Gene-Cytoplasmic Male Sterility

Abstract

Typically, mst represents a condition in which female sex is normal and male sex is nonfunctional. Two types of nuclear genes, fr and ms, control this sterility. The action of ms genes is cytoplasm-insensitive, whereas fr gene action is cytoplasm- dependent and -sensitive as these genes act only in a certain cytoplasm type, the S-cytoplasm. This cytoplasm has specific c-genes which interact with nuclear fr genes to produce mst phenotype so that only S _frfr individuals are mst. Since both nuclear (fr) and cytoplasmic (c) genes are required to produce mst, the sterility is due to g-c interaction and is termed gene-cytoplasmic male sterility (gc-mst). Plants having this sterility have thus two cytoplasm types or plasmatypes viz. N or F (normal or fertile) and S (sterile). N-cytoplasm may have a fertility-restoring gene C. The nuclear fr genes are ineffective in this cytoplasm in causing mst. Likewise, S cytoplasm is ineffective in the presence of Fr nuclear genes or their isoalleles. Thus, fertility to S _frfr genotypes can be restored by either replacement to S- by N-cytoplasm or of fr by Fr genes. This effect of the Fr gene is similar to the action of a suppressor allele in that the S-cytoplasm of restored lines is not permanently freed of mst and if the restoring alleles are removed, the S-cytoplasm expresses the original mst condition. In other words, the cytoplasmic condition is not reversed by the restorer alleles but it is merely suppressed. Furthermore, since normal- restoring nonfertility lines do not become cytoplasmically mst, no nuclear gene responsible for initiating the condition seems to exist. The combination S _frfr producing mst is highly sensitive and delicate, as mutation of S- to N-cytoplasm or any of the fr genes to Fr restores mft in the organism. On the other hand, the combination N _FrFr is an ideal one for stable fertility in an organism because for mst, the mutations of all the Fr genes to fr as well as of N-cytoplasm to S-type are needed. Such simultaneous mutations, although rare, do occur. In maize, pearl millet and sorghum, mutation rates of Fr and c genes are fairly high. Compared to c-genes, fr-gene mutation rate is higher in these as well as in many other crops.