Cucurbit Genetics Cooperative Report 1:31-32 (article 29) 1978
Gynoecious Sex Expression in Cucurbita Resulting from an Interspecific Cross
R. W. Robinson, M. A. Boettger, and J. W. Shail
New York Agricultural Experiment Station, Geneva, NY 14456
The 'Buttercup' and 'Golden Delicious' cultivars of Cucurbita maxima were crossed with the 'Butternut' and 'Waltham Butternut' cultivars of C. moschata. Each parent was monoecious, producing many more staminate than pistillate flowers. Each of the four interspecific hybrids however, was highly female in sex expression. In one year the hybrids were predominantly female, developing many pistillate but also a few staminate flowers. In other years, the hybrids were gynoecious. No differences were noted in reciprocal crosses.
The buff skin color of mature fruit of 'Butternut' and 'Waltham Butternut' was recessive to the green skin of 'Buttercup' and to the orange skin of 'Golden Delicious'. The fruit shape of the hybrids was intermediate to the parents, and the blossom end protrusion of 'Buttercup' fruit was recessive. The fleshy peduncle of the C. maxima parents was also recessive. The deeper orange flesh color of the C. maxima parents was dominant to the lighter flesh color of the C. moschata parents. The C. maxima parents are more tolerant to powdery mildew than the C. moschata parents, and this was a dominant trait. The interspecific hybrids were resistant to the squash vine borer, similar to the C. moschata parents. The interspecific hybrids were very attractive, early in maturity, very high yielding, and exceptionally good in quality.
The desirable features of the interspecific hybrids stimulated interest in developing derived breeding lines, but the lack of staminate flowers restricted selfing the hybrids or using them as pollen parents. Attempts were therefore made to induce staminate flower development by application of gibberellic acid, GA 4/7, at the rate of 50, 100, or 200 ppm, was applied in the first true leaf stage. Other plots received a second application of the same concentration a week later, and three weekly applications of each of the three concentrations were applied to other plots. Each of the GA treatments was successful, stimulating the development of staminate flowers. Pistillate flower formation was reduced by each GA treatment., but not to the extent to interfere with fruit production; the number of pistillate flowers per plant for a 22-day period ranged from 29.0 for untreated hybrids to l5.1 for plants given 3 applications of 200 ppm, and there were no significant differences in fruit production among the treatments. Although the GA treatments increased staminate flower and reduced pistillate flower formation, all treated plants were still predominantly female in sex expression.
The interspecific hybrids were highly sterile. Normal appearing staminate flowers of the hybrids had reduced amounts of pollen, and the pollen of the hybrids averaged only 7.7% stainable with acetocarmine in contrast to over 90% for the parents. There were no significant differences among the GA treatments and the check for pollen stainability. The hybrids produced a high proportion of abnormal staminate flowers. The petals and sepals of these flowers were normal in appearance, but the androecium was reduced in size or absent. Pollen produced by these flowers was scanty and even less fertile than that of normal appearing flowers, averaging only 1.8% stainable. The hybrids had a high degree of female as well as male sterility, as indicated by the poor seed production in reciprocal backcrosses between the hybrids and the parents.
Good fruit set was obtained from self- and sib-pollinations of the interspecific hybrids, but viable F2 seed was not produced. Several months storage of self-pollinated F1 fruit before seed extraction, to permit further development of the seed, did not improve seed viability. Gibberellic acid treatments did not affect fertility of the F1 plants. Viable seed was produced by the hybrids in open-pollination and by backcrosses to 'Buttercup', and the amounts were low and the seeds were very light and flat. These seeds had very poor endosperm development, and germinated better when cultured on White's media with 1 ppm IAA, than when planted on germination blotters.