Expression of the B Genes in Cucurbita

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

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, including “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 the performance of IL-B (tests 10 & 11). Unlike NA159 (test 12) inbred PI-165558 (tests 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; heterogeneity 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 associated with lethality or semi-lethality depending 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.