Heterosis associated with genotype-environment interactions.

SE of hybrid vigor is of great importance in terms of present-day agricultural production. For example GARDNER (1968) states: ". . . Development and use of heterosis has been the most important practical achievement of genetics so far. The monetary value of the increase in corn production due to use of heterosis for a single year has been estimated to exceed the total cost of federal research for plant improvement since 1900." Even though the importance of hybrid vigor is well recognized, the underlying phenomenon, heterosis, remains an enigma of quantitative genetics. Various hypotheses have been developed on purely genetic grounds. In our laboratory we have adopted a different approach-one which views heterosis as not entirely the result of genetic stimuli but rather as the result of the interaction between genetic and environmental stimuli. Such a view is not without precedent, as each of the studies in the following partial list have implicated the environment as a significant factor in the manifestation of heterosis: HANEY, GARTNER and WILSON 1953; GRIFFING 1954; LEWIS 1954; BARNETT and COLEMAN 1960; SHANK and ADAMS 1960; GRIFFING and LANGRIDGE 1963; MCWILLIAM and GRIFFING 1965; PEDERSON 1968; also see LANGRIDGE (1962) far a detailed review of temperature-dependent heterosis. In order to examine the effects of the total stimulus pattern on the manife1tation of heterosis, experiments must be designed so that genetically related 01"ganisms are reared in different but quantitatively related and exactly controlled environmental regimes. Such factorial types of experiment permit a genetic analysis, an environment analysis, and most important of all, an analysis of the contributions of the interactions between ger?etic and environmental stimuli to the observed heterosis. In view of the complexities of the experimental design which requires controlled cultural regimes, it is important to choose a biological system which yields a maximum amount of data while utilizing a minimum amount of time and space in the controlled-environmevt facility. The biological system chosen for this study was that in which the flowering plant Arabidopsis thaliana was grown aseptically in test tubes on nutrient solution solidified with agar. The advantages of this system are: (i) the plants grow in test tubes and hence occupy little space; and (ii) the plants grow quickly,