Breeding of Fruit and Plantation Crops

Male sterility

• Male sterility is characterized by non-functional pollen grains, while female gametes are functional. Male sterility can be classified into three groups viz., genetic male sterility, cytoplasmic male sterility, and cytoplasmic genic male sterility.
 
(a) Genetic male sterility

• Like any other morphological traits, particularly mono and oligogenic, this type of male sterility occurs in plant due to mutation of the fertility locus, situated on chromosomes within the nucleus. In this case, cytoplasm is not involved in bringing the sterility. There could be three possible genotypes for this locus and only one of them is male sterile.
• Fertile (R-line) = RR
• Fertile (B-line) = Rr
• Sterile (A-line) = rr

Sterility maintenance

• By crossing AxB lines, sterile and fertile progenies are produced in equal proportions. For the maintenance of sterile line, the fertile plants need to be quickly removed before the shedding of the pollen grains. The fertile plants can be removed in early stage of plant growth by using marker gene.

Fertility restoration

• Fertile lines can be obtained by crossing A-line with R-line. It can be used in hybrid seed production and genetical studies or for the preservation of variability.

(b) Cytoplasmic male sterility

• It occurs due to the mutation of mitochondria or to due to some other cytoplasmic factors outside the nucleus, resulting in the transformation of the fertile cytoplasm into a sterile one. Nuclear genes are not involved. Further, with two types of cytoplasm i.e. sterile and fertile, at the most, only two kinds of genotypes are possible, one of them is sterile and another fertile. The fertile cytoplasm is denoted by (F – B Line) and sterile cytoplasm is denoted by (f – A line).

Sterility maintenance

• Due to two different types of genotypes, cytoplasmic sterility can be maintained as under:

Fertility restoration

• Since there is no third type of genotype which can act as R-line, as such restoration of fertility is not feasible. However, this does not exhaust all the possibilities of use of cytoplasmic sterile lines.

Uses

• As restoration is not possible, this type of sterility is useful only in crops where the seed is not the desired end product. This is important for horticultural crops where vegetative parts are of economic value.

(c) Cytoplasmic-genic male sterility

• Such sterility arises from the interaction of nuclear gene(s) and conditioning sterility with sterile cytoplasm. The cytoplasmic genic sterility is essentially a cytoplasmic sterility with a provision for restoration of fertility. The fertility is restored by (R) gene present in the nucleus.
• The combination of both nuclear gene(s) and cytoplasmic factors determine the fertility or sterility in such plants. Based on these combinations, there can be maximum of six types of genotypes and only one of them is sterile.

Sterility maintenance

• As visualized by their genetic composition and cytoplasm, only [(rr) f] genotype can maintain the sterility of A-line.

Fertility restoration

• This is achieved by suitable restorer lines which can give rise to all fertile progenies on crossing with A-line. Among the possible six genotypes, only [(RR) F] and [(RR) r] are such restorer or R-line. They produce all fertile progenies.

Uses

• Cytoplasmic-genic male sterile lines are of immense importance in exploitation of hybrid vigour in crops where seed is the desired end product.