Relationship between Gene B and Gene Ses-B in Cucurbita pepo L.

Cucurbit Genetics Cooperative Report 12:79-81 (article 33) 1989

Oved Shifriss
21 Walter Avenue, Highland Park, NJ 08904

Gene B conditions precocious depletion of chlorophyll. And the loss of chlorophyll is often associated with precocious yellow pigmentation. The primary target of B is the fruit (2). But B can also affect other potentially photosynthetic organs, depending on the genetic background and the environment. The “genetic background” is represented by specific nuclear elements. For example, some environmental conditions, the presence of gene Ses-B+ allows the expression of B in leaf blades early in plant development. In contrast, gene Ses-B selectively suppresses the expression of B in leaf blades under a wide range of environmental conditions (3).

The influence of the environment is illustrated in the following. When seed of ‘Jersey Golden Acorn’ (JGA), B/B Ses-B+/Ses-B+, is sown in May in New Brunswick, NJ, the first true leaves are often completely yellow. Similarly, the first true leaves are often completely yellow when seed is sown late in November under greenhouse conditions in New Brunswick. But when the seed is sown in September in Naples, Florida, the first true leaves are completely green. It is assumed that relatively low temperatures or low light intensities trigger the effect of Ses-B+. However, the precise temperature and light conditions necessary to elicit the Ses-B+ effect have not been determined. Moreover, the role of other non-genetic factors cannot yet be excluded.

There are marked variations in sensitivity of B/B lines to environmentally-induced leaf yellowing, a fact that alludes to a more complex genetic basis for this trait. But even a single B/B line, such as JGA, can manifest leaf yellowing in different ways. Examples: (a) Incomplete penetrance and variable expressivity, based on the phenotype of the first true leaf. (b) 100% penetrance and high expressivity, based on the first true leaf, followed by 1 to 3 partially yellow leaves, and then a switch to completely green leaves. (c) The first 3 to 6 leaves are yellow or partially yellow, followed by a distinct variegated phase in which chlorophyll depletion is largely confined to leaf veins, and then a switch to completely green leaves. (d) A prolonged phase of 10 to 30 yellow or partially yellow leaves followed by a switch to green.

Nevertheless, there is little doubt that in some crosses the inheritance of sensitivity is monogenic. It is speculated that Ses-B+ and Ses-B are special regulators of B. In order to study the physical relationship between B and these regulators by breeding experiments two requirements must be met. First, the parental lines must carry alternative alleles. If JGA is to be used as a B/B parent that carries Ses-B+, it is necessary to find a B+/B+ parent that carries Ses-B/Ses-B. Second, one must find an environment in which JGA predictably manifests 100% penetrance and high expressivity of leaf yellowing. Otherwise, it would be extremely difficult, if not impossible, to critically classify segregating generations.

‘Sweet Dumpling’ (SD), a B+/B+ cultivar, was found to carry a strong Ses-B (Shifriss 1982, unpublished). This finding fulfilled the first of the above two requirements. As a result, seed was produced of F₁, BC₁ and F₂, using JGA and SD as parents. At the same time, attempts were made (through the use of growth chambers as well as greenhouse and field facilities) to find an environment that elicits the full effect of Ses-B+. These attempts were largely unsuccessful. In four experiments, JGA manifested incomplete penetrance and variable expressivity of leaf yellowing.

But there was one limited test in which JGA manifested 100% penetrance and high expressivity of leaf yellowing. This test included the parents and the F₂. The F₁ and the BC₁ seedlings were lost by accident. The seed was sown in November of 1983 and the plants were grown for five months under uncontrolled greenhouse conditions at Rutgers University in New Brunswick. In a subsequent sowing, the parents and the F₁ were grown during the summer of 1984, and their fruits are illustrated in Figure 1.

The data are presented in Tables 1, 2, and 3. The key for grades of yellowing in the first true leaves (Table 1) is as follows: 1 = completely green or green with 1 to 2 tiny yellow spots; 5 = yellowing extends over 3/4 of the leaf surface, and 2 to 4 = intermediate grades between 1 and 5. Yellowing appears to reflect a diffused phenomenon rather than an extension of spotting. It is well established that B+/B+ plants can exhibit yellow spotting under some environmental conditions (1).

The data in Tables 1, 2, and 3 suggest that B and Ses-B are non-linked.

Table 1. Limited data on the inheritance of precocious yellow pigmentation.

Breeding materials	Number of seedlings that exhibited different grades of yellowing in the first true leaf
	1	2	3	4	1-4	5	Total	X² (13:3)z	P
P1, JGA	0	0	0	0	0	10	10	—	—
P2 SD	10	0	0	0	10	0	10	—	—
F2	90	2	36	36	164	32	198	0.32	0.50-0.75

^z Testing 164:34

Table 2. Classification of the 90 plants (grade 1, table 1) based on fruit color at later stages of development.

Number of plants that produced:
green fruits	bicolor fruits	yellow fruits	Total	X² (4:2:1)	P
53	232	14	90	0.44	0.75-0.90

Table 3. Classification of the entire F2 based on data in Tables 1 and 2.

Number of seedlings of:
grades 1 to 4 that at later stages produced bicolor or yellow fruits	grade 5 that at later stages produced bicolor or yellow fruits	grade 1 that at later stages produced green fruits exclusively	Total	X² (9:3:4)	P
111z	34	53	198	0.51	0.75-0.90

^z This number was obtained by subtracting 87 (34 + 53) from 198.

Figure 1. Upper left, SD; upper right, JGA; bottom, F1

 

Literature Cited

1. Shifriss, O. 1965. The unpredictable gourds. Amer. Hort. Mag. 44-184-201.
2. Shifriss, O. 1981. Origin, expression and significance of gene B in Cucurbita pepo L. J. Amer. Hort. Sc. 106:220-232.
3. Shifriss, O. 1982. Identification of a selective suppressor gene in Cucurbita pepo L. HortScience 17:637-638.