In this article we will discuss about:- 1. Meaning of Transgenic Male Sterility 2. Features of Transgenic Male Sterility 3. Transfer of Sterility & Restorer Genes 4. Applications 5. Advantages 6. Disadvantages.
Meaning of Transgenic Male Sterility:
In addition to above three types of male sterility, there is another class of male sterility what we call transgenic male sterility. The foreign gene or transgene is also used for induction of male sterility. The male sterility which is induced by the technique of genetic engineering is referred to as transgenic male sterility.
The gene barnase is the first transgene that was used to produce male sterility by Mariani et al. in 1990. It has an effective fertility restoration system in barstar. Transgenic male sterility comes under genetic male sterility and is heritable. This type of sterility has been induced in crop plants like tobacco, rapeseed, maize, etc.
Features of Transgenic Male Sterility:
The main features of transgenic male sterility related to source of gene, types of genes involved, gene integration, nature of gene, material included, maintenance and uses are briefly discussed as follows:
1. Source of Gene:
The gene for inducing transgenic male sterility is generally used from micro-organisms. Transgenic male sterility has been induced in tobacco and rapeseed by transferring gene from Bacillus amyloliquefaciens.
2. Types of Genes Involved:
Transgenic male sterility is controlled by foreign nuclear genes. In this system two types of transgenes, viz., barnase and barstar are used. The gene barnase causes male sterility and the gene barstar restores the fertility. Both these genes have been utilized from the bacterium Bacillus amyloliquefaciens. The male fertility of barnase male sterile is restored by another gene, barstar, of the bacterium B, amyloliquefaciens.
3. Gene Integration:
The gene barnase is integrated in male sterile line called A line and the gene barstar is integrated in restorer line, called R line. Thus, it is a two line system. The cross between A and R line yields fertile F1.
4. Nature of Gene:
The transgenic male sterility is controlled by a dominant gene called barnase. The male sterile plants are always heterozygous. In other words, the transgenic male sterility reported so far is controlled by one dominant gene (barnase). The gene barstar is responsible for restoration of fertility in the hybrid developed using barnase male sterile line.
For commercial exploitation, a dominant genetic male sterility system must have the following important features:
(i) Efficient fertility restoration system,
(ii) Easy maintenance of male sterile lines,
(iii) Easy elimination of male fertile plants from the male sterile line,
(iv) Lack of adverse effects on other traits,
(v) Stable male sterile phenotype over different environments and in genetic backgrounds, and
(vi) Satisfactory performance of the F1 hybrids.
The barnase-bar/barstar transgenic male sterility system appears to satisfy above mentioned requirements.
5. Material Included:
This system consists of three components, viz., A line, B line and R line. Description of these lines is presented as follows:
(i) A Line:
It refers to genetic male sterile line (barnase) which is used as female parent in hybrid seed production. It is a heterozygous line (barnase/-). A male-sterile (ms) line is a group of plants of a plant variety, which are all male-sterile due to the presence of a particular male- sterility gene, say barnase in this case.
A male sterile plant contains a foreign “male-sterility locus” which contains the male-sterility gene S which when expressed in cells of the plant makes the plant male-sterile. Such a male-sterility gene is always a dominant allele at such a foreign male-sterility locus.
(ii) B Line:
It is a maintainer line. It is crossed with A line to maintain the male sterility.
(iii) R Line (Barstar):
This line is used to store fertility in the F1 hybrid. A restorer line is a group of plants of a plant species which all contain the fertility restorer gene say barstar in this case at the same genetic locus. Male fertility is restored to a male-sterile line by introducing the fertility restorer gene in the ms line, e.g. by crossing of the plants of the ms line with plants of the restorer. The cross of the fertility restorer line with male-sterile line restores the male fertility in the F1 progeny. Such plants are obtained from a cross between a male-sterile plant and the restorer plant.
6. Maintenance:
The three components of transgenic male sterility are maintained separately as follows:
(i) A Line:
The transgenic male sterile line (A) is heterozygous (barnase/-) which is maintained by crossing male sterile plants (barnase/-) with normal male fertile lines (-/-) in each generation. From such cross only 50% of the progeny will be male sterile (barnase/-) and the rest 50% will be male fertile. In a hybrid seed production program, the male fertile plants present in the male sterile line are identified and eliminated.
