International Journal of Horticulture, 2017, Vol.7, No.22, 180-204
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recurrent selection are generally synthesized following hybridization and testing of their general combining
ability to develop a synthetic cultivars.
Heterosis breeding:
Cross pollinated and asexually propagated species show moderate to severe inbreeding
depression. Inbreeding is the mating between closely related individuals. Inbreeding depression is defined as the
loss or reduction in vigour and fertility as a result of inbreeding. Inbreeding causes the appearance of lethal and
sub-lethal alleles reduction in vigour, reproductive ability, yield and increases homozygosity.
Heterosis is defined as the superiority of a F
1
hybrid over both its parents in terms of yield or some other
characters.
Manifestation of heterosis:
Cross pollinated species show heterosis when inbred lines are used as parents. In
such species, genetic additive variance is one of the essential source for improvement of hybrid vigour. In some
self-pollinated species, heterosis is applied for the production of hybrid seeds. The superiority of hybrids
(heterosis) over its parents is manifested on increased yield, increased reproductive ability, increase in size and
general vigour, improve quality, early flowering and maturity, greater resistance to insect pests and diseases,
greater adaptability.
Heterosis in ornamental plants:
In marigold, highest heterosis was observed in the crosses ‘Alaska’ x ‘Hawaii’,
‘Alaska’ x ‘Cupid Orange Mum’ and ‘Katrain Local’ x ‘Cupid Orange Mum’ for flower size, flower weight and
flower number, respectively . In another study on diallel crosses involving six parents one hybrid raised from the
cross of ‘Giant Double African Orange’ x ‘Cracker Jack’ was reported promising for commercial purposes. A
hybrid, ‘MS-8’ x ‘Pusa Narangi Gainda’ developed by using apetalous male sterile lines was found promising
in terms of higher flower yield.
In antirrhinum, F
1
dwarf hybrids are developed using parents
Antirrhinum majus
and
A. glutinosum
. Over
dominance was observed in the inheritance of all characters.
In balsam, heterosis was reported involving five parents in a diallele set of crosses for various characters.
Maximum heterosis was found for numbers of flowers per plant followed by branches per plant.
In China Aster, considerable heterosis was recorded involving 12 parents in a diallele set of crosses for all the
characters. Three crosses ‘Shell Pink’ x Azure Blue’, ‘AST-20’ x ‘Azure Blue’ and ‘AST-20’ x ‘AST-16’ were
developed for manifestation of heterosis in terms of flower size, numbers of flowers per plant and stalk length.
In hollyhock, five F
1
hybrids namely ‘Pusa Pink Beauty’, ‘Pusa Yellow Beauty’, ‘Pusa Pastel Pink’, ‘Pusa
Apricot Supreme’ and ‘Pusa Pastel Pink Supreme’ were developed.
In single multiflora and grandiflora types of petunia, heterosis involving diallele crosses using eight parental lines
were studied for all characters.
Mutation breeding:
Mutation is the sudden heritable changes occurred in an organism exception to Mendelian
segregation and recombination and the mutated individual is called mutant. The term mutation was first
mentioned by De Vries (1900). Mutation may be caused by spontaneous and induced and the result of a change in
the gene or chromosomes or change in the cytoplasmic genes.
There are various types of mutation found in ornamental plants.
Gene or point mutations:
Mutation due to changes in the base sequences of genes are called gene or point
mutations. It is used directly as improved variety, to increase variability in allogamous species, for cross breeding,
to induce mutation in inbred lines, to induce male sterility and sport development.
