Inbreeding and selection
- In dioecious lines, suitable male plants are selected from the same progeny which have resemblance to female plants in vegetative characters, such as stem and leaf colour, stem thickness and height at flowering etc. Progenies raised from S1 inbreds are screened and desired male and female plants are selected for further sibmating i.e., crossing between the female plant and male plant of the same cultivar. The process is to be continued for 7-8 generations to achieve uniformity for a group of characters. In this method, the progeny will have male and female in equal proportion. Many dioecious cultivars have been bred by this method.
- Development of cultivar with high papain content was started at TNAU, Agricultural College and Research Institute, Coimbatore. As a result of intensive breeding programme, four cultivars i.e., CO1, CO2, CO5 and CO6 were developed through inbred selection which are dioeicous(Rao, 1974, Sundarajan and Krishnan,1984, Shanmugavelu et al., 1988) at Coimbatore. Recently during 2011, CO 8 papaya was developed in the dioecious group through sib-mating of CO2 variety, which is a red-fleshed variety and this variety is unique for the red pulp colour which is not exist in any of the dioecious papaya varieties developed. Systematic work for breeding of uniform varieties with high yield and good quality for wider adaptability was started at IARI Regional Station, Pusa, and Bihar in 1966.
- As a result of inbreeding and selection for 8 generations during 1966-1982, uniform lines of Pusa Delicious, Pusa Majesty, Pusa Giant and Pusa Dwarf with desirable attributes were developed. At Coorg, Aiyappa and Nanjappa (1959) selected Coorg Honey Dew which is gynodioecious. Based on the two season evaluation of papaya germplasm in Nainital, cultivars Barwani Red, Coorg Honey Dew, BR 1, 2, 3, 4, Coimbatore 1B and Washington have been found promising for commercial cultivation. Considerable genetic variability was found in many plants with respect to fruit characteristics like fruiting height, fruit weight, size shape, TSS, taste and number of fruits per tree.
Important cultivars of papaya developed through selection are as under
| CO1
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Plant is dwarf; fruit is round, selected from cultivar Ranchi
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| CO2
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It is pure line selection from local type, suitable for papain production in terms of high enzyme activity. Dual purpose variety for dessert fruit as well as papain
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| CO3
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It is a hybrid between CO2 x Sun Rise Solo, plant is vigorous, fruit is medium in size with good keeping quality.
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| CO4
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It is a hybrid between CO1 x Washington, fruit is large, keeping quality is good, and flesh colour is yellow.
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| CO5
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It is a selection from Washington; it is also good for papain production in terms of high latex yield
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| CO6
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It is a selection from ‘Giant. Fruits are bigger weighing 2.5 to 3.0 kg. Suitable for papain and dessert purpose
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| CO7
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It is gynodioecious hybrid between CP75 x Coorg Honey Dew. It has red flesh. Fruit cavity round
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| CO8
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It is a red-fleshed dioecious variety bred through selective sib-mating in the population of CO.2. Suitable for dessert purpose, processing and papain production
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| Coorg Honey Dew
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It is gynodioecious selection from Honey Dew at Coorg\
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| Pusa Majesty
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It is selection from cv. Ranchi. It is gynodioecious variety suitable for high papain.
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| Pusa Delicious
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A gynodioecious selection from cv. Ranchi, it has good fruit
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Induction of polyploidy
- Hofmeyer (1945) reported on polyploidy in papaya. They found that the quality of tetraploid fruit was better than the diploid and it was also compact with small seed cavity. But tetraploids were less fertile than diploid as indicated by comparative seed count. However, according to Singh (1955) there was complete sterility in both female and male tetraploids and expressed doubt about their commercial utilization. Further, Zerpa (1957) reported that colchicine induced tetraploid hermaphrodite plants, which were used as male parent in a cross with a female diploid produced a few seeds without endosperm, by embryo culture, two triploid plants were obtained which turned out to be hermaphrodite.
