Fungal Distribution and Importance ,Fungal Structure,Fungal Reproduction

   

  Unlike protists, fungi are primarily terrestrial organisms. They have a global distribution from polar to tropical regions. Fungi are saprophytes, securing nutrients from dead organic material by re­ leasing degradative enzymes into the environment.  

     This enables their absorption of the soluble products-a process sometimes called osmotrophy. Fungi are important decomposers. They degrade complex organic materials in the environment to simple organic compounds and inorganic molecules. In this way, carbon, nitrogen, phosphorus, and other critical constituents of dead organisms are released and made available for living organisms. Many fungi are pathogenic, with over 5,000 species known to attack economically valuable crops and many other plants.

 About20 new human fungal pathogens are documented each year. Conversely, fungi also form beneficial relationships with other or­ ganisms. For example, the vast majority of vascular plant roots form  important  associations  with  fungi  called mycorrhizae.

Mycorrhizae 

Fungi, especially yeasts (single-celled fungi), are essential to many industrial processes involving fermentation. Examples in­ clude the making of bread, wine, beer, cheeses, and soy sauce. They are also important in the commercial production of many organic acids (citric, gallic) and certain drugs (ergometrine, cor­ tisone), and in the manufacture of many antibiotics (penicillin, griseofulvin) and the immunosuppressive drug cyclosporine. 

 In addition, fungi are important research tools in the study of fun­damental biological processes. Cytologists, geneticists, bio­ chemists, biophysicists, and microbiologists regularly use fungi in their research. The yeast Saccharomyces cerevisiae  is  the best understood eukaryotic cell. It has been a valuable model organism in the study of cell biology, genetics, and cancer.

Fungal Structure

The body or vegetative structure of a fungus is called a thallus (pl., thalli). It varies in complexity and size. Single-cell microscopic fungi are referred to as yeasts, while multicellular masses are called molds.

 Fungi also include macroscopic puffballs and mushrooms. Like most bacteria, fungi possess cell walls; however,fungal cell walls are usually made of chitin. Chitin is a strong but flexible nitrogen containing polysaccharide consisting of N-acetylglucosamine residues. Instead of chitin, some fungal cell walls are composed of other polysaccharides such as man­ nans, galactosans, or cellulose.

A yeast is a unicellular fungus with a single nucleus that reproduces either asexually by budding and transverse division or sexually through spore formation. Each bud that separates can grow into a new cell, and some group together to form colo­ nies. 

 Generally yeast cells are larger than bacteria and are com­monly spherical to egg-shaped. They lack flagella and cilia but have most other eukaryotic organelles. The thallus of a mold consists of long, branched, threadlike filaments of cells called hyphae (s., hypha; Greek hyphe, web) that form a tangled mass called a mycelium (pl., mycelia). In some fungi, protoplasm streams through hyphae,uninterrupted by cross walls.

 These hyphae are called coenocytic or aseptate hy­phae. The hyphae of other fungi  have cross walls called septa {s., septum) with either a single pore  or multiple pores  that enable cyto­plasmic streaming. These hyphae are termed septate hyphae.

Hyphae are composed of an outer cell wall and an inner lumen, which contains the cytosol and organelles. A plasma membrane sur­ rounds the cytoplasm and lies next to the cell wall. The filamentous nature of hyphae results in a large surface area relative to the volume of cytoplasm. This makes adequate nutrient absorption possible.

Fungal Reproduction

Reproduction in fungi can be either asexual or sexual.

 Asexual reproduction is accomplished in several ways: 

               1) a parent cell undergoes mitosis and divides into two daughter cells by a cen­ tral constriction and formation of a new cell  wall    

               2) mitosis in vegetative cells may be concurrent with budding to produce a daughter cell. 

                This is very common in yeasts. The formation of asexual spores often accompanies asexual reproduction and is usually used as a means of dispersal. There are many types of asexual spores, each with its own name. Arthroconidia (arthrospores) are formed when hyphae frag­ ment through splitting of the cell wall or septum. Sporangiospores develop within a sac (sporangium; pl., sporangia) at a hyphal tip. CONIDIOSPORES are spores that are not enclosed in a sac but produced at the tips or sides of the hy­pha. BLASTOSPORES are  produced  from a  vegeta­tive mother cell budding.

Sexual reproduction in fungi involves the fusion of compat­ible nuclei. Homothallic fungal species are self-fertilizing and produce sexually compatible gametes on the same mycelium. Heterothallic species require outcrossing between different but sexually compatible mycelia. Depending on the species, sexualfusion may occur between haploid gametes, gamete-producing bodies called gametangia, or hyphae. Sometimes both the cyto­plasm and haploid nuclei fuse immediately to produce the dip­loid zygote,  as seen in higher  eukaryotes.  Usually,   however,there is a delay between cytoplasmic and  nuclear  fusion.  

