36 Papaya
evaluation, the best one can be grown and inter-crossed. After the inter-crossing of
superior plants, selection can again be practiced in the new population. The recur-
ring population improvement concept leads to the name ‘recurrent’ selection. This
can be used for selecting superior types in dioecious as well as gynodioecious lines.
Smith (1970) has amply emphasised the role of selection and controlled pollination
in papaya. According to Dwivedi (1998) indirect selection of plants on the basis of
higher number and weight of seeds per fruit, higher number of fruits and plant and
low to moderate fruit length is more effective in increasing fruit yield than direct
selection. In addition, selection of fruits for higher peel weight, fruit weight and
diameter is expected to improve pulp weight in papaya (Dwivedi and Jha 1999).
2.7.3 Hybrid Breeding
2.7.3.1 Heterosis
Exploitation of heterosis, that is, producing F 1 progenies superior to the better parent
involved in a cross has ample prospect in papaya improvement. Dai (1960) reported
heterosis in the cross between ‘Philippine’ and ‘Solo’ varieties. The F 1 exhibited
reduced number of seeds and enhanced vigour. A high positive heterosis for fruit size
and number of seeds was observed by Sah and Shanmugavelu (1975) in the cross,
CO-1 × Coorg Honeydew at Coimbatore. Iyer and Subramanyam (1981) observed
heterosis for the vegetative characters, fruit yield and its components. Heterosis
up to 11.4% for yield was obtained in the cross, Solo Yellow × Washington. High
relative heterosis for fruits in certain crosses was observed in the combination,
Solo Yellow Sweet × Washington; Pink Flesh Sweet × Coorg Honey; Pink Flesh
Sweet × Washington and Thailand × Washington. High heterosis for potential eco-
nomic competitiveness was noticed in Thailand × Washington. In crosses between
Pusa Delicious × Halflong and Pusa Delicious × Homestead, high heterotic responses
(47.34% and 39.77%, respectively) were observed with respect to fruit yield over
mid-parent. Similarly, enough heterosis (31.29%) existed in the progeny obtained
from Pusa Delicious × Homestead with respect to fruit yield over better parent. As
regards fruit length, crossing between Washington × Ramnagar Local showed bet-
ter heterosis, that is, 44.63% over mid-parent and 36.92% over better parent (Ram,
1982). Ram et al. (1999) have reported positive heterosis for fruit yield and number
of fruits per plant, however, the heterosis was negative for characters like, single fruit
weight (average), fruiting length, first fruiting node, plant girth, plant height and days
to first flowering. Dwivedi (1998) has also indicated a negative influence of fruit
length on fruit yield and pulp weight.
2.7.3.2 Combining Ability
In order to make hybrids a reality, identification of highly productive heterotic com-
binations is very important. It might be logical to assume that the greatest heterosis
would be displayed by crosses of pure inbreds which have undergone maximum
inbreeding depression. Heterosis dependent on combining ability can occur in any
combination of pure inbreds.
General combining ability (GCA) is expressed in the progeny of an inbred crossed
with many genotypes and is primarily the result of additive gene action. To test