CONGRUITY BACKCROSSING AS A MEANS OF CREATING GENETIC-VARIABILITY IN SELF POLLINATED CROPS - SEED MORPHOLOGY OF PHASEOLUS-VULGARIS L AND PHASEOLUS-ACUTIFOLIUS GRAY,A. HYBRIDS

Seeds of early generations of three reciprocal congruity-backcross (CB C) pedigrees, developed by backcrossing Phaseolus vulgaris - I? acutif olius hybrids to each of the parent species in alternate generations, exhibited a preponderance of traits (size, shape, color, and pattern) of the cytoplasmic parent. The large size of 'Red Cloud' (V-1), the P. vulgaris parent common to all of the pedigrees, dominated pedigrees w ith V-1 as the cytoplasmic parent, while the small size and rounded or square shapes of the tepary parents, wild P. acutifolius var. acutifo lius PI 263590 or G400445 (A(19)), wild P. acutifolius var. latifolius PI 406622 (A(10)), or cultivated P. acutifolius var. latifolius 'Sero wi' PI 319443 (A(9)), were the majority phenotypes when P. acutifolius was the cytoplasmic parent. Continuing through the second cycle of CB C, that is the second backcross with each of the parent species or the fourth backcross, began an amelioration of the apparent cytoplasmic e ffect on gene expression, as reciprocal pedigrees became more alike, u sually with intermediate expression of parental traits or the appearan ce of new traits. The large seed size of V-1 was recovered in hybrids with P. acutifolius cytoplasm and the kidney shape of V-1 became rare in hybrids with P. vulgaris cytoplasm. Although the tepary-bean parent s represented two subspecies and both cultivated and wild P. acutifoli us, the three sets of reciprocal-hybrid pedigrees with P. vulgaris 'Re d Cloud' are surprisingly similar. It may be that the 'exotic' parent used to develop a CBC pedigree should be selected more for combining a bility in the interspecific cross than for specific economic traits. W hile the number of generations (six or more) required to produce ferti le, intermediate CBC hybrids (that did not require embryo rescue) may preclude routine use of this method by practical plant breeders, the c rossability of advanced hybrids with both parental species and the amo unt of variability apparent in advanced-hybrids progenies suggests tha t CBC would be valuable for maintaining 'exotic' germplasm in immediat ely useful forms.