MITOCHONDRIAL HETEROSIS IN MAIZE1p2

M A N Y hybrids between genetic pure lines in maize exhibit heterosis. The search for the genetic basis of heterosis has been carried on for many years and by many workers. Despite these efforts, an adequate, specific biochemical explanation of heterosis has not been presented. JONES (1952) has noted the interaction of nuclear genes and cytoplasm, as evidenced by differences in reciprocal crosses of corn and other species. WILKIE (1 964) has discussed the physiological complementation of cytoplasms (mitochondria) in producing heterokaryon vigor in micro-organisms. Work with heterotic interspecific hybrids of primrose species has revealed a striking difference between reciprocal hybrids which indicated a cytoplasmic influence on hybrid vigor in this case (OEHLKERS 1964). It has been shown that mitochondria from maize exhibit heterosis in terms of nitrogen content (HANSON, HAGEMAN and FISHER 1960). We demonstrated mitochondrial heterosis in terms of biochemical activity ( MCDANIEL and SARKISSIAN 1966b). Further study showed the mitochondria of a maize hybrid were polymorphic ( SARKISSIAN and MCDANIEL 1967). The combination of mitochondria isolated from the parents of hybrids was found to yield a mitochondrial mixture which showed complementation and approached the activity of mitochondria from the hybrid ( MCDANIEL and SARKISSIAN 1966b; SARKISSIAN and SRIVASTAVA 1967). As mitochondria are of great importance in cellular metabolism, the relation of these subcellular, cytoplasmic organelles to heterosis deserves further study. The oxidation of carbolhydrates, fatty acids and amino acids by the KREBS tricarboxylic acid cycle of the mitochondrion provides much of the ATP needed by the organism for energy-requiring reactions, as well as reduced cofactors and carbon skeletons for syinthetic reactions. Heterosis has been shown to be a phenomenon of growth, being expressed as more rapid growth rates exhibited by heterotic hybrids. As growth is dependent on the production of available energy and utilization of carbon skeletons, and other biochemical products, the rate and efficiency with which the energy captured by photosynthesis can be liberated and used, could be a limiting factor in the growth process.