Fungi

Introduction

• Fungi are eukaryotes
• Nearly all multicellular (yeasts are unicellular)
• Distinguished from other kingdoms by:
  • modes of nutrition
  • structural organization
  • growth
  • reproduction

The Characteristics of Fungi

• Body form 
  • Unicellular 
  • Filamentous (tube-like strands called hypha (singular) or hyphae (plural) 
  • Mycelium: aggregate of hyphae 
  • Sclerotium: hardened mass of mycelium that generally serves as an overwintering stage. 
  • multicellular, such as mycelial cords, rhizomorphs, and fruit bodies (mushrooms) 
• Cell wall present, composed of cellulose and/or chitin. 
• Eukaryotes - true nucleus and other organelles present. 
• No obligate anaerobes. 
• Fungi grow in almost every habitat imaginable, as long as there is some type of organic matter present. 
• Heterotrophy - 'other food’ 
  • Saprophytes or saprobes: feed on dead tissues or organic waste (decomposers)
  • Symbionts: mutually beneficial relationship between a fungus and another organism
  • Parasites: feeding on living tissue of a host.  
  • Pathogens: parasites that cause disease 

Nutrition

• Absorptive nutrition: enables fungi to live as decomposers and symbionts
• Fungi are heterotrophs that acquire nutrients by absorption
• Secrete hydrolytic enzymes and acids to decompose complex molecules into simpler ones that can be absorbed
• Specialized into three main types:
  • Saprobes: absorb nutrients from dead organic material
  • Parasitic fungi: absorb nutrients from cells of living hosts; some are pathogenic
  • Mutualistic fungi: absorb nutrients from a host, but reciprocate to benefit the host

Heterotrophic Absorbers

• Fungi get carbon from organic sources
• Hyphal tips release enzymes
• Enzymatic breakdown of substrate
• Products diffuse back into hyphae
• Fungus is often hidden from view. It grows through its food source (substratum)

Septate vs Aseptate

• Fungal hyphae may be septate or aseptate
• Hyphae of septate fungi: divided into cells by crosswalls called septa
• Hyphae of aseptate fungi: lack cross walls (coenocytic)
• Parasitic fungi have modified hyphae called haustoria, which penetrate the host tissue but remain outside cell membrane

Hyphal Growth

• Hyphae grow from their tips
• Mycelium: extensive, feeding web of hyphae
• Mycelia are the ecologically active bodies of fungi

Mycorrhizae

• Mutualism between:

  • Fungus: nutrient & water uptake for plant)
  • Plant: carbohydrate for fungus)

• Extremely important ecological role of fungi!

• Mycorrhizae: mutually beneficial relationships between fungi and plant roots

• Ectomycorrhizal fungi: form sheaths of hyphae over a root and also grow into the extracellular spaces of the root cortex

• Arbuscular mycorrhizal fungi: extend hyphae through the cell walls of root cells and into tubes formed by invagination of the root cell membrane

Lichens

• Lichens: mutually beneficial relationship between fungus and alga/cyanobacterium
• Mutualism between
  • Fungus: structure
  • Alga or cyanobacterium: provides food
• Three main types of lichens: 
  • Crustose lichens: form flat crusty plates.  
  • Foliose lichens: leafy in appearance, although lobed or branched structures are not true leaves. 
  • Fruticose lichens: even more finely branched and may hang down like beards from branches or grow up from the ground like tiny shrubs.  

Lichen Internal Structure

• Lichens are nature’s biological monitors of pollution and air quality
  • Thalli act like sponges
  • Some species more sensitive to pollution
  • Which species are present can indicate air quality
  • Most resistant species can also be analyzed for pollutants, including bioaccumulation of heavy metals and radioactive isotopes

Fungal Spores

• Spores: asexual (product of mitosis) or sexual (product of meiosis) in origin. 
• Purpose of Spores 
  • Allows the fungus to move to new food source
  • Resistant stage: allows fungus to survive periods of adversity
  • Means of introducing new genetic combinations into a population
• Fungi reproduce by releasing spores
• Usually unicellular, haploid and of various shapes and sizes
• Spores are produced either sexually (by meiosis) or asexually (by mitosis)
  • In favorable conditions, fungi produce enormous numbers of spores asexually
  • For many fungi, sexual reproduction only occurs as a contingency - results in greater genetic diversity
• Spores are the agent of dispersal responsible for geographic distribution of fungi:
  • Carried by wind or water
  • Germinate in moist places with appropriate substrata
• Spores are formed:
  • Directly on hyphae
  • Inside sporangia
  • Fruiting bodies

Sexual Reproduction

• Fungal nuclei are normally haploid, with the exception of transient diploid stages formed during the sexual life cycles

• Sexual reproduction requires the fusion of hyphae from different mating types

• Fungi use sexual signaling molecules called pheromones to communicate their mating type

• Plasmogamy: is the union of cytoplasm from two parent mycelia

• In most fungi, the haploid nuclei from each parent do not fuse right away; they coexist in the mycelium, called a heterokaryon

• In some fungi, the haploid nuclei pair off two to a cell; such a mycelium is said to be dikaryotic

The Move to Land

• Fungi were among the earliest colonizers of land and probably formed mutualistic relationships with early land plants
• Molecular analyses have helped clarify evolutionary relationships among fungal groups, although areas of uncertainty remain

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