Origin, Expression, and Significance of Gene B in Cucurbita pepo L.1

Abstract

Two systems of fruit pigmentation exist in Cucurbita pepo . In the standard B + -system, the onset of color differentiation (orange, yellow, cream, or white) occurs after the “bud phase.” In the precocious B -system, the onset of color occurs during the bud phase. The source of the precocious system is a group of bicolor-fruited cultivars of the ornamental gourds. The yellow portion of a bicolor fruit is precociously pigmented, but the extent of precocious pigmentation may vary greatly even among fruits borne on the same plant. Starting with a single bicolor-fruited plant of the ‘Pear’ gourd, a program of self-reproduction and selection was initiated in an attempt to establish true-breeding lines which differ in extent of precocious fruit pigmentation. Many plants of successive inbred generations proved to be unstable genetically or phenotypically or both. But 2 genetically stable plants of known breeding history were identified as BB and B w B w ( B w for a “weak” allele). Inbred BB produces completely yellow fruits in one environment and somewhat variable fruits in another environment. Inbred B w B w produces fruits which exhibit a wide range of variation in extent of precocious pigmentation in different environments. Substitution of B for B + in several edible cultivars has revealed the following facts: 1) BB lines of some backgrounds produce completely yellow fruits in diverse environments. 2) BB + heterozygotes of different backgrounds differ from one another in extent of precocious fruit pigmentation and in the range of fruit variation on the same plant. 3) B is stable in some backgrounds and unstable in other backgrounds. 4) B can exert “secondary” effects on plant growth, sexuality, and fruit quality, some of which are detrimental and others, beneficial. 5) The secondary effects are separable in breeding experiments. It is suggested that the initial source of B and its precocious alleles is genetic instability which occurs spontaneously in some stocks during gametogenesis; that the extent of precocious fruit pigmentation depends on time at which B (or its allele) blocks chlorophyll synthesis during the bud phase; that the determination of this time is affected by the “strength” of the B alleles present, their dosage, modifier genes, and non-genetic factors; that the secondary effects of B result from different interactions between B and other genes; and that superior precocious cultivars could be developed in which the beneficical effects of B are enhanced and its detrimental effects are suppressed through selection of new gene recombinations.