Infertility is widely used in fungi as a criterion for distinguishing
sibling species and mating populations. Whether a cross is called fert
ile or sterile may depend on the frame of reference. The terms are rel
ative rather than absolute. Failure to recognize this had led to appar
ent contradictions where none exist. In many situations the unqualifie
d terms are insufficiently precise and descriptive modifiers will be r
equired, for example, ''highly or sparingly fertile, nearly or complet
ely barren.'' Precision is also needed in describing the reduction in
fertility that is characteristic of certain genotypes when crosses are
made between individuals of the same species.