Flower Variegation in Hybrids between NICOTIANA TABACUM and N. OTOPHORA.

HE fate of genes and chromosomes following interspecific gene transfer Tdeserves detailed study. Genes of one species often can be introduced into another without evident changes of their action and with little effect upon their stability. ANDERSON'S (1949) widely known work on introgression after interspecific hybridization could not have been successful had he been dealing with unstable genetic units; but in his investigations gene expression in advanced hybrid generations served to identify the genotype of an ancestral species. As an example which demonstrates considerable stability one might mention the genus Triticum and its allies. In this group instability was found very rarely even though the experiences of hybridization were vast and varied. Even here the literature is not completely devoid of examples of genetic instability in hybrids: PLOTNIKOWA (1932) and SACHS (1952) encountered somatic chromosome losses and KIHARA (1951) described one case of unstable gene expression in Triticum hybrids. In other grass hybrids numerous cases of instability are on record. Thus, WALTERS (1957) found extensive chromosome breakage in hybrids of Bromus, NIELSEK ( 1962) observed widespread occurrence of various cytological aberrations in several hybrids between grass species, and MANGELSDORF (1958) described the mutagenic effect of crossing maize and teosinte. Among the dicotyledons one of the earliest known cases of genetic instability following hybridization was described by HARLAND (1937) in Gossypium purpurascens. The gene for petal spot of this taxon was found to become increasingly mutable (or perhaps only weak and variable in expression) when other genes were progressively replaced by those from G. hirsutum in backcrosses to the latter. In the same genus, GERSTEL and PHILLIPS (1958) found that a leaf shape allele which was highly unstable in the species G. arboreum became stable in hybrids with G. anomalum. These examples suggest that the relative stability of a gene may be under control of the residual genotype. Following an early observation by KOSTOFF (1935) reports on instability in hybrids between species of Nicotiana have been especially numerous. Most of the cases of genetic and developmental instability described in the genus were found in hybrids between distantly related species belonging to different taxonomic sections. In hybrids between N . tabacum (section Genuinae) and N .