Cucurbit Genetics Cooperative Report 13:50-54 (Article 21) 1990

Relationship Between the B Genes of Two Cucurbita Species, III.

Oved Shifriss

21 Walter Avenue, Highland Park, NJ 08904

The main subject of this series of reports is the relationship between the gene for precocious depletion of chlorophyll in ovaries of C. pepo and the gene for precocious depletion of chlorophyll in stems and ovaries of C. maxima. Precocious depletion of chlorophyll is associated with white, yellow, or golden color, singly or in combination, depending upon other genes.

The "precocious" gene of C. pepo was formally designated by symbol B for potential bicolor fruit variation. And the "precocious" gene of C. maxima was tentatively designated by the same symbol because it too is potentially associated with bicolor fruit variation. Over 20 years ago, in the absence of a breeding test for allelomorphism, I treated the two genes as different alleles at the same locus. This was done for sake of temporary convenience in nomenclature, but not without misgivings.

The absence of a breeding test for allelomorphism was due to the fact that the two species are isolated from one another by strong genetic barriers. These barriers were circumvented through interspecific transfers of the two "precocious" genes to C. moschata. As a result, two new inbreds of C. moschata were established: NJ-B and IL-B. NJ-B carries the "precocious" gene of C. pepo, and IL-B carries the "precocious" gene of C. maxima (Ref. 1; see also Table 1 in the present report).

Now that we have a breeding test for allelomorphism, through the cross NJ-B x IL-B, it seems that my early assumption that the two genes are alleles was false, and that these genes are non-linked (Refs 1 and 2; see also additional data in the following report, IV). But one cannot yet exclude the possibility that the two genes belong to the same "gene family".

Although the evidence for non-linked relationship is fairly convincing, some mysteries remain. The mysteries are largely due to epigenetic and external factors that profoundly modify the expression of the "precocious" genes during plant development. And these factors indirectly introduce difficulties in classification. The objective of this report is to present data on the breeding behavior of a single plant, 123-66, whose phenotype was difficult to classify with certainty.

Plant 123-66. Two F₂ segregates of the cross NJ-B x IL-B were described in the 1989 report as "having precociously pigmented stems, green ovaries and green leaves". The two plants were listed in a group of nine unclassified F₂ individuals (see last paragraph in reference 2), and one of the two was 123-66. This plant was lightly pigmented overall. It produced small, oblate fruits (4.5 x 7 cm) that turned from green to tan at maturity. Apart from fruit size and shape, its fruit color development was similar to that of 'Butterbush' (Table 1). Precocious depletion of chlorophyll in stems was mainly confined to the base of the plant.

The reason for listing 123-66 among the unclassified plants was that I had some reservation of its true phenotype. Specifically, I observed only six ovaries in this plant and indeed all were green. Experience has shown, however, that some B plants produce in sequence as many as ten green ovaries before they produce precociously pigmented bicolor ovaries. Furthermore, certain virus infections can transform potentially yellow ovaries into green ones. Since 123-66 with green ovaries and precociously pigmented stems posed a challenging question, I decided to study its breeding behavior. The question was this: Is plant 123-66 a cross-over product?

Key to phenotypic symbols used in classification. For each plant, the pigmentation of ovaries, stems, petioles and leaf blades was represented by a combination of letters. The key to these phenotypic symbols was as follows: B = leaf blade; G = green; L = low level of expressivity, implying that precocious depletion of chlorophyll was not extensive; O = ovary; P = petiole; PDC = precocious depletion of chlorophyll; S = stem; T = turn, implying that ovaries turned from green to another color at anthesis or about 2 days after anthesis, i.e., chlorophyll depletion occurred early in fruit development, but not at very early pre-anthesis stages; U = uniform pigmentation over the entire surface of the ovary or fruit. Thus: the combination GO, GS, GP, GB refers to all-green plants; GO, PDC-SL, GP, GB describes plants in which the ovaries are green, the stems exhibit a low level of expressivity with respect to precocious depletion of chlorophyll and the petioles and blades are green; GOT, PDC-S, GP, GB refers to plants in which ovaries turn early in fruit development from green to another color, the stems exhibit a high level of expressivity of precocious depletion of chlorophyll (extensive pigmentation) and the petioles and leaves are green; and PDC-UO, PDC-S, PDC-P, GB describes plants whose ovaries are precociously and uniformly pigmented, stems and petioles are also precociously pigmented and blades are green. In this latter phenotype, the leaf blades are often partially PDC, i.e., fluctuating from GB to PDC-BL..

Results and discussion. The results are presented in Table 2. The proportion of phenotypes in the offspring of 123-66 does not disagree with a monhybrid ratio of 1:2:1 (test #1), n=49, X²=1.16, df=2, P=0.50-0.70; note that I excluded from the total a single deviant plant to which I'll refer later), suggesting that 123-66 was heterozygous for the "precocious" gene of C. maxima, and that in heterozygotes the effect of this gene on ovaries appears as a recessive trait whereas its effect on stems appears as a partially dominant trait. This suggestion is supported by test #3 (n=256, deviation X²=0.09, df=2, P=0.95-0.98; heterogeneity X²=3.74, df=6, P=0.70-0.80) as well as by all other relevant tests.

Conclusion: 123-66 did not originate as a cross-over product. Reason: In the offspring of 123-66, plants homozygous for the "precocious" gene exhibited the dual effects of this gene rather than one exclusively, the effect on stems.

