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Cucurbit Genetics Cooperative Report 14:113-115 (article 40) 1991

Expression of the B Genes in Cucurbita.

Oved Shifriss

21 Walter Avenue, Highlnd Park, NJ 08904

B1 vs B2. Table 1 shows the effect of the two B genes on different plant organs. B1B1 stands for B1B1B2+ B2+ in tests 1 to 6, and B2B2 stands for stands for B1+B1+B2B2 in tests 7 to 11. It is not certain yet whether NA159 (test 12), a derivative of NJ-B x IL-B is B1+B1+B2B2 or B1B1B2B2 . Inbreds similar in their phenotype to NA159 were obtained from crosses of B1+B1+B2B2 (e.g., PI 1165558) x B1+B1+B2+B2+ (e.g., 'Buttercup') and these are presumably B1+B1+B2B2 .

JGA (tests 1 & 2) carries a modifier gene, Ses - B1+, that enables B1 to "turn on" in leaf blades in response to relatively low temperature (below 15 ˚ C). In contrast, JGA, II carries Ses - B1 that selectively suppresses the action of B1 in leaf blades.Circumstantial evidence suggests that a similar mechanism, includig "selective activators" and "selective suppressors" is responsible for the regulation of B2 in stems. But the regulators of B2 in stems have not been identified. Furthermore, the control mechanism in this case may be more complicated, because the expression of B2 in stem,s can be intense and stable evn at very high temperatures, as shown by theperformance of IL-B (tests 10 & 11). Unlike NA159 (test 12) inbred PI-165558 (ttests 7 & 8) is vigorous and very productive. This is largely due to the fact that the expressivity of B2 is low (or incomplete) and fluctuating in organs other than the ovary and pedicel.

Phenotypic variation among B2B2+heterozygotes. The data in Table 2 shows that heterozygotes can exhibit eithera relativelynarrow or a relatively wide range of phenotypic variations. Thus, the expression of B2 in fruits and stems is incompletely dominant in heterozygotes of some crosses (tests 3 & 4). In heterozygotes of other backgrounds, the expression of B2 ranges from incomplete dominance in both fruits and stems to incomplete dominance in fruits but complete recessiveness in stems (tests 5 to 9). The progeny of 807-34 (test 5) gives an excellent fit to a 1:2:1 ratio when all heterozygotes are grouped together (22 B2B2: 56 B2B2+ : X2 = 0.4; P = 0.80-0.90). Moreover, selection of heterozygotes for a very low level of expressivity is effective (test 9). Note also that all the 53 (22 + 6 + 7 + 8 + 10) B2B2 segregates in progenies of tests 5 to 9 exhibited a lower level of expressivity of B2 than IL-B (test 1), the original B2B2 parent. The lower expression of B2 in these 53 individuals was manifested by their green petioles.

Expressivity of B2 is influenced by the environment, but the genetic background is a major factor, implying that modifiers are at work.

The basis for the excess of green-fruited plants in progenies of tests 7 & 8 is not known. However, the pooled data from progenies of tests 5 to 9 give a fairly good fit to 3:1 ratio (202:53; deviation X2 = 2.42; P = 0.10-0.20; heterogenity X2 = 0.69; P = 0.95-0.98.

The above data support R.A. Fisher's hypothesis on the role of modifier genes in the evolution of dominance as do the data of several previous workers. For review of evolution of dominance see Chapter 8 in P.M. Sheppard. 1960. Natural Selection and Heredity. Harper Torchbooks (a 1959 revision of the original 1958 edition.).

Table 1. Effect of B1 and B2 on different plant organs in Cucurbita. Relationship of effect to genetic background and environment. Effect - chlorophyll deficiency; + = organ was effected - = organ was not effected.