  Thisproduces a dikaryotic STAGE in which cells contain two separate haploid  nuclei  (N  +  N), one  from  each  parent.After a period of dikaryotic existence, the two nuclei fuse and undergo meiosis to yield haploid spores. This is seen in both ascomycetes and basidiomycetes, so these are sometimes re­ ferred to as dikaryotic fungi.

Fungal spores, both asexual and sexual, are important for fusionHaploid stage (N)several reasons. They enable fungi to survive environmental stresses such as desiccation, nutrient limitation, and extreme temperatures, although they are not as stress resistant as bacterial endospores. 

They aid in fungal dissemination, which helps ex­ plain their wide distribution. Because spores are often small and light, they can remain suspended in air for long periods  and  are often spread by adhering to the bodies of insects and other animals.  

     The bright colors and fluffy textures of many molds often are due to their aerial hyphae and spores. Finally, the size, shape, color, and number of spores are useful in the identification of fungal species.

Eumycetozoa,slime molds,

First described in the 1880s, members of Eumycetozoa or "slime molds" have been classified as plants, animals, and fungi. 

 As we examine their morphology and behavior, the source of this con­ fusion should become apparent.

 Recent analysis of certain pro­ teins (e.g., elongation factor EF-1, a-tubulin, and actin) as well as physiological, behavioral, biochemical, and developmental data point to a monophyletic group.

 Eumycetozoa includes Myxogas­ tria and Dictyostelia. 

The acellular slime  mold  (Myxogastria) life cycle includes a distinctive stage when the organisms exist as streaming masses of colorful protoplasm. 

The protoplasm creeps along in amoeboid fashion over moist, rotting logs, leaves, and other organic matter, which it degrades .

 Acellular slime molds derive their name from the presence of a large, mul­ tinucleate mass called a plasmodium; there can be as many as 10,000 synchronously dividing nuclei within a single plasmo­ dium.

 Feeding is by endocytosis.

 When starved or dried, the plasmodium develops ornate fruiting bodies.

 As these mature, they form stalks with cellulose walls that are resistant to environmental stressors .

 When conditions im­ prove, spores germinate and release haploid amoeboflagellates. These fuse, and as the resulting zygotes feed, nuclear divisions give rise to the multinucleate plasmodium.

Cellular slime molds (Dictyostelia) are strictly amoeboid and use endocytosis to feed on bacteria and yeasts. 

Their com­ plex life cycle involves true multicellularity, despite their primi­ tive evolutionary status .

 The species Dictyostelium discoideum is an attractive model organism.

 During its life cy­ cle, a pseudoplasmodium is formed. 

This consists of many, many individual vegetative cells moving together as a mass. Thus it differs from the acellular slime mold's true plasmodium. 

The pseudoplasmodium forms when starved cells release cyclic AMP and a specific glycoprotein, which serve as molecular sig­ nals.

 Other cells sense these compounds and respond by forming an aggregate around the signal-producing cells. 

In this way, large, motile, multicellular slugs develop and serve asprecursors to fruiting body formation.

 Fruiting body morpho­ genesis commences when the slug stops and cells pile on top of each other.

 Cells at the bottom of this vertically oriented struc­ ture form a stalk by secreting cellulose, while cells at the tip differ­ entiate into spores .

 Germinated spores become vegetative amoebae to start this asexual cycle anew.

Dictyostelium spp. display complex behaviors. 

In addition to farming, described in the chapter opening story, they also differ­ entiate to resemble primitive immune cells.

 During slug forma­ tion, some cells become "sentinel cells" and vanquish harmful bacteria.

 Sentinel cells accomplish this by producing proteins that are similar to those involved in immune responses in higher or­ ganisms.

 These cells roam within the slug as if patrolling for pathogenic bacteria such as Legionella pneumophila, which are known to infect Dictyostelium spp.

 Sentinel cells have been found in several species related to Dictyostelium discoideum; immunolo­ gists are not too surprised, noting that all multicellular organisms need protection against bacterial pathogens.

Sexual reproduction in D. discoideum involves the formation of special spores call macrocysts.

 These arise by a form of conju­ gation that has some unusual features. 

First, a group of amoebae become enclosed within a wall of cellulose.Conjugation occurs between members of different mating types, of which there are three, as well as those capable of self-fertilization. 

Mating type (denoted as Type I, II, or III) is controlled by the nucleotide se­ quence of a single gene. Following conjugation, a large amoeba forms and cannibalizes the remaining amoebae. 

The now-giant amoeba matures into a macrocyst. Macrocysts can remain dor­ mant within their cellulose walls for extended periods. Vegetative growth resumes after the diploid nucleus undergoes meiosis to generate haploid amoebae.