Detection of Four Lethality Types in Interspecific Crosses among Nicotiana Species through the Use of Three Rescue Methods for Lethality

Abstract

Hybrid lethality was surveyed in 186 cross combinations among 17 species from 9 sections of the genus Nicotiana. The hybrid seedlings of 14 cross combinations expressed lethality characterized by one of four symptoms and the lethality was classified into four types as follows: browning of the shoot apex and the root tip (Type I), browning of the hypocotyl and the roots (Type II), yellowing of the true leaves (Type III) and formation of multiple shoots (Type IV). Three kinds of rescue methods for the lethality were applied to the 4 cross combinations expressing different types of the lethality as follows: (1) culture of hybrid seedlings under high temperature conditions to suppress the lethality, (2) culture of the cotyledonary segments of the hybrid seedlings to obtain regenerants overcoming the lethality and (3) culture of the hybrid seedlings on a medium containing cytokinin to obtain regenerants overcoming the lethality. The 1st and 2nd methods were effective in the cross of N. nudicaulis×N. tabacum expressing the lethality Type I. All the methods were effective in the cross of N. paniculata × N. suaveolens expressing the lethality Type II. Only the Ist method was effective in the cross of N. glutinosa× N. repanda expressing the lethality Type III. None of the methods were effective in the cross of N. paniculata × N. alata expressing the lethality Type IV. Each lethality type exhibited a specific set of responses to the three rescue methods. These findings suggest that the lethality observed in interspecific crosses of Nicotiana differs in the physiological processes leading to death.