials.Nuclei are smaller as compared to those of higher plants. Nuclei undergo intranuclear spindle formation ( karyochoresis).Food reserve is glycogen ( animal starch) and oils . Volutin granules are present.

Cell structure

Microscopic Organization

An environmental isolate of  Penicillium--- 1.hypha 2 conidiophore 3.phialide  4. conidia 5. septa

 

Most fungi grow as hyphae, which are cylindrical, thread-like structures 2–10 um in diameter and up to several centimeters in length. Hyphae grow at their tips (apices); new hyphae are typically formed by emergence of new tips along existing hyphae by a process called branching,  The combination of apical growth and branching/forking leads to the development of a myceliumHyphae can be either septate or  coenocytic: septate hyphae are divided into compartments separated by cross walls. Septa have pores that allow cytoplasm, organelles, and sometimes nuclei to pass through; an example is the dolipore septum in the fungi of the phylum Basidiomycota. Coenocytic hyphae are essentially multinucleate supercells.

Although fungi are opisthokonts—a grouping of evolutionarily related organisms broadly characterized by a single posterior flagellum- all phyla except for the chyrids have lost their posterior flagella.

Reproduction

The reproduction in Fungi are of the following types.viz: vegetative, asexual, sexual.

Vegetative Reproduction:

Reproduction of fungus in vegetative means take place by fragmentation, fission or budding.

Fragmentation: A small fragment of hypha detached from the parent mycelium develop into a new individual. Fragmentation is also known as disarticulation.