Cucurbit Genetics Cooperative Report 1:6-8 (article 5) 1978
An Interspecific Cross Between Cucumber (Cucumis sativus) and Muskmelon (Cucumis melo)
B. J. van der Knaap and A. C. de Ruiter
Deruiterzonen B. V., Bleiswijk, The Netherlands
In 1971, we started a program for making an interspecific cross between a cucumber and melon. If such a cross would succeed, it would enable us to exchange certain genes between the cucumber and the melon, especially resistance to powdery mildew, Sphaerotheca fuliginea. Although there are many sources of powdery mildew resistance found within the cucumber species, C. sativus, this resistance is generally believed to be based on 3 recessive genes, while the resistance of the melon species is based on 1 dominant gene. It is obvious that it would be much more convenient to work with the monogenic dominant resistance from the melon.
The pickle variety used as the female parent was susceptible to powdery mildew, and the male parent was a powdery mildew resistant breeding line of muskmelon. During a four week period in the greenhouse; we made 1,000 pollinations in 26 different combinations, which can be divided into 3 groups: -we varied the ages of both female and male flowers from 2 days before till two days after anthesis; -we used tetraploid and diploid cucumber lines both as female or male parent; -we pollinated using pollen mixtures containing 50% pollen of the male and 50% pollen of the female parent.
It has been the last group which yielded the hybrid plants. The seeds we have collected from certain mixed pollinations (C. sativus X (C. sativus + C. melo)) were plump and resulted in approximately 2% hybrid plants after planting.
The F1 hybrid plants raised from the seeds of the mixed pollinations were quite different from both the parent lines. The shape of the leaves was more or less similar to the muskmelon and the color of the leaves resembled that of the pickling cucumber. The fruits were intermediate and looked like a pear. As far as the vitality is concerned the F1 plants showed a completely different habit. The growth was very poor and the plants did not get taller than 100 to 120 cm, where as the growing- point was very small with extremely short internodes, like a rosette. We decided to call these F1 plants "MEGURK", derived from the Dutch words meloen (melon) and augurk (pickling cucumber).
The majority of the megurk plants were found to have 19 chromosomes. All of the megurk plants were fertile and produced viable seed when selfed. The F2 progeny were phenotypically similar to the megurk F1 plants, and the only segregation which has been observed was for the number of chromosomes, varying from 14 to 19. No segregation was found in either the F3 or F4.
All backcrosses made to the muskmelon failed to set fruit, whereas those made to the pickling cucumber using the megurk as the female parent, yielded fruits with plump seeds. The BC1 to the cucumber consisted entirely of cucumber phenotypes. After selfing the BC1 plants a segregation of 243 cucumber and 44 megurk phenotypes were found, all had 14 chromosomes.
We can not explain the lack of segregation for phenotype in the F2, F3, and F4. Because of the high degree of fertility of the F1 megurk plants it is reasonable to assume that the cucumber and melon chromosomes are closely related.
Although the male muskmelon parent of the original cross was monogenic dominant mildew resistant, the F1 plants all were very susceptible. This means that the genes for susceptibility of the cucumber are more dominant in comparison with the melon gene for resistance. The F2, F3, and F4 progenies showed some resistance to powdery mildew, but none were completely resistant.
In the various backcross progenies of megurk to cucumber, we hope to get a translocation of the melon chromosome segment carrying the gene for mildew resistance into a cucumber chromosome. In order to enlarge the chance of the occurrence of this desired recombination we set up a program covering 3 different methods:
A. Colchicine treatment of megurk seeds in order to get an amphidiploid genotype which might enable us to make backcrosses to melon (if necessary to tetraploid melon). There is a chance that the dominant Pm gene from muskmelon would be expressed if present twice in the amphidiploid. We have not succeeded in getting any recombination in this way.
B. Irradiation of the megurk seed. These irradiations took place at the I.T.A.L. at Wageningen in 1975. The doses varied from O to 10 krad Nf (fast neutrons). In the M1 generation we have found one off-type plant, which resembled the cucumber parent as far as the growth and plant habit is concerned. After crossing this plant to megurk followed by selfing, we found in the F2 generation: -cucumber type - megurk plants, -megurk plants, unchanged, -plants of a type in between the cucumber-type megurk and the standard megurk.
The cucumber-type megurk plants had much longer internodes than the standard megurks, were very susceptible to damping off, and had long tendrils. For several months they formed only male flowers. Later, they formed hermaphroditic flowers, which were superior and whose carpels had not joined, so the nuclei were easy to see. After this transient period they formed female flowers. Unfortunately, the plants that survived were so old that the pollinated flowers aborted and the plants died.
Spraying the cucumber-type megurk plants in a younger stage with ethephon has not stimulated early production of female flowers. These plants probably have a higher internal level of gibberellic acid, though we did not check it. Chromosome counts of the cucumber-type megurk plants have failed. The so called in-between-type plants were not susceptible to damping-off, were monoecious and had rudimentary tendrils.
C. Continued irradiation of flowering megurk plants. In 1975 we placed nine groups of flowering megurk plants on different distances from 137Cs-y source. The distances varied from 1 to 9 m, which corresponded with dose rates varying from 910-956 to 16-17 rad per day. The Cs source was operating for 20 or 21 hours per day. The necessary pollinations have been done during the same period of irradiation. The M2 and M3 generations of the treated plants yielded some promising genotypes: We found one progeny with very long (115 cm) hypocotyls, one progeny with distinct larger leaves and one progeny that contained one plant with different leaf shape, called sunflower leaf-type. Unfortunately the latter died before yielding any viable seeds.
Although we have tried to repeat the original cross several times under the same circumstances and with the same material, we never succeeded in establishing this cross. Obviously the success of the 1971 cross has been a lucky shot. Because we do not have too much seed of the different progenies mentioned earlier, we must note that there is no seed available.
Our research program for the future is: - Screening the various new types of Megurk plants for resistance to powdery mildew; inbreeding of all new types of Megurk plants; - Crossing the various new types with each other and also with the pickle and melon parent.