Genetic and molecular analyses of mitochondrial inheritance and cytoplasmic male sterility in alfalfa.

Abstract

Biparental inheritance of plastids has been demonstrated in approximately one-third of angiosperm species examined, although the species studied may not be representative of angiosperms in general. Biparental inheritance of mitochondria in angiosperms has been observed in only one case. Since essential cellular functions, as well as several important agronomic traits, are encoded by plant extranuclear genomes, biparental inheritance of organelles has important implications for plant genetics and breeding. Evidence of biparental inheritance of plastids in alfalfa (Medicago sativa L.) suggests the possibility that mitochondria may likewise be inherited biparentally. The objectives of this dissertation were to determine: (1) the inheritance of mitochondria in alfalfa (uniparental or biparental), and (2) the inheritance of cytoplasmic male sterility and the extent of male fertility restoration in single and population crosses of alfalfa. Several large RNA molecules observed in alfalfa mitochondrial preparations were inherited biparentally. These molecules were unaffected by RNase A added to preparations of intact mitochondria indicating that the RNA's were contained within an RNase impermeable compartment. Linear sucrose gradient purification failed to separate the RNA's from mitochondria and examination of sucrose gradient fractions using transmission electron microscopy (performed by J. K. Brown, Dept. of Plant Pathology, Univ. of Arizona) revealed that mitochondrial preparations were free of contamination by virus-like particles and other organelles. These results indicated that the large RNA's were contained within the mitochondrion. The inheritance of large mitochondrial RNA's in alfalfa provided evidence that mitochondria are inherited biparentally in this species. Association of cytoplasmic male sterility with a particular organelle has not been determined in alfalfa, although the mitochondrion has been implicated in several other species. Analysis of progeny from single and population crosses provided evidence of biparental inheritance of cytoplasmic male sterility. Biparental cytoplasmic inheritance as well as nuclear inheritance may influence male fertility restoration.