Cucurbit Genetics Cooperative Report 17:120-121 (article 35) 1994
On the Selective Activator of Gene B2 in C. moschata
21 Walter Avenue, Highland Park, NJ 08904
Gene B2 originated as mutation in C. maxima (1,2). This gene conditions precocious depletion of chlorophyll in fruits of all known genetic backgrounds. But in some backgrounds, B2 is also expressed in one or more other aerial organs. Results of breeding experiments suggested the existence in C. maxima or nuclear elements that selectively activate or selectively suppress the expression of B2 in aerial organs other than fruit.
This report provides evidence for the existence of a selective activator or selective activators of B2 in C moschata, a species in which B2 has not been reported to occur naturally. The evidence is based on results obtained from two interspecific crosses of C. maxima x C. moschata involving different cultivars. In each case, the cultivar of C. moschata was used as a pollen parent in the initial crossing as well as the recurrent pollen parent in repeated back-crosses.
The first cross involved PI 165558, B2 B2, of C. maxima and 'Waltham Butternut', B2+ B2+ , of C. moschata. Both the initial cross and all the backcrosses were made by A.M. Rhodes (personal communication). The present writer merely extracted a B2 B2 inbred of C, moschata from one of Rhodes' sixth backcross progenies. This new B2 B2 inbred is known as the IL-B breeding line (2). In his transfer of B2 from C. maxima to C. moschata, Rhodes employed a special "bridge" the nature of which will be described elsewhere.
PI 165558 is an introduction from India. In this accession B2 is expressed in both fruits and stems. However, unlike the uniformly golden pigmentation and high phenotypic stability of B2 expression in fruits, the golden pigmentation in stems is subject to phenotypic plasticity, i.e. under varying field conditions the stems often exhibit a mosaic phenotype, being predominantly golden but with some green patches or stripes (3).
In IL-B, the expression of B2 is consistently stable in both fruits and stems. When compared to PI 165558 stems, the IL-B stems are particularly striking in their uniformly golden pigmentation and high phenotypic stability under varying field conditions. What then is the cause for the high phenotypic stability of B2 expression in IL-B stems?
The second interspecific cross was made in part as an attempt to find an answer to the above question. This cross involved 'Pink Banana', B2 B2, of C. maxima and 'Chirimen', B2+ B2+, of C. moschata. 'Pink Banana' was represented by an advanced inbred, PB#1. In this inbred, the fruits are precociously pigmented and pink in color, but the stems are persistently green. 'Chirimen (4) was represented by an inbred obtained from Theodore H. Superak. When used as pollen parent, 'Chirimen' is known to be cross-compatible with C. maxima. The F1 is vigorous, self-sterile but moderately cross-fertile to "Chirimen' as pollen parent. The F1 plants bear precociously pigmented golden fruits. The stems of these F1 plants are predominantly but not persistently green, i.e. under some field conditions a portion of one of the stems may undergo chlorophyll depletion and appear golden. Field rooting of rare golden shoots and their subsequent growth in a warm greenhouse led to F1 plants that produced precociously pigmented fruits and persistently green stems.
A sample of 159 BC1 plants was observed for a period of three months. Over 90% of this sample manifested different types of developmental abnormalities, including leaf distortion and varying degrees of stunting. However, with the passage of time, 82 plants recovered sufficiently to allow decisive classification of both fruits and stems. Of these 82 plants, 62 had precociously-pigmented fruits and stems, and 20 had precociously pigmented fruits and persistently green stems. In addition, there were 29 plants that appeared to have persistently green fruits and green stems, but the classification of some of them was not entirely certain. Of the remaining 48 plants, 13 died prematurely and 35 were too stunted and abnormal to permit and abnormal to permit adequate classification.
Two additional backcrosses and some selfing resulted in the isolation of two distinct breeding lines: one bearing precociously-pigmented golden fruits and uniformly golden stems, and the other bearing precociously-pigmented golden fruits and persistently green stems. Although the level of fertility of these lines remains to be determined, it is clear that 'Chirimen' carries a genetic material (one or more nuclear elements) capable of strongly activating B2 in stems. Furthermore, one cannot exclude the possibility that the strong selective activator ofB2 in IL-B was derived from 'Waltham Butternut' rather than from PI 165558. Is the strong selective activator of B2 widely spread in C, moschata? What function, if any, does it have in a B2+ B2+ species?
- Shifriss, O. 1990. Developmental aspects of the B genes in Cucurbita. In: Bates, D.M., R.W. Robinson and C. Jeffrey, eds. Biology and Utilization of the Cucurbitaceae. Cornell University, Ithica, NY.
- Shifriss, O. 1991. The two B genes in Cucurbita are unlinked. Cucurbit Genetics Coop. Rept. 14:116-122.
- Shifriss, O. 1965. The unpredictable gourds. Amer. Hort. Mag. 44-184-210.
- Tapley, W.T., W.D. Enzie, and G.P. Van Enseltine. 1937. The Vegetables of New York. Vol. 1, part 4. The Cucurbits. N.Y. Agr. Exp. Sta., Geneva, NY.
Acknowledgements: I thank Rogers NK Seed Co. for enabling me to conduct this study.