Some of the economically important crop plants normally reproduce asexually and propagate through, runners, stolons, bulb, corm, rhizome, tuber etc. Many others like roses, apple, pear, plum, mango etc are cultivated through cuttings, gratings’ etc.
These plants are genetically highly heterozygous and do not breed true sexually. In vegetative propagation however it is always true breeding, because it takes place by mitosis.
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There are many clones or genotypic varieties in the vegetative propagating plants. These clones are selected and are bred (sexually) in order to improve the quality further. There are two methods to sexually breed vegetative propagated plants. These are
Clonal selection:
In a population of the mixed type there will be several clones. Those with desirable traits are selected and propagated by vegetative means. This method helps in the perpetuation of a desired trait, but it does not add anything new to the existing genotype.
Hybridization:
This is resorted to in order to combine the desired traits distributed in various clones. Sometimes, traits distributed even in different species or genera are also sought to be combined. Each clone is crossed to produce a population of F.
Since the clones themselves are genetically heterozygous, segregation takes place in the F itself. Usually F, are not suited to produce F2 as characters might separate once again. Hybridization in these plants is of the following types.
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Interspecific crosses:
Two varieties or clones belonging to the same species and differing in one or more traits are crossed since there is maximum homogeneity of the chromosomes between the two variants; the chances of success in the cross are definite.
Many varieties of vegetable crops (Raddish, Carrot, Cauliflower, Cabbage, Knol Khol etc) are improved by intraspecific crosses.
Interspecific crosses:
Crosses are made between the individuals belonging to two different species. Usually species growing in the same ecological habitat, geographical area are selected as there will be a fair chance of success in the cross.
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For e.g., Orchard, and Ornamental plants are improved by interspecific hybridization, followed by back crossing and selection. In this way many genes and gene blocks are transferred from one individual to the other.
One of the greatest draw backs in interspecific crosses is hybrid sterility brought about due to genome non homogeneity. This can be overcome partially or completely either by back crossing or induction of polyploidy.
Interspecific crosses have been employed in tomato (Lycopersicum esculentum x L.pimpinellifolium).
In L esculentum fruits are large, flattened and crack on opening. The taste is acidic. L pimpinellifolium is the wild plant possessing small reddish fruits. The plant is also disease resistant. The hybrids between these two plants are known to possess a combination of many desirable traits.
In Sorghum, a cross between (S.helpense X S.durra) x S.durra had yielded a hybrid with increased height and girth of the stem, panicle, leaf size etc. Similarly in cotton also a cross between Gossypium arboreum X G.herbaceum has yielded a variety of cotton (H 190) useful in many respects.
When a plant can be propagated vegetative, hybrid sterility is no hinderance as the plant need not reproduce sexually. F, individuals are propagated by means of cuttings, graftings etc.
Roses, Plums, Cherries etc are propagated by this method. In sugar cane also (Saccharum officinarum x S.spontaneum) crosses between species have yielded useful hybrids.
Intergeneric crosses:
When a cross is made between two different genera it is known as intergeneric (Raphanussativus X Brassica oleracea). Hybrid non viability and sterility are much more pronounced in intergeneric crosses than in interspecific ones as the genomes are entirely different.
Usually intergeneric crosses are not employed as the chances of success are very meagre. Polyploid individuals however have a fair degree of success. Due to the sheer size of the genome, in a cross with a related genus, there is likelihood of some amount of chromosomal homology, resulting in a successful hybrid.
In maize, wheat etc., crosses have been made with related genera to obtain successful hybrids. The genus Euchlaena has two species E.mexicana and E.perennis which have partial homology with the chromosomes of maize.
An intergeneric cross between maize and Euchlaena has helped in transferring the disease resistant gene to the maize.
Crosses of similar type have been attempted in wheat which has helped in tracing the origin of the common bread wheat.