34 Papaya
2.7 Breeding
2.7.1 Objectives
The main breeding objective is to develop cultivars with higher yield of better qual-
ity fruits. Six principal characters namely, yield, colour and texture of the flesh, fruit
size, sweetness and storage are considered the most desirable traits in all breeding
endeavours. Papaya plants suffer greatly from several viral diseases. Resistance to
the virus infection would indeed be the first priority in breeding. Some other prob-
lems that need to be solved to enhance the probability of increased fruit and latex
production are
- Elimination of ambisexual andromonoecious or hermaphrodite forms that
tend to become female sterile at certain time of the year or show a tendency
towards stamen carpelloidy. - Development of homozygous hermaphrodite forms by possible elimination
of the zygotic lethal factor or by in vitro culture of embryo. - Inducing a sex-linked vegetative character for eliminating unwanted sex
forms in the early seedling stage. - Early and low bearing with short internodes.
- Selection for inflorescences of moderate length (about 7.5–10.0 cm) bearing
single fruit, to rule out crowding with resulting misshapen fruits. - Selection for an ovarian cavity circular in transverse section, with an easily
separable placenta. - Uniformity of fruit shape, texture and flavour for export trade.
- Breeding of dioecious cultivars for regions where andromonoecious types
are excessively sexually ambivalent. - Breeding to expand regions of production by hybridising with species that
are more cold tolerant (e.g. C. pubescens) or tolerant to excessive soil mois-
ture or salinity. - Breeding for increased latex yield for papain production.
2.7.2 methodS
2.7.2.1 Backcross Breeding
Backcross breeding method has been frequently used in papaya improvement
programmes. Its objective is to add easily heritable desirable alleles from a non-
recurrent parent to the genetic background of a recurrent parent. Since papaya is a
highly cross-pollinated crop, a great deal of variation exists in shape, size, quality,
taste, colour and flavour of the fruit. With backcrossing, increased homozygosity
for recurrent parent alleles will occur leading to fair uniformity in desirable traits.
The rate with which this takes place for each locus can be calculated by the formula
(2b − 1)/2b where b is the number of backcrosses. For example, in the BC 6 (the sixth
generation of backcrossing) 127/128 or 99.22% of the segregating loci for a particular
trait will be homozygous. The outstanding dioecious papaya genotypes like, Pusa
Dwarf and Hortus Gold, rapidly degenerate due to out-crossing. Backcrossing is