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Cucurbit Genetics Cooperative Report 14:66-68 (article 25) 1991

Techniques to Overcome Barrier of Interspecific Hybridization in Cucumis

M. Chatterjee and T. A. More

Division of Vegetable Crops, Indian Agricultural Research Institute, New Delhi, India 110012

Our efforts to achieve the interspecific hybrid in Cucumis species through conventional breeding procedures were unsuccessful becaue of the existence of a pre-fertilization barrier. The barrier was characterized by non-germination of pollen even up to 72 h after pollination. To overcome this problem, techniques like bud pollination, use of mentor pollen, brush pollination, growth regulator application, and streatment of stigma with organic solvent, suggested in other crop plants and in Cucumis were employed to find out their potential to overcome the barriers of interspecific hybridization.

Studies on receptivity of bud of C. figarei indicated that the stigma of C. figarei is receiptive 24 h before the anthesis. Hence C. figarei x C melo (M4), C. zeyheri x C. melo (M4), C. meeussii x C. melo (M4), C. meeusii x C. melo (M4) and their reciprocal crosses were attempted at bud stage (24 h prior to anthesis). However, this technique was ineffective in overcoming the barriers. Brush pollination (scratching of stigma before pollination) was also ineffective. On the other hand, techniques like mentor pollination as suggested by Oost and den Nijs, 1979 in Cucumis, application of growth regulator, and treatment of stigma with organic solvent proved to be useful to obtain fruit set in interspecific crosses. Mentor pollen was prepared by exposing the C. figarei pollen for 5-6 h in sunlight and then storing them for 24 h at room temperature. The dried anthers were thenput on a filter paper and the mass of pollen was broken up with spatula, after which these were transferred into a glass vial. Pollinations of pre-bagged flowers were done with (a) compatible pollen (self), (b) mentor pollen only, and (c) mentor pollen + compatible pollen (C. melo [PM]). The viable pollen of C. melo was mixed with mentor pollen in approximate proportions of 2 parts mentor to 1 part viable. When C. figarei was pollinated with copatible pollen of C. figarei, 30% fruit set was obtained, when C. figarei was pollinated with mentor pollen only, no fruit set was obtained thereby indicating that the mentor pollen was completely non-viable. When C. figarei was pollinated with mentor pollen + compatible pollen (PM), i.e., C. figarei x (C. figarei* + C. melo [PM]), 8% fruit set was obtained (Table 1).

Application of benzyl adenine (1%) to the base of ovaries after pollination also proved to be successful in securing the cross C. figarei x C. melo (PM).BA (1%) in lanolin-water paste (7:3 w/v) was prepared and applied to the base of ovaries with a disposable tuberculin syringe (minus needle) based on the method described by Loy (1982). Four per cent fruit set was obtained in thie cross, the fruit was small in size and the pedicel was thick at the point of its attachment. The fruit was harvested 40 days after pollination and it contained 85 seeds. In the reciprocal cross, BA application resulted in fruit set, but the fruits were abnormal in shape and they collapsed after 10 days of pollination. At this stage, the fruits were devoid of seed showing the parthenocarpic development of these fruits. In the cross of C. m,eeussi x C. melo (PM), three fruits were set when BA was applied at the base of the ovary prior to pollination. However, only one fruit reached maturity. It was completely normal with respect to fruit shape and size. It was harvested after 40 days and it contained 10 seeds. Six seeds were abnormal in shape, twisted and were empty and the remaining four seeds were normally developed and well filled. The other two fruits collapsed after 12 days of pollination and no seeds were formed in them, indicating that they were developed parthenocarpically. In the reciprocal cross, no fruit set was obtained, probably due to the presence of unilateral incompatibility.

In the cross C. figarei x C. melo (M4) 2% fruit set was obtained when pollination was done after washing the stigma with organic solvent (n-hexane) as per the method described by Willing and Pryor (1976) in Populus.

It is inferred that it is possible to obtain interspecific hybrid(s) by employing techniques like mentor pollination, growth regulator (BA) and use of organic solvent (n-hexant) in the crosses mentioned above. However, confirmation that these actually were interspecific hybrids is needed.

Table 1. Per cent fruit set obtained in interspecific cross in Cucumis through various techniques.

Experiments
Pollen Sourcez
No. of flowers pollinated
No. of fruits obtained
% Fruit set
No. of fruits attaining maturity
Average no. of seeds per fruit
A. MENTOR POLLEN
Crosses:
C. figarei
C. figarei
C. figarei x (C. figarei* + C. melo (PM)
B. GROWTH REGULATOR (BA 1%)y
Crosses:
C. figarei x C. melo (PM)
-
25
1
4
1
85
C. melo (PM) x C. figarei
-
50
3
6
0x
0
C. meeusii x C. melo (PM)
-
25
3
12
1w
10
C. ORGANIC SOLVENT (n-hexane)y
Crosses:
C. figarei x C. melo (PM)
-
50
4
8
4
250
C. figarei x C. melo (M4)
-
100
2
2
2
200

zS.P.: Self pollen, M.P. : Mentor pollen, M.P. + C.P. : Mentor pollen + compatible pollen (C. melo [PM]), C. figarei*: Pollen made non-viable by storing for 24 h (M.P.).
yPM: Pusa Madhuras, M4: Monoecious 4.
xA: 6 per cent fruits were obtained, however, all of them developed parthenocarpically for up to 10 days after pollination and then abscised.
wB: Remaining 2 fruis developed parthenocarpically for up to 12 days and then collapsed.  

Literature Cited

  1. Loy, J.B. 1982. Comparison of the effectiveness of BA and AVG in promoting fruit set in muskmelon. Cucurbit Genetics Cooperative 5:23.
  2. Oost, E.H. and A.P.M. den Nijs. 1979. Mentor pollen a tool in interspecific hybridization in Cucumis. Cucurbit Genetics Cooperative 2:43-44.
  3. Willing, R.R. and R.D. Pryor. 1976. Interspecific hybridization in Poplar. Theor. Appl. Genet. 17:141-151.
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