Hybrid lethality is a type of postzygotic isolation and is observed in some species of Nicotiana in association with genes encoded on the Q chromosome. Tezuka et al. (pp. 267–276) make interspecific crosses of eight wild species with cultivated tobacco, N. tabacum, and find only one, N. fragrans, that produces 100 % viable hybrids. They confirm that one or more genes on the Q chromosome of N. tabacum are responsible for hybrid lethality, but the effect can be suppressed if the seedlings are grown at elevated temperatures.