Hybridization
- A few hybrid varieties have been developed by the intervarietal or intergeneric hybridization. But still there is great scope for development of superior cultivars with better quality and yield. At TNAU, Coimbatore three varieties have been developed viz. CO3 (CO2 x Sunrise Solo), CO4 (CO1 x Washington) and CO7 (CP.75 x Coorg Honey Dew). At IIHR Bangalore, two hybrids IIHR-39 named as Surya (Sun Rise Solo x Pink Flesh Sweet), and IIHR-54 (Waimanalo x Pink Flesh Sweet) were developed (Dinesh and Yadav 1998), Hybrid HPSC-3 (Tripura local x Honey dew) was developed by the ICAR Research Complex Tripura (Singh and Sharma,1996). Cultivar Cariflora was developed by crossing K2xK3 line of papaya which is tolerant to PRSV (Conover et al.,1986).
- Inter-specific hybridization was also attempted in the genus Carica. The cross between the Carica papaya and Carica caulifora did not form mature seed but immature embryos could be germinated and grown by embryo culture. F1 hybrids of Carica papaya x carica pubescens and Carica papaya x Carica quercifolia were vegetatively vigorous. Carica papaya x Carica cauliflora F1 progenies are slow growing and those of Carica papaya x Carica stipulata developed apical necrosis before reaching maturity. Iyer and Subramanyam (1984) attempted interspecific hybridization and reported that F1hybrids of C.cauliflora x C.monoica when crossed with C.papaya gave fertile hybrids, although the C.cauliflora and C.monoica are incompatible with C.papaya but hybrids of C.cauliflora x C.monoica are compatible.
Heterosis breeding
- Dai (1960) reported heterosis in the cross between Philippines x Solo varieties. F1 hybrid tended to have reduced seed number and enhanced plant vigour. Heterosis up to 111.4% for yield and yield traits was obtained in Solo yellow x Washington whereas high heterosis for potential economic competitiveness was noticed in Thailand x Washington (Iyer and Subramanyam, 1981). At IIHR, Bangalore, an F1 hybrid namely, Surya (Sun Rise Solo x Pink Flesh sweet) was released recently. It is gynodioecious in nature and produces about 75-80 fruits of medium size weighing about 600-800g. the flesh is red in colour, firm, sweet to taste with a TSS of 14º brix.
Mutation breeding
- Ram and Majumder (1981) developed a dwarf mutant line by treating papaya seed with 15K gamma rays. Initially, 3 dwarf plants were isolated from M2 population. Repeated sibmating among the dwarf plants helped in establishing a homozygous dwarf line Pusa Nanha.
Biotechnology
- In-vitro propagation and genetic engineering technique can serve as a potential tool to overcome major constraints in Carica papaya.
Embryo culture
- In papaya, incompatibility is mainly due to the failure of endosperm formation. The hybrid embryo resulting from interspecific cross of C.papaya and C.cauliflora has been successfully rescued on White’s medium in 30 days by Yung (1986).
Transgenic papaya
- Transgenic papaya has been developed against Papaya Ring Spot Virus (PRSV) using coat protein mediated resistance in University of Hawaii by Dennis Gonsalves. The coat protein gene from PRSV was isolated, cloned and used for transforming papaya to provide resistance against the severe strain of the same virus. The target cultivars used in transforming papaya were the Red fleshed, Sun Set Solo and the Yellow Fleshed Kapoho Solo. Transformation with coat protein gene was done using micro projectile bombardment technique using embryogenic tissues of papaya.
- Two transgenic lines Sun UP from Sun Set Solo and UH Rainbow from Kapoho were developed which have shown excellent resistance to PRSV. Sun UP, which is homozygous for CP (Coat Protein gene) was resistant to most isolate of PRSV, from other geographical locations except Taiwan’s YK isolate of PRSV. Rainbow was found susceptible to PRSV isolates from outside Hawaii but was resistant to the severe strain of Hawaiian PRSV (HA isolates).
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