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 BwBw (Bw for a “weak” allele). Inbred BB produces completely yellow fruits in one environment and somewhat variable fruits in another environment. Inbred BwBw 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.

Identification of Modifier Genes Affecting the Extent of Precocious Fruit Pigmentation in Cucurbita pepo L.1

“…Materials. The sources and characteristics of the 2 inbred parents -'Table King' (B*/B+, green fruits) and 'Precocious Small Sugar' (BIB, yellow fruits) -have been described previously (4).…”

“…bicolor-fruited plants. This group defied precise classification into subgroups because it involved not only quantitative variation between plants but also quantitative variation among fruits borne on the same plant (3,4). Moreover, even the group of yellow-fruited segregates was not homogeneous.…”

“…1). This is a slightly modified and expanded scale of that used previously (3,4). The phenotype of each plant was measured by the mean color grade of 5 to 10 fruits.…”

“…yFor the theoretical basis of these and other tri-hybrid ratios see Figs. 4 and 5 and the text under Results and Discussion.…”

“…Previous studies have shown that precocious fruit pigmentation is conditioned by gene B and that the extent of this pigmentation depends on the strength of the precocious alleles present, their dosage, the genetic background, and non-genetic factors (3,4). The evidence for the background effect suggested that modifier genes influence the expression of B, but these hypothetical modifiers have not been identified before.…”