Test
Species
Inbred
Genotype
Environmentz
Stems
Petioles
Leaf blades
Pedicel
Ovary
Calyx of staminate flowers
1
C. pepo
JGAy
B1B1
Spring, NJ
-
-
+v
+
+
-
2
"
"
"
Fall, FL
-
-
-
+
+
-
3
"
JGA, II
"
Spring, NJ
-
-
-
+
+
-
4
"
"
"
S & F, FL
-
-
-
+
+
-
5
C. moschata
NJ-B
"
Spring, NJ
-
-
-
?
+
-
6
"
"
"
S & F, FL
-
-
-
?
+
-
7
C.maxima
PI-165558x
B2B2
Spring, FL
+
+
+uv
+
+
+
8
"
"
"
Fall, FL
-
+
+u
+
+
+
9
"
NA701
"
S & F, FL
-
-
-
+
+
+
10
C. moschata
IL-B
"
Winter, NJ
+
+
+v
+
+
?
11
"
"
"
S & F, FL
+
+
-
+
+
?
12
"
NA159w
"
S & F, FL
+
+
+
+
+
+

z Spring, NJ = Late May field transplanting in New Brunswick; Winter, NJ = greenhouse culture in New Brunswick
Spring, FL = Early March field transplanting in Naples; Fall, FL = late September field transplanting in Naples..
y 'Jersey Golden Acorn' (JGA) is the source of B1 in inbreds JGA, II and NJ-B.
x PI-165558 is the source of B2 in inbreds NA701, IL-B and NA159.
w The whole-plant phenotype of inbred NA159 is manifested as severe yellowing. The extent of yellowing may be partial or complete, and may be assiciated with lethality or semi-lethality dependiong on local environmental variations.
v Leaf blades exhibit varying degrees of diffused yellowing.
uv Leaf blades often appear as "galaxies' of golden spots or exhibit varying degrees of diffused yellowing.
uLeaf blades often appear as "galaxies" of golden spots.

Table 2. Phenotype and breeding behavior of some B2B2+ heterozygotes in Cucurbita moschata. Naples, Florida.

Parents
Offspring
Testz Pollination of breeding material Phenotypesx Genotypes Growing season Growing season

PDC-UO

PDC-S

PDC-P

GB

PDC-U0

PDC-S

GP

GB

GOT

PDC-SL

GP

GB

GOT

GS

GP

GB

GOT-VL

GS

GP

GB

GO

GS

GP

GB

Number of classified plants
1
IL-B, self NA124, self
PDC-UO, PDC-S, PDC-P, GB
B2B2
Fall, '89
S & F, '90
15
0
0
0
0
0
15
2
F1, IL-B x NA124
GO,GS, GP, GB
B2+B2+
"
"
0
0
0
0
0
25
25
3
BC1, F1x NA124
-
B2B2+
"
"
0
0
25V
0
0
0
25
4
BC1,F1x NA 124
-
-
"
Spring, '90
0
0
294V
0
0
264
558u
5
807-34, self
GOT, GS, GP, GB
B2B2+
"
"
0
22
41
13
2
28
106t
6
807-34-8, self
GOT, PDC-SL, GP, GB
"
Spring'90
Fall, '90
0
6
0
16
0
8
30
7
807-34-14, self
GOT-L, GS, GP, GB
"
"
"
0
7
0
14
0
20s
41
8
807-34-34, self
GOT, GS. GP, GB
"
"
"
0
8
0
13
0
17s
30
9
807-34-110. self
GOT-VL, GS, GP, GB
"
"
"
0
10
0
0
18
12
40

z Tests 1 to 4 serve as checks.
y IL-3 is an inbred that carries B2 of PE-165558 (Table 1); NA 124 is a dark green inbred derived from the cros IL-B X NJ-B (Table 1); 807-34 is an F2 segregate of IL-B X NJ-B.
x See Table 1 in the preceding report for the key to phenotypic symbols. Additional symbols: B = leaf blades; P = petioles; VL = very low level of expressivity, chlorophyll deficiency being limited to a small area (5 - 10 mm in diameter) affecting the proximal end of the fruit and the pedicel.
v Includes a few plants that are close to the PDC-UO, PDC-S, GP, GB phenotype.
u In addition to 558, 68 plants died before they could be classified with certainty.
t In addition to 106, 4 plants died before they could be classified with certainty.
s Classification was uncertain.

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Page citation: Wehner, T.C., Cucurbit Genetics Cooperative;
Created by T.C. Wehner and T. Ng, 1 June 2005; design by C.T. Glenn;
send questions to T.C. Wehner; last revised on 14 December, 2009