Male sterility refers to the absence of functional pollen grains in other wise hermaphrodite flowers. Male sterility is of great value in the production of hybrid seed. Male sterility is of two types:
Genetic and cytoplasmic. Cytoplasmic male sterility is termed cytoplasmic-genetic when restorer genes are known. There are several natural mechanisms that control the mode of pollination in crop plants. Two of these mechanisms, self-incompatibility and male sterility are of special significance because of their utilization in hybrid seed production. Male sterility is characterized by nonfunctional pollen grains, while female gametes function normally.
Male sterility is classified into three groups:
Genetic male sterility
Genetic male sterility is ordinarily governed by a single recessive gene, ms, but dominant genes governing male sterility are also known, e.g., in safflower. Male sterility alleles arise spontaneously or may be artificially induced. A male sterile line may be maintained by crossing it with heterozygous male fertile plants. Such a mating produces 1:1 male sterile and male fertile plants.
Genetic male sterility may be used in hybrid seed production. The progeny from ms msxMs ms crosses are used as female, and are interplanted with a homozygous male fertile (Ms Ms) pollinator. The genotypes of the ms ms and Ms ms lines are identical except for the ms locus, i.e., they are isogenic and are known as male sterile (A) and maintainer (B) lines, respectively. The female line would, therefore, contain both male sterile and male fertile plants; the latter must be identified and removed before fertile plants; the latter must be identified and removed before pollen shedding. Pollen dispersal from the male (pollinator) line should be good for a satisfactory seed set in the female line. In India, it is being used for hybrid seed production of arhar (C.catan) by some private seed companies. It is use in several other crops, e.g., cotton, barley, tomato, sunflower, cucurbits etc., but it is not yet practically feasible.
Cytoplasmic Male Sterility
This type of male sterility is determined by the cytoplasm. Since the cytoylasm of a zygote comes primarily from egg cell, the progeny of such male sterile plants would always be male sterile. The male sterile line is maintained by crossing it with the pollinator strain used as the recurrent parent in the backcross programme since it s nuclear genotype is identical with that of the male sterile line. Such a male fertile line is known as the maintainer line or B line as it is used to maintain the male sterile line. The male sterile line is also known as A line. It is observe in some important plant crops viz. Maize, (Zea mays), Nicotiama tabacum, Triticum aestivum, Gossypium hrisutum, Sorghum bicolor, Helianthus annus, Oryza sativa.
Cytoplasmic male sterility may be utilized for producing hybrid seed in certain ornamental species, or in species where a vegetative part is of economic value. But in those crop plants where seed is the economic part, it is of no use because the hybrid progeny would be male sterile.
Cytoplasmic-Genetic male sterility
This is a case of cytoplasmic male sterility where a nuclear gene for restoring fertility in the male sterile line is known. The fertility restorer gene R, is dominant and is found in certain strains of the species, or may be transferred from a related species e.g., in wheat. This gene restores male fertility in the male sterile line, hence it is known as restorer gene. The cases of cytoplasmic male sterility would be included in the cytoplasmic genetic system as and when restorer genes for them would be discovered. This system is known in maize, jowar, bajra, sunflower, rice and wheat.
The plants would be male sterile in the presence of male sterile cytoplasm if the nuclear genotype were rr, but would be male fertile if the nucleus were Rr or RR. The development of new restorer strains is somewhat indirect. First a restorer strain (R) is crossed with a male sterile line (A). the resulting male fertile plants are used as the female parent in repeated backcrosses with the strain (C), used as the recurrent parent, to which the transfer of restorer gene is desired. In each generation, male sterile plants are discarded, and the male fertile plants are used as females for backcrossing to the strain C. This acts as a selection device for the restorer gene R during the backcross programme. At the end of the backcross programme, a restorer line isogenic to the strain C would be recovered.
The cytoplasmic-genetic male sterility is used commercially to produce hybrid seed in maize, bajra and jowar. A triple cross may be produced by crossing single cross with a fertility restoring inbred so that all the plants in the triple cross would be male fertile.
Origin of Male Sterile Cytoplasm
Mutant male sterile cytoplasms arise spontaneously in low frequencies. Mutant cytoplasms have been isolated in maize, bajra and sunflower.
Transfer of the full somatic chromo-some complement of a crop species, through repeated backcrossing into the cytoplasm of a related wild species often leads to cytoplasmic male sterility. In cross-pollinated crop species, the male sterile cytoplasms have generally originated through mutation, while in self-pollinated crops they have been transferred from related species.
Ethidium bromide is a potent mutagen for cytoplasmic genes or plasmagenes. Male sterile cytoplasm may be induced by seed treatment with ethidium bromide e.g., Petunia.
Limitation of Cytoplasmic-Genetic Male Sterility
for e.g., the Texas cytoplasm in maize, by far the most successful cytoplasm commercially, slightly retards growth, yield (2.4%) plant height and leaf number; induces earlier silking and delayed pollen shedding; and makes the plants highly susceptible to Helminthosporium leaf blight. Restorer genes only restore male fertility; they are unable to remove the side effects of the male sterile cytoplasms.
In many cases, restoration of fertility is not satisfactory and cannot be used in the production of hybrid seed.
Natural pollination is often not satisfactory, except in wind-pollinated crops like maize. This reduces the production of hybrid seed, and thereby increases its cost. In some species e.g., Capsicum, this has prevented the use of male sterility in hybrid seed production. Poor pollination would always be a major problem in self-pollinators e.g. wheat.