CHARACTERIZATION OF MALE STERILE LINES

 

Introduction

Heterosis is mainly expressed in first filial/hybrid generation (F₁) vigour will be generally reduced in F₂ and later generations. To exploit the heterosis in self and cross-pollinated crops; larger quantity of F₁ hybrid seed is required. Therefore, it is very difficult to produce such huge quantity of seeds by using hand emasculation. In order to overcome the difficulties, a breeder needs three lines viz., A, B, and R lines. The details of three lines are as under

1.  Male sterile line (MS line or A-line)

2. Maintainer line (B-line)

3. Restorer line (R-line):

 

Male sterile line: The line which has absence of pollen grains or non functional pollen grains with functional stigma called as male sterile line.

Maintainer line: The line, which is used to maintain the male sterility, is known as maintainer line.

Restorer line: The line, which is used to restore the male fertility to male sterile line, is called restorer line.

 

CHARACTERISTICS OF A, B, AND R-LINES 

Characteristics of MS–Line/A-line

·      Anthers are abnormal, only abortive pollen exists within them.

·      Some times, there is absence of pollen or rudimentary pollen.

·      No seed set on selfing.

Characteristics of Maintainer line/B-line

·      Anthers are normal.

·      Pollens are functional.

·      Maintainer line serves as a pollinator to pollinate MS line.

·      The progeny of A x B is male sterile.

·      It does not interfere with the agronomic attributes of CMS (A) line. 

Characteristics of Restorer line/R-line

·      Anthers are normal.

·      Pollens are functional.

·      Seed set by selfing.

·      Restorer line serves as the pollinator to produced fertile F₁ progeny.

Requirement of A-line

·      Male sterile line is characterized by the absence of pollen grains or rudimentary pollens.

·      Male sterile line should be agronomically superior.

·      Male sterile line should be stable i.e., inherited from generation to generation without any change in pollen sterility.           

·      Synchronous flowering: The flowering time of MS- line should match with pollen parents. 

Requirement of Maintainer line (B-line)  

·      A maintainer line should produce abundant pollen for multiplication of MS- line.

·      It should be pure with uniform populations.

·      The flowering time of maintainer line should match with MS- line.

·      Maintainer line should be agronomically superior   

Requirement of Restorer line (R-line)

·      The restorer line should produce abundant pollen.

·      It should have strong restoring ability.

·      Restorer line should have good agronomic characters, good combining ability and significant usable heterosis in its hybrids.

·      The flowering time should synchronize with that of the MS-line.

 

Adaptability observations

The observations are recorded on growth, vigour, uniformity, and agronomic characters, resistance to diseases and insects and flowering habits in particular.

 

Identification of male sterility

           The male sterility is identified by the following methods

 

1.      Visual observations

Inspect the field during flowering season. Observe colour, size and plumpness of the anthers in the male sterile plants. Shake the panicle or spikes to know weather the anthers are shed from pollen or not.

 

2.      Bagging

 At the time of flowering, the panicle spike or buds are bagged. After a few days, the seed setting is recorded in the bagged panicles or buds. If there is no seed setting, it means plant is completely male sterile.

 

1.      Microscopic observations

·      Take some samples of anthers from flower

·      Crush some anthers on the slide with the tip of the needle

·      Put a few drops of I-KI solution on the crushed anthers

·      Examine under the microscope

·      Normally, fertile pollen grains take stain with I₂-KI, where as the sterile pollen grains are empty and do not take the stain.

Identification of Restorer line

Restorer gene

           The genes, which restore the male fertility to the male sterile line, are called restorer genes. The fertility restorer gene is mostly dominant and found in certain strain/varieties of a species or sometimes may be transferred into other strains.

Source

           It has been observed that restoring genes do exist in the nucleus of the female parent, which provides the male sterile cytoplasm. The number of varieties which are closely related to original parent possess restoring genes.

 

Identification

·         Make large number of test crosses of individual plants with

      MS-line in pairs.

·         Harvest the seeds from each cross.

·         Grow the F₁ plants from each cross.

·         Examine their fertility status. If the percentage of the dehiscent anthers is over 99% or normal fertile pollens are over 80% then it is restorer line.

·         Then back cross the F₁ generation with MS-line and again examine the anther dehiscence and fertility status. If both the things are still normal, then it is clear that male parent possesses the restoring male fertility capacity to male sterile parent.

·         If segregation occurs for fertility in F₁ hybrid, it indicates that the male parent is not pure.

·         If this is the situation, then individual plants are again selected among the progenies of male parents and test crossed till no segregation occurs.

 

Study Questions:

 

1. Explain with figure how do you identify the male sterile line, maintainer line and restorer line from the field of crop plant?