- Cultivated apple has been classified as pumila group. Majority of the cultivated apples are diploids (2n=34) and few are triploids (2n=51). Delicious group of apples are very popular and occupy 50-70 per cent area in the states of Himachal Pradesh, Jammu & Kashmir, Uttar Pradesh and North- East hills.
Breeding objectives
- Apple is grown as a composite tree consisting of rootstock, scion and occasionally interstem. Thus genetic improvement must involve both rootstock and scion. The scion breeding objectives are to evolve varieties, red in colour with early maturity, high yield, superior dessert and storage quality and resistance to scab. Besides, a new wave of clonal rootstocks capable of surviving under wide range of environmental conditions, inducing precocity, enhancing productivity and fruit quality in scion are required to be bred.
Genetics of apple
- Malus has 25 to 30 species and several sub-species, many of which are cultivated as ornamental trees for their profuse blossoms and attractive fruits. Many of the species intercross freely and semi self incompatibility is common. Trees grown from collection of Malus are frequently inter-specific or inter-varietal hybrids. The cultivated apple is botanically Malus domestica Borkh.
- The majority of cultivated apples are diploids (2n=34). There has been a belief that they are complex polyploids, being partly tetraploids and partly hexaploids with the basic number of x=7 which is common in Rosaceae. The hypothesis is based on the associations and behavior of chromosomes and six sets of three chromosomes. So, they are functionally diploids. Among the cultivars, there are also triploids (2n=51).
- Triploids appear to be more common in cultivated apples, accounting for about 10 per cent of the commonly grown cultivars. Some triploid varieties are Baldwin, Gravenstein, Rhode Island Greening, Blenheim Orange and Mutsu, Jupiter and Jonagold, these are more vigorous and tend to have larger fruits but produce poor pollen and require diploids to pollinate them. These are useless as parents for breeding as they produce few seeds and give rise to weak seedlings.
Sterility and Incompatibility
- Sterility and incompatibility are two main causes of unfruitfulness in apple. The generational sterility is caused by the failure of any of the processes concerned with the development of pollen, embryo sac, embryo and endosperm. This is common in triploids and some diploids. Gagnieu (1951) concluded that the segregation suggests a simple disomic inheritance of four different and possibly allelomorphic genes P1 P2 P3 and P4. Sexual incompatibility which is due to the failure of the pollen, although functional, to grow down the style and bring about fertilization is widespread in the apple. Self incompatibility is particularly common, although cases of cross incompatibility are also known.
Apomixis
- Facultative apomixis is characteristic of a number of Malus species which are probably of hybrid origin but does not appear to occur among the cultivated apples. The apomictic species which have been investigated are polyploids. Malus sikkimensis (Hook). Koehne is a triploid, M.coronaria (L.) Mill., M.hupenhensis (Pamp.) Rehd., M.lancefolia Rehd. M.platycarpa Rehd., M.toringoides (Rehd.,) Hugs are known in triploid and tetraploid forms. M.sergenti Rehd is known in diploid, triploid, tetraploid and pentaploid forms.
- Under normal circumstances, these species reproduce themselves freely by apomictic seeds but most of them can produce sexual hybrids if crossed with sexual diploids. Seedlings from these apomictic species are not necessarily identical and a certain amount of variation can be found. The importance of this character in Malus species is that seedlings of some are sufficiently uniform to enable their use as rootstocks which are virus – free.
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