It is important to point out that I observed 15 F1 plants of each of the crosses IL-B x Black Line and IL-B x 'Butterbush' (see Table 1; note that both Black Line and 'Butterbush' are standards), and all 30 plants exhibited precociously pigmented stems and fruits. In these hybrids, the "precocious" gene of C. maxima is partially dominant with respect to its dual effects. Question: Does the F2 of NJ-B x IL-B segregate for elements that selectively switch one of the dual effects of this gene from dominant to recessive expression?

As to the deviant plant (123-66-26) found in the offspring of 123-66 (test #1), it is clear (test #7) that this is a mutant whose phenotype is intermediate between the "precocious" homozygote and the heterozygote. This is perhaps another example supporting the suggestion made years ago that "precocious" loci are labile.

Acknowledgment. I thank Tom V. Williams and Raymond B. Volin of Northrup King Co. for providing facilities for this study. I am also grateful to Leslie Skubic, John Arnold and Peter Vorsatz for technical assistance.

Literature Cited

1. Shifriss, O. 1986. Relationship Between the B genes in Two Cucurbita Species. Cucurbit Genetics Coop. 9:97-99.
2. Shifriss, O., R. G. Volin and T. V. Williams. 1989. Relationship Between the B Genes of Two Cucurbita Species, II. Cucurbit Genetics Coop. 12:75-78.

Table 1. Description of some breeding materials in C. moschata.
Breeding Materials	Description
Black Line	A vine breeding line homozygous for genetic material conditioning normal synthesis of chlorophyll. It is considered as standard. Stems and fruits are green at early stages becoming darkgreen or "black" later in development. Fruits are similar to 'Butternut' in size and shape. Originated in an F2 segregate (124-56) of the cross NJ-B x IL-B.
'Butterbush'	An inbred of 'Burpee Butterbush' homozygous for genetic material conditioning normal synthesis of chlorophyll. It is considered as another standard. But unlike Black Line, it exhibits an overall light green pigmentation. Fruits turn from green to tan 14-16 days following anthesis.
IL-B	A vine inbred that carries a gene for precocious depletion of chlorophyll in stems and fruits. This gene was transferred to C. moschata from P.I. 165558 of C. maximaz through innovative series of backcrosses. Stems and fruits are precociously yellow at early stages, becoming intensely golden late in development. Fruits are similar to 'Butternut' in size and shape. Leaf blades are green under field conditions in New Brunswick, NJ, and Naples, Florida.
MP	A clone derived from an F1 plant, 7356-14, of the cross NJ-B x IL-B. Stems and fruits are precociously yellow gradually becoming light golden. Fruits are of medium size and bell-shaped. Leaf blades exhibit precocious yellow pigmentation along their midrib ("midrib pattern") in short days.
NJ-B	A bush inbred that carries gene B for precocious depletion of chlorophyll in fruits. This gene was transferred to C. moschata from 'Jersey Golden Acorn' (JGA) of C. pepo through the pedigree method of selection following the cross JGA x 'Butterbush'. Fruits are precociously yellow at early preanthesis stages, turning white and later tan at maturity. The fruits are small and oblate (3.5 x 7.0 cm). Stems and leaves are green.
NOMP	A clone derived from an F1 plant, 7356-1, of the cross NJ-B x IL-B. It is similar to MP except that its leaf blades do not exhibit precocious yellow pigmentation along their midrib.
zThis gene of C. maxima was tentatively designated by symbol B; but its physical and functional relationship to gene B of C. pepo is not known.

 

Table 2. Breeding tests of selection 123-66 and its offspring. This selection was an F2 segregate of the cross NJ-B x IL-B (Table 1). The F2 was grown in fall 1988; the offspring of 123-66 was grown in spring 1989 and all other progenies were grown in fall 1989. Field data, Naples, Florida.
		Offspring
	Parent	Phenotypic Classesz
Test	Pedigree, Phenotype and mode of reproduction	PDC-UOy PDC-S PDC-P GB	GOT PDC-S GP GB	GO PDC-SL GP GB	GO GS GP GB	Total
1	123-66:GO,PDC-SL,GP,GBy,	13	1x	21	15	50
2	123-66-1:GO,GS,GP,GB,	0	0	0	10	10
3	123-66-8:GO,PDC-SL,GP,GB,	9	0	25	16	50
	123-66-21:GO, PDC-SL,GP,GB,	13	0	27	10	50
	123-66-30:GO,PDC-SL,GP,GB,	12	0	17	8	37
	123-66-49:GO,PDC-SL,GP,GB,	29	0	60	30	119
	Test 3 pooled	63	0	129	64	256
4	123-66-1x123-66-8	0	0	4	6	10
	123-66-1x123-66-21	0	0	7	3	10
	123-66-1x123-66-30	0	0	4	6	10
	Test 4 pooled	0	0	15	15	30
5	123-66-23:PDC-UO, PDC-S,PDC-P,GB,	5	0	0	0	5
	123-66-32:PDC-UO, PDC-S,PDC-P,GB,	5	0	0	0	5
	123-66-44:PDC-UO, PDC-S,PDC-P,GB,	5	0	0	0	5
	Test 5 pooled	15	0	0	0	15
6	123-66-23x123-66-1	0	0	5	0	5
	123-66-44x123-66-1	0	0	5	0	5
	Test 6 pooled	0	0	10	0	10
7	123-66-26:GOT,PDC-S,GP,GB,	0	106	0	0	106
zThe fruits of all the plants of these classes were indistinguishable at maturity, being uniformly tan.
yThe key to phenotypic symbols is given in the text.
xThe pedigree of this plant was 123-66-26. See test 7 (above) for the breeding behavior.