(ii) R Line:
Transgenic plants expressing barstar are male fertile without any phenotypic effect, and are easily maintained in the homozygous state by self-pollination. When a homozygous barstar male fertile line is crossed with a barnase male sterile line, all the progeny plants are male fertile since barstar gene product effectively inhibits the barnase RNAse in barnase-bar/barstar plants.
(iii) B Line:
This is a normal homozygous male fertile line which is maintained by self-pollination.
7. Uses:
Transgenic male sterility can be used for the production of hybrid seed in both seed propagated and vegetatively propagated species.
Transfer of Sterility & Restorer Genes:
Initially, the male sterility foreign gene (barnase) and male fertility restorer gene barstar are transferred to A line and R line by the technique of genetic engineering and the transformed cells are regenerated into a mature plant. Subsequently, the resulting transformed plants can be used in a conventional breeding scheme to produce more transformed plants with the same characteristics or to introduce the male-sterility gene, or the fertility-restorer gene in other varieties of the same or related plant species.
Thus the male-sterility gene or the fertility-restorer gene when introduced into a particular line or variety of a plant species can always be introduced into any other line or variety by backcrossing. The male sterility and fertility restorer genes are transferred to female and male parent by separate backcross program.
Barnase is the ribonuclease which is secreted by Bacillus amyloliquefaciens and barstar is the inhibitor of barnase that is produced by the same microorganism. The barnase gene can be used for the construction of a sterility gene while the barstar gene can be used for the construction of a fertility-restorer gene.
Experiments with oilseed rape and maize have shown that a chimeric barstar gene can fully restore the male fertility of male sterile lines in which the male sterility was due to the presence of a chimeric barnase gene.
The restorer capacity refers to the ability and efficiency of a restorer line to restore male fertility efficiently to a wide variety of ms lines. It is directly related to the amount of barstar produced in the stamen, particularly in the tapetum cells, of the restorer plants. Restorer capacity is important because an efficient restorer line can be used for the restoration of male fertility to a wide variety of male-sterile lines.
Plant species in which the synthetic barstar DNAs are used include especially oilseed rape, maize and rice. Restorer genes comprising the synthetic barstar DNAs may be used for producing restorer lines in different plant species but are particularly useful for producing restorer lines in cereals, especially in corn, rice and wheat.
Applications of Transgenic Male Sterility:
For hybrid seed production, it is necessary to ensure that fertility of the hybrid plants is fully restored. In systems in which the male sterility is under genetic control this requires the existence and use of genes that can restore male fertility. The development of hybrid cultivars is mainly dependent on the availability of suitable and effective sterility and restorer genes.
For the production of hybrid seed the male-sterile line is also called the female or first parent line, and the male- fertile (restorer) line is also called the male or second parent line. Transgenic male sterility/fertility system has shown commercial promise in maize and oilseed rape. Now it is also available in cauliflower, chicory, tomato and wheat.
In India, transgenic lines expressing barnase and barstar genes have been developed in Brassica juncea. These lines are stable and provide a complete and usable male sterility-restorer system. This barnase-barstar system is expected to promote hybrid development in Brassica juncea.
Advantages of Transgenic Male Sterility:
i. Early identification of male sterile plants is very easy.
ii. Transgenic male sterility can be used for the production of hybrid seed in both seed propagated crops and vegetatively propagated species.
iii. Transgenic male sterility is stable and provides a complete and usable male sterility- restorer system.
iv. Transgenic male sterility generally does not have undesirable agronomic characters.
Disadvantages of Transgenic Male Sterility:
There are four main disadvantages of transgenic male sterility.
i. Transgenic male sterility is induced by the technique of genetic engineering which is an expensive method.
ii. This type of male sterility has been used in very few crops so far.
iii. Crosses between male sterile plants (barnase) and normal male fertile plants are made every year for maintaining the transgenic male sterility.
iv. Transgenic male sterility requires more area and labor, because the breeder has to maintain three types of populations, viz., A, B and R lines.
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