Cytpoplasmic Genetic Male Sterility

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The male sterility which  is governed by both nuclear and cytoplasmic genes is known as Cytoplasmic Genetic Male Sterility or CGMS.

While CMS is controlled by an extra-nuclear genome, nuclear gene may have the capability to restore fertility. When nuclear restoration of fertility genes (“Rf”) is available for a CMS system in any crop, it is cytoplasmic-genetic male sterility

The sterility is manifested by the influence of both nuclear and cytoplasmic genes. There are commonly two types of cytoplasms, N (normal) and S (sterile). The phenomenon is controlled by the interaction of a genetic factor (S) present in the cytoplasm and nuclear gene (s). There are also restorers of fertility (Rf) genes, which are distinct from genetic male sterility genes. The Rf genes do not have any expression of their own unless the sterile cytoplasm is present. Rf genes are required to restore fertility in cytoplasm which causes sterility.
Thus a combination of N cytoplasm with rfrf and S cytoplasm with Rf-produces fertiles; while S cytoplasm with rfrf produces only male steriles.
N cytoplasm along with rfrfFertile
S cytoplasm with rfrfOnly male sterile

 

Another feature of these systems is that Rf mutations (i.e., mutations to rf or no fertility restoration) are frequent, so N cytoplasm with Rfrf is best for stable fertility.

Examples – It is observed in maize, sorghum, bajara, sunflower, rice and wheat.

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 restorar gene.

Application of CGMS in Plant Breeding

  • It is used in commercial production of hybrid seeds in maize, sorghum and bajra.

  • It is widely exploited in crop plants for hybrid breeding due to the convenience of controlling sterility expression by manipulating the gene-cytoplasm combinations in any selected genotype.
  • Incorporation of these systems for male sterility evades the need for emasculation in cross pollinated species thus encouraging cross breeding producing only hybrid seeds under natural conditions. This system is known in cotton, maize, jowar, sunflowe, rice and wheat etc.

Limitations of CGMS

  • Undesirable effects of the cytoplasm
  • Unsatisfactory fertility restoration
  • Unsatisfactory pollination
  • Spontaneous reversion
  • Modifying genes
  • Contribution of cytoplasm by male gamete
  • Environmental effects
  • Non availability of suitable restorer line

Advantages of CGMS

  • Hybrid seed production. E.g.: maize, rice etc.
  • Convenience of controlling sterility expression by manipulating the gene-cytoplasm combinations in any selected genotype.
  • Facilitate cross breeding producing only hybrid seeds under natural conditions.
  • Saves lot of time money and labour.

Points to know

  • Also known as nucleoplasmic male sterility.
  • Common in many plant species across plant kingdom.
  • Male sterility determined by single recessive gene.
  • There are restorer fertility gene distinct from genetic male sterility.