The Fungi

Fungi are eukaryotic organisms
Almost all are multicellular organism
They used to be grouped with plant in the two kingdom system
They do not have chlorophyll (heterotrophic)
They are unique organisms that differ from other eukaryotes in term of:
- Nutritional modes
- Structural organization
- Growth and reproduction
Their multicellularity differ fro animal and plant in that partition between nucleated compartment or cell are either absent or partial
In other words, the cytoplasm is continuous
Consequently, each compartment has more than one nucleus
Therefore, for many fungi, the term of multinucleate description is more accurate that multicellular


Nutrition and habitat

All fungi are heterotrophs (lack of chlorophyll)
The aquire their nutrients by absorption (small nutrients are absorbed from the surrounding)
Fungi digest their food outside their body by secreting hydrolytic enzymes into the foods
Based on their absorptive mode fungi can be classified as:
– Saprobic fungi
Absorb nutrients from non living organic materials
Examples of foods: fallen log, waste of live organisms, and animal corpses
– Parasitic fungi
Absorb nutrients from living hosts
Some are pathogenic fungi in human (infect lung)
– Mutualistic fungi
They absorb nutrients from living organisms, but they also give some benefit to their hosts
Example: Mycorhiza
Their habitat is very wide
– Mostly occupy terrestrial environment
– Some of them are associated symbiotically with many organism (such as Lichen: association of fungi and algae)

Structure

Except in yeast, the body of fungi is constructed by a basic building unit called hyphae (singular, hypha)
These hyphae form an interwoven mat called mycelium (the ‘feeding’ network of a fungus)
Most fungal hypha is divided into cells by crosswall called septa (singular septum)
The septa generally have pores large enough to allow ribosome, mitochondria, or nuclei to travel from cell to cell
Their cell wall mainly composed of chitin
Some fungal hyphae are aseptate (their cells are not divided into cells by cross walls)
This type of hyphae are called coenocitic
Some parasitic fungi have their hyphae modified as haustoria
Haustoria is nutrient-absorbing hyphal tips that penetrate the tissue of the host

Growth and reproduction

Fungi reproduce by releasing spores that are produced either sexually or asexually
Their spore come in all shape and size
In the favorable conditions, fungi will produce enormous number of spore asexually
These will be carried by wind or water and germinate when landing on a moist surface
Sexual spores will be produced when there is some change in the environment
– This mode will produce greater genetic diversity among the offsprings (for adaptation in such an environmental condition)
Syngamy (sexual union of cells from two individuals)
This occurs in two stages:
– Plasmogamy
The fusion of cytoplasm
After plasmogamy, the nuclei from each parent pair up but not fuse forming a dikaryon (two nuclei)
– Karyogamy
The fusion of nuclei
Examples of spores produced by sexual mode:
– Ascospores (produced by group of Ascomycota)
Stored in ascus, normally contain 8 spores
– Basidiopores (produced by group of Basidiomycota)
Located in the basidium, normally contain 4 spores

Division of fungi

More than 100,000 species of fungi have been known until recently (thousand is reported yearly)
In this chapter these have been divided into 3 major division based on:
– Structure involved in the plasmogamy
– The time spent as a dikaryon
– The location of caryogamy
The three division are name after the sexual cells in which karyogamy occurs

Division zygomycota

About 600 species have been described
Mostly terrestrial fungi
Live in soils, decaying plants, and animal materials
Some form mycorrhizae (Mutualistic association with plant roots)
Example of this kind of fungi:
– Rhizopus stolonifer
– Pilobolus (decompose animal dung)
The life cycle of Rhizopus

Division Ascomycota

More than 600,000 species have been described
Also called sac fungi
They range in size and complexity
They include some of the most devastating plant pathogens
Many are important saprobes, particularly of plant materials
About a half form symbiotic association with algae (lichen)
Some form mycorrhiza
The defining feature is the production of sexual spore in sac-like asci (singular ascus)
They bear their sexual spores in microscopic fruiting bodies called ascocarp
Their spores are not located in the sporangia (Naked spores or also called conidia)
Examples:
Hypoxylon multiforme
Morchella esculenta (Fig. 28.4c)
Saccharomyces cerevisiae
Neurospora sp.
Penicillium sp.
Trichoderma sp.

Division Basidiomycota

App. 25,000 species have been described
Includes mushroom, shelve fungi, rust fungi, and puffball fungi (Fig. 28.6)
The name derived from basidium
The club shaped of the basidium give rise to common name club fungus
The mycelium can grow into fruiting body called basidiocarp (in certain conditions)
A mushroom is an example of a basidiocarp


(a) Hygrophorous (Mycorrhizal with oak)
(b) Shelve fungi
(c) Puffball fungi (Lycoperdon gemmatum)

Examples of Basidiomycota
– Volvariela volvacea
– Auricularia polytricha
– Pleurotus sp. (edible fungus)
– Amanita phalloides (Poisonous)
– Exobasidium vexans (parasite in tea tree)
– Corticium salmonella (parasite in the tree trunk, especially fruit producer)