Mitochondrial Polymorphism in Maize. II. Further Evidence of Correlation of Mitochondrial Complementation and Heterosis.

HAT heterosis is a genetic phenomenon cannot be doubted. And yet, in spite of our awareness of this phenomenon for many years, genetic mechanisms responsible for heterosis have not been demonstrated operationally ( SARKISSIAN 1967). That is, it has not been possible to define heterosis-superior growth of a hybrid-in terms of actual operations o r experimental treatments that may have been applied to heterotic and nonheterotic individuals with the aim of testing the behavior of the hybrid and the genetic control of such behavior. We submit that it is primarily for this reason that knowledge of the intimate mechanisms of heterosis has not flourished. It has been demonstirated recently that mitochondria isolated from a heterotic single-cross hybrid of maize exhibit heterosis in terms of oxygen uptake and phosphorylation, while mitochondria from a nonheterotic hybrid do not (McDANIEL and SARKISSIAN 1966). It was also shown that 1:l mixtures of mitochondria of the parents of the heterotic hybrid approached the mitochondria of the hybrid in their activity while mixtures of parents of the nonheterotic hybrid did not. It seemed to us that such mitochondrial complementation indicated that: (1 ) maize lines which upon crossing produce a heterotic hybrid possess dissimilar mitochondria and since the complementing mixture of mitochondria approaches mitochondria of the hybrid in activity, the hybrid is polymophic with regard to its mitochondria, (2) the phenomenon of complementation may be an operational means for study of heterosis. Polymorphism of mitochondria of maize has been demonstrated ( SARKISSIAN and MCDANIEL 1967). The second alternative noted above needs further examination since the implications of mitochondrial complementation in vitro are very significant. Does complementation by mitochondria of two lines of maize suggest good combining ability of those lines? Of course it is realized that heterosis may be observed in different measurable attributes in various organisms. Our work is based on seedling heterosis which is measurable in terms of rate of germination and growth of the seedling. These processes are probably dependent in large part on superior mitochondrial activity; thus mitochondrial complementation in vitro may be indicative of the activity of mitochondria of the hybrid, especially since ’ \uppoi ted br Yatiimal Science Foundation Grant GB 5518 1’iemit ddrea , Institute of I ife Science, I‘exas X&\I Uni\eiaity, College Stahon, l exas 77843