7.4 Diversity Among Fungi

Introduction

The kingdom fungi are a diverse group comprising of more than 100,000 known species, most of which are terrestrial. The ancestry of fungi that dated back to about 570 million years ago, has not been determined. They are thought to have evolved from red algae as they both lack flagella at any stage of their life cycles.

Definition

Diversity Among Fungi: The kingdom fungi are a diverse group comprising of more than 100,000 known species, most of which are terrestrial.

In simple words: Fungi are a very diverse group with many different types.

Core Concept

• The kingdom fungi comprise more than 100,000 known species.
• Most fungi are terrestrial.
• The ancestry of fungi dates back to about 570 million years ago.
• Their ancestry has not been determined.
• They are thought to have evolved from red algae because both lack flagella at any stage of their life cycles.

Detailed Explanation

Classification of Fungi

Fungi are classified into four main groups based primarily on the methods of reproduction and types of sexual reproductive structures.

These groups also differ in the types of hyphae and some other structures.

• Zygomycota (Conjugating Fungi)
• Ascomycota (Sac Fungi)
• Basidiomycota (Club Fungi)
• Deuteromycota (Imperfect Fungi)

1. Zygomycota (Conjugating Fungi)

Definition: Zygomycota, also known as conjugating fungi, are a group of typically fast-growing fungi that can be found in various habitats, including soil and decaying organic matter.

Key Points:

• Commonly they are saprotrophs on bakery goods, vegetables and fruits.
• They are known for their important role as decomposers, breaking down organic material in ecosystems.
• However, some Zygomycota members can be pathogenic to plants or animals, making them significant in both ecological and medical contexts.
• They have non-septate hyphae forming a well-developed and branching mycelium.

Asexual Reproduction:

• Asexual reproduction takes place by sporangia containing conidia or spores.
• Example: *Rhizopus nigricans* (Black bread mold).

Sexual Reproduction:

• Sexual reproduction takes place by conjugation.
• Conjugation occurs only between a member of a plus (+) and a minus (–) strain.
• When hyphae (stolon) of opposite mating strains meet, hormones are produced that cause the tips of the hyphae to come together.
• This fusion results in formation of specialized gamete producing structures called gametangia.
• These structures become separated from rest of the mycelium by the formation of septa.
• A diploid zygote is thus formed by fusion of haploid nuclei from opposite strains.
• The zygote develops into a zygospore with a thick and resistant wall to bear unfavorable conditions.
• Zygospores germinate under favorable conditions and divide by meiosis.
• Hyphae emerge from zygospore breaking its wall.
• The tip of the hypha develops into a sporangium containing many nuclei.
• When mature, the wall of the sporangium ruptures, spores are liberated and then the spores grow into new strain of mycelium.
• Thus, the life cycle of Rhizopus is continued.

2. Ascomycota (Sac Fungi)

Definition: Ascomycota, commonly referred to as sac fungi, is one of the largest and most diverse phyla within the fungal kingdom.

Key Points:

• Ascomycota display remarkable diversity in terms of habitat and lifestyle.
• They can be found in various ecosystems, from soil and wood to symbiotic relationships with plants and lichens.
• Many economically important fungi belong to this group, including:
• Morel and truffle mushrooms
• Brewers' and bakers' yeast
• Several plant pathogens
• They are named after their characteristic reproductive structures, known as asci (singular: ascus), which are sac-like cells containing sexually produced spores.
• Their hyphae usually have septa, but the cross walls are perforated so that cytoplasm can move from one compartment to the other.
• Ascomycetes reproduce both asexually and sexually.

Asexual Reproduction:

• Asexual reproduction involves production of spores called conidia (singular conidium) or conidiospores (Greek: konis, dust and spora, seed).
• Conidia vary in shape, size and maybe multicellular.
• There are no sporangia in Ascomycetes.
• The conidia develop directly on the tips of modified aerial hyphae called conidiophores.
• When released conidia are blown away by wind.
• Conidia occur in various shapes, sizes and colors in different species.
• The color of conidia is what gives the characteristic brown, blue, pink or other tint to many of these molds.
• In unicellular yeasts, asexual reproduction takes place by budding.
• In this process a small projection (bud) grows and eventually separates from the parent cell.
• Each bud can grow into a new yeast cell.
• Yeast also reproduces asexually by fission.

