An unusual, partially-fertile population of diploid Vitis vinifera x V. rotundifolia F1 hybrids is described. These hybrids are partially fertile with varieties of both parental species.It is suggested that the success of hybridization, as measured by the fertility of the F1 hybrids, depends on the V. vinifera clone originally used as a female parent.In meiosis of these F1 plants, average bivalent formation varies from 13.1 to 15.6 per cell at MI. There is a correlation in the diploid VR hybrids between chromosomal pairing at MI and fertility of the vines.Some V. rotundifolia characters are dominant in diploid F1 hybrids.In regard to crossability, the F1 hybrids are reciprocally crossable with V. vinifera varieties, but can only serve as female parents with V. rotundifolia. Evidence from breeding tests indicates that incompatibility between V. rotundifolia x V. vinifera is not due to cytoplasmic inheritance, but is caused by nuclear factors
