🥶 Male Sterility: GMS, CMS, CGMS, and Chemical Methods

Why Male Sterility Matters in Agriculture

Hybrid maize, hybrid rice, hybrid bajra, hybrid sunflower — all of India's commercial hybrid crops rely on male sterility systems for cost-effective seed production. Without male sterility, producing hybrid seed would require manual emasculation of every flower on the mother plant — impractical on a commercial scale. The CMS-based three-line system (A-line, B-line, R-line) is the backbone of the hybrid seed industry.

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• Male sterility means non-functional pollen grains. Here female gametes are normally functional. Male sterility is a crucial phenomenon in plant breeding, primarily used for hybrid seed production without the labour-intensive process of hand emasculation. It is classified into four groups:
• Genetic Male Sterility (GMS) or Nuclear Male Sterility
  • Temperature-sensitive GMS
  • Photoperiod-sensitive GMS
  • Transgenic GMS
• Cytoplasmic Male Sterility (CMS)
• Cytoplasmic-Genetic Male Sterility (CGMS)
• Chemically-Induced Male Sterility
• Male Sterility is used for hybrid seed production in many crops because it is a means of genetic emasculation. ^{UPPSC 2021} By eliminating functional pollen, male sterility removes the need for manual removal of anthers, making large-scale hybrid seed production economically viable.

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Genetic (Nuclear) Male Sterility

• GMS/NMS is ordinarily governed by a single recessive gene ms but also by dominant genes. Male sterility alleles are occurred spontaneously or are induced artificially. Since the gene is nuclear in origin, it follows standard Mendelian inheritance patterns.
• Genetic male sterility is simple and ecofriendly technique of hybrid seed production. ^{UPPSC 2021}
• When a male sterile plant (ms ms) is crossed with the male fertile (Ms Ms) plant, the product F1 (Ms ms) is male fertile. All F1 plants are fertile because the dominant Ms allele masks the recessive ms allele.
• In F2, 3 Fertile : 1 Sterile is obtained. A male sterile line is maintained by crossing with heterozygous male fertile (Ms ms). This mating produces 1 : 1 male sterile & male fertile plants. This 1:1 ratio is the basis for maintaining the male sterile line in a breeding programme.

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Maintenance of a male sterile line

• During the maintenance of male sterile lines, sibmating (i.e. brother-sister mating) is achieved through natural pollination. The male sterile plants receive pollen from their male fertile sibling plants growing alongside them.
• In some cases ms gene expression is affected by the specified range of temperature and or photoperiod regime. Such GMS is environment sensitive and found in Rice, tomato, wheat etc. This environmental sensitivity can be exploited -- the line is maintained under one set of conditions and hybrid seed is produced under another.
• Temperature-sensitive Genetic Male sterility:
  • Complete male sterility is produced by the ms gene at higher temperatures
  • E.g. 23.3 degrees C or higher for rice TGMS line Pei-Ai 645. This type of genetic male sterility is being used in the China to develop hybrid rice. At lower temperatures, fertility is restored, allowing the line to be self-maintained.
• Photoperiod-sensitive Genetic male sterility (PGMS):
  • In some cases, ms gene expression is drastically affected by the prevailing photoperiod, provided the temp. is within the critical range.
  • E.g. the critical temp. is 23-29 degrees C for rice for PGMS. Within this critical temp. range, complete sterility is found in rice plants grown under long day conditions (i.e. day length more than 13 hours 45 minutes). But under short day conditions almost normal fertility is found. This type PGMS is being utilized in the China to develop hybrid rice. The two-line hybrid system in rice is based on TGMS and PGMS.

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Transgenic Genetic male sterility

• Transgene is such a gene which is introduced into the genome of an organisms by recombinant DNA technology or genetic engineering. Many transgenes cause male sterility. These are found in maize, cauliflower, tomato, wheat & chicory but this system is not yet used commercially. Transgenic approaches offer precise control over male sterility but face regulatory and public acceptance challenges.
• The genetic male sterility is being used for hybrid rice development in China. GMS is being successfully used in castor in U.S.A. In India GSM is being used for hybrid seed production of Arhar by private companies like Maharashtra Hybrid seed Co. Ltd.

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Cytoplasmic Male Sterility (CMS)