Viroids, prions and lichens

T.O. Diener (1971) discovered the viroid, an infectious agent smaller than a virus that causes potato spindle tuber disease.

A viroid is free RNA with no protein coat, which is what separates it from a virus; its RNA is of low molecular weight.

Prions are infectious agents made of abnormally folded protein, about the size of viruses, that transmit certain neurological diseases.

Prion diseases include bovine spongiform encephalopathy (BSE, mad cow disease) in cattle and its human counterpart, Creutzfeldt-Jakob disease (CJD).

Lichens are symbiotic, mutually useful associations between algae and fungi. The algal partner is the phycobiont (autotrophic) and the fungal partner is the mycobiont (heterotrophic).

In a lichen the alga makes food for the fungus, while the fungus gives shelter and absorbs water and mineral nutrients for the alga. The two are so closely joined that a lichen looks like a single organism.

Lichens are very good indicators of pollution because they do not grow in polluted areas.