GENERAL CHARACTERISTICS OF FUNGI

 INTRODUCTION

Fungi are eukaryotic spore bearing organisms that lack chlorophyll and generally reproduce both sexually and asexually. They are of great practical and scientific importance. One of the reasons for this is that many fungi are of microscopic cellular dimensions. Fungi have a diversity of morphological appearances depending on the species. Fungi comprise the molds, mushrooms and yeasts. Molds are filamentous and multicellular while yeasts are unicellular. They are widely distributed and found wherever moisture is present. They are of great importance to man in both beneficial and harmful ways. This unit examines the general characteristics of fungi, the distribution, morphology, nutrition and reproduction of fungi.

Definition of Fungi

Fungi are eukaryotic spore bearing organisms that lack chlorophyll and generally reproduce both sexually and asexually.

Distinguishing Characteristics of Fungi

They are large, diverse and widespread group of organisms, the molds, mushrooms and yeasts.

1.  Fungi are Eucaryotic. They are members of the domain Eucarya.  
2.  They contain a membrane-enclosed nucleus and several other organelles. 
3.  They have no chlorophyll. 
4.  They are chemo organotrophic organisms. 
5.  The body of the fungi is called thallus. 
6. The thallus may consist of a single cell as found in yeasts. 
7. The thallus may consist of filaments, 5 to 10µm across which are commonly branched as found in molds. 
8. The yeast cell or mold filament is surrounded by a true cell wall (exception is the slime mould which have a thallus consisting of a naked amoeboid mass of protoplasm). 
9. Some fungi are dimorphic, that is they exist in two forms. Some pathogenic fungi of humans and other animals have a unicellular and yeast like form in their host but when growing saprobically in soil or on a laboratory medium they have a filamentous mold form. 
10.  Habitat distribution of fungi is diverse. Some are aquatic, living primarily in fresh water and a few marine fungi are terrestrial. They inhabit soil and dead plant. Some are parasitic, inhabiting and infecting living hosts either plants or animals. Some form beneficial relationships with other organisms as mycorrhizae. 
11. The study of fungi is known as mycology. 

Structure and Forms of Fungi

The body or vegetative structure of a fungus is called a thallus (plural thalli). It varies in complexity and size ranging from the single cell microscopic yeasts to multicellular moulds and mushrooms. The fungal cell is usually enclosed in a cell wall of chitin.

 Yeasts

1. They are unicellular fungi that have a single nucleus. '
2. They are commonly egg-shaped but some are elongated and some spherical. Yeasts have no flagella or other organelles of locomotion. 
3. They possess most of the other eukaryotic organelles. 
4. Yeast cells are larger than most bacteria. Yeasts vary considerably in size ranging from 1 to 5µm in width and from 5 to 30µm or more in length. 
5. They reproduce asexually by budding and traverse division or sexually through spore formation.

Fig. 1: A Diagram of a Yeast Cell 

Source: Wikimedia Commons by Frankie Robertson using Inkscape (2009) 

Molds

The thallus of a mold consists of long branched threadlike filaments of cells called hyphae. These hyphae form a mycelium which is a tangled mass or tissue like aggregation of hyphae.

Hyphae

1.  Each hypha is about 5 to 10µm wide. Hyphae are composed of an outer tube like wall surrounding a cavity the Lumen which is filled or lined by protoplasm. Between the protoplasm and the wall is the plasmalemma, a double layer membrane which surrounds the protoplasm. 
2.  The hyphal wall consists of microfibrils composed of hemicelluloses or chitin. True cellulose occurs only in the walls of lower fungi. 
3. Wall matrix material in which the microfibrils are embedded consists of proteins, lipids and other substances. Growth of a hypha is distal near the tip. 

The Mycelium

1. The mycelium is a complex of several filaments called hyphae (singular, hypha). New hyphae generally arise from a germinated spore. The germinated spore puts out a germ tube or tubes which elongate to form hyphae. These hyphae form a tangled mass or tissue like aggregation. 

