Why Viruses Sit on the Border Between Living and Non-Living

The fuzzy growth on stale bread, the mushrooms sold in a market and the viruses behind many infections are all studied in this part of classification. Fungi form a kingdom of their own, while viruses, viroids and prions sit outside the five kingdoms altogether.

This lesson covers the features and classes of fungi, their importance, lichens and mycorrhiza, the structure of viruses, and viroids and prions.

Fungi are heterotrophic eukaryotes, mostly made of thread-like hyphae with chitin cell walls, that reproduce vegetatively, asexually by spores and sexually through plasmogamy, karyogamy and meiosis; they are grouped into Zygomycetes, Ascomycetes, Basidiomycetes and Deuteromycetes.

General features:

- Body of hyphae forming a network called the mycelium
- Cell walls of chitin; nutrition saprophytic, parasitic or symbiotic
- Some hyphae are coenocytic, with many nuclei in continuous cytoplasm

Reproduction:

- Vegetative — fragmentation, fission and budding
- Asexual — spores such as conidia, sporangiospores and zoospores
- Sexual — plasmogamy (fusion of cytoplasm), karyogamy (fusion of nuclei) and meiosis; in ascomycetes and basidiomycetes a dikaryotic stage, with two nuclei per cell, comes between plasmogamy and karyogamy

The four classes:

- Zygomycetes — coenocytic hyphae; sexual spores are zygospores; Mucor, Rhizopus
- Ascomycetes (sac fungi) — septate hyphae; sexual spores are ascospores formed in asci; Penicillium, Aspergillus, Neurospora, yeast
- Basidiomycetes (club fungi) — sexual spores are basidiospores formed on basidia; mushrooms, Puccinia (rust), Ustilago (smut)
- Deuteromycetes (imperfect fungi) — no known sexual stage; Alternaria, Colletotrichum, Trichoderma

An everyday example. Button mushrooms grown in dark sheds are basidiomycetes; the part eaten is the fruiting body, while the mycelium spreads unseen through the compost.

The substance. Deuteromycetes are a holding group — once the sexual stage of a species is found, it is usually moved into Ascomycetes or Basidiomycetes.

Fungi give us antibiotics, bread and edible mushrooms but also cause crop and human diseases; lichens are partnerships between an alga and a fungus, and mycorrhiza is a partnership between a fungus and the roots of a higher plant.

Useful fungi:

- Penicillium yields the antibiotic penicillin
- Yeast ferments sugar, raising bread dough and producing alcohol
- Some fungi ripen cheese, and many mushrooms are eaten
- Neurospora is widely used in genetic research

Harmful fungi:

- Puccinia causes rust of wheat and Ustilago causes smut
- Albugo causes white rust of mustard
- Some fungi cause ringworm in humans

Lichens. An alga, the phycobiont, makes food by photosynthesis; a fungus, the mycobiont, provides shelter and absorbs water and minerals. The partners live so closely that a lichen looks like a single organism. Lichens do not grow in polluted areas, so they are good indicators of air pollution.

Mycorrhiza. Fungal hyphae around or inside plant roots absorb phosphorus and water from the soil for the plant, which supplies sugars in return.

An everyday example. Grey-green crusts on tree bark in a clean hill station are often lichens — their absence on trees in a busy city is a sign of polluted air.

The substance. Both lichens and mycorrhiza are examples of mutualism — each partner gains something it cannot easily obtain alone.

A virus is a non-cellular particle of nucleic acid — DNA or RNA — inside a protein coat called the capsid, made of subunits called capsomeres; it behaves as non-living outside a host cell but uses the cell's machinery to multiply inside it.

Structure:

- Genetic material — either RNA or DNA, never both; single- or double-stranded
- Capsid — a protein coat of capsomeres arranged in helical or polyhedral patterns
- Plant viruses usually have single-stranded RNA, as in the tobacco mosaic virus
- Bacteriophages, the viruses that infect bacteria, usually have double-stranded DNA

Living features. They multiply inside a host, carry genetic material and can mutate.

Non-living features. They have no cell structure and no metabolism of their own, and they can be crystallised.

Key contributions:

- Ivanowsky showed that the agent of mosaic disease of tobacco could pass through filters that hold back bacteria
- Beijerinck showed that extract from infected tobacco plants could infect healthy plants, and called the agent Contagium vivum fluidum, an infectious living fluid
- Stanley showed that viruses can be crystallised and that the crystals consist largely of proteins

An everyday example. Yellow and green mosaic patches on the leaves of tomato or chilli plants are often a sign of a viral infection.

The substance. Antibiotics do not work against viruses — antibiotics attack bacterial structures such as cell walls, which viruses do not have.

Viroids are infectious agents made only of a small, free RNA without a protein coat, while prions are infectious agents made only of abnormally folded protein.

Viroids:

- Smaller than viruses
- RNA of low molecular weight, with no protein coat
- Example: potato spindle tuber disease

Prions:

- Contain no nucleic acid at all
- An abnormally folded protein makes normal proteins fold the same wrong way, damaging nerve tissue
- Bovine spongiform encephalopathy (BSE), commonly called mad cow disease, in cattle
- Creutzfeldt–Jakob disease (CJD) in humans

Comparing the agents:

- Virus: nucleic acid plus a protein coat
- Viroid: nucleic acid only
- Prion: protein only

An everyday example. Potato farmers who find long, thin, cracked tubers may be seeing the effect of a viroid, which can spread through infected seed potatoes and farm tools.

The substance. Prions break the rule that infectious agents need genetic material — they spread by changing the shape of existing proteins, not by copying genes.