Cucurbit Genetics Cooperative Report 2:17-18 (article 11) 1979
Isozyme Analysis of the Megurk
R. W. Robinson, J. T. Puchalski and A. C. de Ruiter
York State Agricultural Experiment Station, Geneva, NY 14456
and DeRuiter Zonen, The Netherlands
A possible cross between the cucumber and muskmelon has been named the "megurk", after the Dutch words for melon and cucumber (1). The high self fertility of the F1 megurk plants, their complete compatibility with cucumber but not with muskmelon, the lack of segregation in the F2 and subsequent generations, the lack of segregants with the dominant gene for powdery mildew resistance gene that was possessed by the muskmelon parent, and the considerable differences between sativus and melo that indicate these species are not closely related--all these considerations raised questions as to the authenticity of this cross.
We used comparative electrophoretic analysis of isozymes; to resolve this question. Cucumis sativus is considerably different from melo in every enzyme system studied (2). If the megurk really has both melo and sativus genes, this would be expected to be reflected in its isozymes.
Megurk plants and the parental cucumber and muskmelon lines were grown in the field at Geneva, NY in 1978. The megurk plants did show an outward resemblance to both the cucumber and the muskmelon parent. They were similar in disease resistance to the cucumber parent, resistant to cucumber mosaic virus and very susceptible to powdery mildew whereas the muskmelon parent was susceptible to CMV but resistant to powdery mildew. The megurk had shorter internodes than the cucumber parent, earlier maturity, rounded leaves somewhat like those of a muskmelon plant, and short, wide fruit intermediate in shape to cucumber and muskmelon fruit.
The megurk crossed readily with cucumber but not with muskmelon. All crosses were made with the megurk as the maternal parent, since it was gynoecious.
Extracts from young leaves of the megurk and its putative parents were compared for esterase and peroxidase isozymes. In every case, the isozymes of the megurk plants were identical to the cucumber and quite distinct from the muskmelon plants. Accordingly, it is concluded that the megurk is entirely C. sativus, and is not derived from an interspecific cross.
Conclusive evidence concerning the origin of the megurk is not yet available, but it has all the earmarks of being a mutant conditioned by a single recessive gene. It is not more unlike the cucumber than different cucumber mutants that one of us (RWR) has induced with radiation and chemicals.
The ratios of megurk to normal plants that have been reported (1) are not in agreement with Mendelian monogenic ratios, yet they do not entirely conflict with a single recessive gene hypothesis. The original cross, in which a mixture of pollen from several cucumber and muskmelon plants was applied to the stigma of the cucumber, resulted in 2% megurk plants. If we may assume that the muskmelon pollen was not functional in this cross and only one of the several cucumber plants was heterozygous for a spontaneous mutation, the proportion of megurk plants in the next generation would depend on the proportion of self and sib pollination. If the heterozygous cucumber plant was pollinated with an equal mixture of its own pollen and that of 10 normal cucumber plants, approximately 2% of the progeny should be mutants. The failure to obtain any megurk plants when the experiment was repeated can be explained on the basis of none of the cucumber plants in the second experiment being heterozygous for the gene, which is probably very rare in occurrence.
The report (1) that the Fl of the cross of the megurk and cucumber was entirely normal is consistent with the megurk being a recessive mutant. The proportion of 243 normal cucumbers to 44 megurks in the F2 of the cross does not agree with a 3:1 ratio, and more evidence is needed to fully understand the genetic basis of the interesting megurk.
- Van der Knapp, B. J. and A. C. de Ruiter. 1978. An interspecific cross between cucumber (Cucumis sativus) and muskmelon (Cucumis melo). Cucurbit Genetics Coop. Rpt. 1:6-8.
- Puchalski, J. T., R. W. Robinson, and J. W. Shail. 1978. Comparative electrophoresis of isozymes of Cucumis species. Cucurbit Genetics Coop. Rpt. 1:39- 40.