Fig. 2: Rhizopus stolonifer 

Source: Retrieved from The Backyard Nature Website at 

file://G:\Bread mold fungus, Rhizopus stolonifer.htm. 

In some fungi, protoplasm streams through hyphae uninterrupted by cross walls, these hyphae are called coenocytic or aseptate. The hyphae of others have cross walls called septa (s. septum) with either single pore or multiple pores that enables cytoplasmic streaming. These hyphae are termed septate.

Summarily, hyphae can be said to occur in three forms:

1. Nonseptate or coenocytic; such hyphae have no septa. 
2. Septate with uninucleate cells. 
3. Septate with multinucleate cells. Each cell has more than one nucleus in each compartment. 

Nutrition and Metabolism

Most fungi are saprobes, securing their nutrients from dead organic matters. They release hydrolytic exo-enzymes that digest external substrates and absorb the soluble products. They are also chemoorganoheterotrophs, i.e. they use organic compounds as a source of carbon, electrons and energy.

Fungi are usually aerobic; however, some yeasts are facultatively anaerobic and can obtain their energy by fermentation. Obligately anaerobic fungi are found in the rumen of cattle.

Reproduction

Reproduction in fungi can either be asexual or sexual.

Asexual Reproduction

Asexual reproduction is a type of reproduction involving only one parent that produces genetically identical offspring by budding or by the division of a single cell or the entire organism into two or more parts. Asexual reproduction, also called somatic or vegetative reproduction is accomplished in several ways and does not involve the fusion/union of nuclei, sex cells or sex organs. It may be accomplished by:

1. fission of somatic cells yielding two similar daughter cells
2. budding each bud a small outgrowth of the parent cell develops into a new individual 
3.  fragmentation or disjointing of the hyphal cells each fragment becoming a new organism 
4.  spore formation. 

There are several types of asexual spores each with a name.

1. Sporangiospores: These are single-celled spores formed within sacs called sporangia (singular: sporangium) at the end of special hyphae called sporangiospores). 
2.  There are two types of sporangiospores: Aplanospores which are non-motile and zoospores which are motile. Motility is due to the presence of flagella. 
3.  Condiospores or conidia (singular, conidium). These are formed at the tip or side of a hypha. Single celled conidia are called microconidia while large multicelled conidia are called macroconidia. 
4. Oidia (singular oidium) or arthrosopores: These are single- celled spores formed by disjointing of hyphal cells. 

Fig. 3: Different types of Asexual Spores 

Source: (http://mb0804mycology.wordpress.com/2008/07/29/reproduction-of-fungi/) 

Sexual Reproduction

Sexual reproduction is a type of reproduction in which two parents give rise to offspring that have unique combinations of genes inherited from the gametes of the two parents. It is carried out by fusion of the compatible nuclei of two parent cells. The process of sexual reproduction begins with the joining of two cells and fusion of their protoplast (plasmogamy) thus enabling the two haploid nuclei of two mating types to fuse together (karyogamy) to form a diploid nucleus. This is followed by meiosis, which again reduces the number of chromosomes to the haploid number.

The sex organelles of fungi if present are called gametangia. They may form differentiated sex cells called gametes or may contain instead one or more gamete nuclei. If the male and female gametangia are morphologically different, the male gametangium is called the antheridium (plural antheridia) and the female gamentangium is called the Oogonium (Oogonia).

Methods of sexual reproduction include:

1. Gametic copulation: This is the fusion of naked gametes, one or both of which are motile. 
2. Gamete-gametangial copulation: Two gametangia came into contact but do not fuse; the male nucleus migrate through a pore or fertilization to be into the female gamentangium. 
3. Gametangial copulation: Two gamentangia or their protoplast fuse and give rise to a zygote that develops into a resting spore. 
4.  Somatic copulation: Fusion of somatic or vegetative cells. 
5.  Spermatization: Union of a special male structure called a spermatium (plural spermatia) with a female receptive structure. The spermatum empties its content into the female during plasmogamy.