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.
T. Tezuka, T. Kuboyama, T. Matsuda, W. Marubashi, 2010, 'Seven of eight species in Nicotiana section Suaveolentes have common factors leading to hybrid lethality in crosses with Nicotiana tabacum', Annals of Botany, vol. 106, no. 2, pp. 267-276 http://dx.doi.org/10.1093/aob/mcq114