PhD

Pumpkin Mosaic Virus (PMV) and Yellow Vein Mosaic Virus (YVMV) are the most devastating diseases of pumpkin in Kerala and the only way to manage this is to use virus resistant varieties. Investigations were carried out at Sugarcane Research Station, Thiruvalla, Kerala Agricultural University to identify source(s) of resistance to PMV and YVMV, to study the genetics and biochemical mechanism of mosaic virus resistance and to identify desirable segregants possessing combined resistance to PMV and YVMV.
Six species of Cucurbita (103 genotypes) were used for the study. Only one accession Nigerian Local (NL), introduced from Cornell University possessed high level of resistance to both the virus.
The mosaic resistance confirmation studies namely, back inoculation, grafting and multi-environment study clearly established that the reaction in NL can be considered as stable. Neither the inoculated leaves nor the uninoculated subsequent leaves of the susceptible variety Ambili showed symptoms when back inoculated with sap from NL in case of PMV. Through back inoculation, viruses could not be isolated from this genotype thereby proving that the resistance is not symptomless carrier type of tolerance. NL did not develop any disease symptom even after grafting on the YVMV infected symptomatic root stock. The environmental fluctuations also did not influence the resistance expression of NL against both PMV and YVMV.
Resistance to PMV was evaluated in five parents and their F1, F2 and backcross generations of four cross combinations namely, Amb x NL, AC x NL and Co1 x NL. On an average 96.25 per cent plants in the F1 showed susceptibility. The gene for susceptibility to PMV had only 95 percent penetrance in PV x NL. Under different genetic background the role of gene modifiers in the disease susceptibility expression of the major gene in AC x NL can not be ruled out. The F2 segregation ratio in all the crosses was in agreement with the Mendelian genetic ratio of 3:1 (susceptible : resistant). The test cross (F1 back crossed to NL) confirmed this with a genetic ratio of 1:1 (resistant : susceptible) in all the four crosses studied. The inheritance studies in six generations of all the four cross combinations clearly revealed that the resistance to PMV in pumpkin is controlled basically by a single recessive gene. The role of gene modifiers affecting the expression of this major gene is to be investigated further.
Reaction to YVMV in the six generations i.e., P1, P2, F1, F2, B1 and B2 of the four cross combinations under study revealed that the resistance is controlled by a single dominant gene. In the F1 on an average out of 40 plants only 32 were resistant. Hence the influence of gene modifiers in the resistant expression of this dominant gene for resistance to YVMV in heterozygous conditions can not be ruled out and is to be further analysed. The reciprocal crosses also behaved in a similar fashion. Out of 300 F2 plants in each cross, a mean number of 220 plants were resistant and 80 susceptible, fitting in a 3 : 1 ratio indicating the monogenic dominance of resistance. When F1 was back crossed to the susceptible parent, the segregation of resistance and susceptibility was in equal proportion i.e., out of a mean number of 100 B1 plants inoculated 48 were resistant. This very well fitted into 1 : 1 ratio of resistance : susceptibility. So resistance to YVMV is governed basically by a dominant gene, slightly influenced by gene modifiers.
Although NL is a good source of resistance to both the viruses under study, the seed germination was as low as 19 per cent. Through seed coat removal, 62 per cent seed germination (43%increase) was noted in NL.
The isozyme pattern of peroxidase and esterase enzymes in susceptible and resistant genotype of pumpkin as well as their F1 hybrids was analysed, before inoculation and after inoculation of both viruses. The resistant and susceptible genotypes showed clear cut differences in their peroxidase profile. The susceptible parents had three bands namely, PRX1, PRX3 and PRX6, while the resistant parent NL expressed only one isoform-PRX1 (0.083) before virus inoculation. The hybrid PV x NL exhibited six bands with three additional bands, PRX2, PRX3 and PRX5. The additional band PRX2 can be attributed to the earliness in growth and development of this hybrid.
After inoculation of PMV, the band PRX1 expressed before inoculation was not expressed in both the resistant and susceptible parents as well as in their F1 hybrids. In the resistant parent NL an additional band, PRX5 (Rm value 0.463) was noted. This particular isoform might be responsible for resistance to PMV in NL. After infection of YVMV, the resistant parent as well as the resistant F1 hybrids had the additional band PRX4 (Rm value 0.392).
There were six bands of esterase in the susceptible parents and only three in the resistant before virus inoculation. The resistant parent after inoculation of PVM, exhibited three additional bands EST1 (0.368), EST4 (0.632) and EST5 (0.721). Consequent to YVMV infection, the additional band EST 4 (Rm value 0.632) observed in NL was thick and the specific activity of this particular isoform might also be responsible for the resistance mechanism.
Consequent to virus infection, the total system of esterase was affected, while peroxidase was comparatively stable. The resistant expression of NL against PMV infection can be attributed to the expression of the additional band PRX5 (0.463) and the resistant mechanism against YVMV in NL as well as in the F1 hybrids can be due to the activity of the isoform PRX4 (0.392).
Sequential inoculation with PMV followed by YVMV was carried out in 25 seedlings each in the F1, F2, B1 and B2 generations of the four cross combinations namely, Amb x NL, PV x NL, AC x NL, and Co1 x NL to identify plants with combined resistance to both the viruses.
In the F1s, eight plants were identified as resistant to both the viruses i.e., one in Amb x NL, two in PV x NL, three in AC x NL and two in Co1 x NL. In the F2, B1 and B2 populations, there were 17, 9 and 37 seedlings respectively, resistant to both the viruses.
All the 71 seedlings possessing combined resistance to PMV and YVMV were further evaluated in the field for their biometric and horticultural characters. The studies in general indicated earliness of F1s compared to F2s, B1s and B2s. On an average, the fruit yield in terms of number and weight of fruits in all the crosses was maximum for the F1 plants.
In all the crosses, the plants belonging to the F1, F2, B1 and B2 generations produced fruits with external fruit colour as well as flesh colour of either the female parent or the male parent or with a blend of these two colours.
Based on resistance to PMV and YVMV, fruit smoothness, non-incidence of other mosaic viruses and yield of fruits, eight most promising superior segregants from among the four cross combinations were selected for further improvement. They included four in Amb x NL, two from Co1 x NL and one each from PV x NL and AC x NL.