Sexual Reproduction:

• Sexual reproduction takes place after two hyphae grow together and their cytoplasm blend.
• The nuclei from parent hyphae pair in this cytoplasm but do not fuse.
• New dikaryotic hyphae develop from the fused structure.
• The n + n hyphae form a fruiting body known as ascocarp.
• It is a reproductive structure where spores are produced and released.
• Ascocarps vary greatly in shape, for instance cup shaped in cup fungi, flask shaped in molds while stalked and crown by shaped structures in morels.
• The asci develop in the ascocarp surrounded by sterile hyphae.
• Within an ascus, the two nuclei fuse and form a diploid zygote that undergoes meiosis to form four haploid nuclei.
• This process is usually followed by one mitotic division of each of the four nuclei, resulting in eight haploid nuclei.
• Each haploid nucleus develops into an ascospore.
• Thus, there are usually eight haploid ascospores within the ascus.
• In most ascomycetes the asci become swollen as they mature.
• The mature asci burst liberating the ascospores.
• If spores land in a suitable location, they germinate to form a new mycelium, e.g., in yeasts, neurospora etc.

3. Basidiomycota (Club Fungi)

Definition: Basidiomycota, commonly known as club fungi, is a diverse and ecologically important phylum within the Kingdom Fungi.

Key Points:

• Basidiomycota includes some of the most recognizable and economically significant fungi, such as:
• Mushrooms
• Rust
• Smut
• Puffballs
• Toadstools
• Shelf fungi
• These fungi can be found in various habitats, often forming mycorrhizal relationships with plants or decomposing organic matter.
• Basidiomycota fungi are distinguished by the presence of filamentous hyphae.
• Most species reproduce sexually with basidia, which are specialized spore-forming structures.
• Basidiomycota are named after these unique spore forming structures.
• Each basidium is a club-shaped organ.
• It is a hyphal cell that typically generates groups of four spores on its tip, referred to as basidiospores.
• Each individual fungus produces millions of basidiospores.
• Each basidiospore has the potential to give rise to new primary mycelium.
• Hyphae of primary mycelium are composed of monokaryotic (n) cells.
• The mycelium of a basidiomycote e.g., *Agaricus* (Mushroom), consists of mass of white, branched, thread like hyphae that occur mostly belowground.
• The hyphae are divided into cells by septa.
• The septa are perforated and allow cytoplasmic streaming between cells.

Sexual Reproduction:

• Although club fungi occasionally do produce conidiospores asexually, they usually reproduce sexually.
• A hyphae of a primary mycelium fuses with the monokaryotic (n) hyphae of a different mating type.
• The nuclei from parent hyphae pair in this cytoplasm but do not fuse.
• In this way a secondary mycelium with dikaryotic (n+n) hyphae is produced in which each cell contains two nuclei.
• The secondary mycelium grows into a compact mass called buttons.
• Each button grows into a fruiting body known as mushroom.
• A mushroom which consists of a stalk and a cap is more formally referred to as basidiocarp.
• The walled off ends of tightly packed hyphae become the club shaped basidia.
• The lower surface of the cap usually consists of many thin perpendicular plates called gills that radiate from the stalk to the edge of the cap.
• On the gills of the mushroom haploid nuclei of the dikaryotic cells fuse to form diploid zygotes.
• Meiosis then take place forming four haploid nuclei that move into finger like projections forming basidiospore, which are released later.

4. Deuteromycota (Imperfect Fungi)

Definition: These fungi are called "imperfect fungi" owing to the absence of sexual stage in their life cycle.

Key Points:

• They reproduce asexually by forming conidiospores.
• The cellular morphology and biochemistry of imperfect fungi trace their ancestry to sac fungi, which have lost the ability to reproduce sexually.
• These fungi lead a saprophytic or parasitic life on plants.
• They have a well known economic importance.
• Examples: *Penicillium, Aspergillus, Alternaria, Fusarium*.
• Despite absence of sexual reproduction, imperfect fungi have a genetic recombination mechanism called Parasexuality.
• It facilitates the exchange of genetic material between fungal cells by exchanging portions of chromosomes of two nuclei lying in the same hypha, fostering genetic diversity and adaptability.