The Environmental Induction of Heritable Changes in NICOTIANA RUSTICA Parental and Selection Lines.

NVIRONMENTAL participation in the construction of an individual's phenoE type was first recognised in 1909 by JOHANNSEN working with the dwarf bean Phaseolus vulgaris. Since then it has been proved that the effects of the environment are in general transitory, lasting only for the lifetime of the individual concerned. Two exceptions to this rule have been discovered, namely induced mutation and certain types of conditioning. However, the former, irrespective of whether it is chromosomal or extrachromosomal in origin, usually reveals itself not in the parents, which merely act as carriers, but in their descendants. A further distinction arises in so far as chromosomal mutation is a random process, whereas conditioning, like many induced extrachromosomal mutations, is a nonrandom process. This article will be concerned primarily with those conditioned responses which are transmitted either partly or wholly through successive asexual or sexual generations in the absence of the stimuli; a phenomenon which will be referred to as transmutation. Conditioning as such may be regarded as a directed alteration in the development of the phenotype of an individual resulting from the action of external agents. Examples of conditioning occur frequently, especially in the animal kingdom, ranging in appearance from the phenocopies observed in Drosophila, poultry and man, where the effects on the foetus of the mother contracting German measles or taking the drug thalidomide may be viewed in this light (see GOLDSCHMIDT 1955; INGALLS 1957), through dauermodifications (see DARLINGTON and MATHER 1952) , to transmutation as manifested in Paramecium ( SONNEBORN 1950; BEALE 1954), in peas (HIGHKIN 1958) and in flax (DURRANT 1962; DURRANT and TYSON 1964). Despite this diversity, all these various forms have one outstanding feature in common, namely that they arose as a result of applying suitable external stimuli during the early, critical stages of development. Phenotypic changes of this type could be mediated by the cytoplasm, a fact demonstrated by JINKS (1954) in Aspergillus nidulans and by BREESE, HAYWARD and THOMAS (1965) with natural populations of perennial ryegrass. However, for a fuller discussion of the respective roles of the nucleus and cytoplasm in development and differentiation see JINKS (1964). The present paper will be confined largely to experiments conducted with