Hybrid Lethality in the Genus Nicotiana

Abstract

Reproductive isolation is a mechanism that separates species. It is considered to play a crucial role in the evolution of animals and plants. Reproductive isolation is divided into two types of barriers, namely prezygotic and postzygotic. In plants, a typical prezygotic barrier observed after pollination is pollen–pistil incongruity (or incompatibility). Specifically, when the pollen of one species is rejected by the pistil of another species, but the reciprocal cross is successful, the incongruity is called unilateral incongruity. Postzygotic barriers include seed abortion, and hybrid lethality and hybrid sterility in the F1 generation. When F1 hybrids are normal but their F2 progeny contains lethal or sterile individuals, this phenomenon is called hybrid breakdown and is discriminated from abnormalities in the F1 generation. Whereas these prezygotic and postzygotic barriers contribute to speciation, they are obstacles for plant breeders, especially in breeding programs involving wide hybridization. Hybrid plants from normal parents sometimes show weak growth or die before maturity. Several terms have been used to describe these phenomena, i.e., hybrid lethality, hybrid weakness, hybrid necrosis and hybrid inviability. Hybrid lethality is observed in certain cross combinations in many plant species (Bomblies & Weigel, 2007). In this chapter, I review studies of hybrid lethality in the genus Nicotiana.