Describing fungi or algae that require mating with a different strain or individual to reproduce sexually, rather than reproducing by themselves.
From Greek 'hetero-' (different) and 'thallos' (shoot, branch). Developed in mycology and algae studies in the early 20th century to describe sexual compatibility systems.
Most molds and fungi you see are heterothallic—they can't reproduce with themselves and need a 'partner' of a different mating type, which is why bread mold can mysteriously appear when two different spores randomly land nearby and decide to make a family.
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