Relationship between the B Genes of Two Cucurbita Species, II

Cucurbit Genetics Cooperative Report 12:75-77 (article 32) 1989

Oved Shifriss, Raymond B. Volin and Tom V. Williams
21 Walter Avenue, Highland Park, NJ, Northrup King Co., 10290 Greenway Rd., Naples, Fl 33962

The primary effect of a B gene is precocious depletion of chlorophyll in young fruits prior to anthesis (1). Genes conditioning this effect exist in both C. pepo and C. maxima. It is practically impossible to study the relationship between the B of C. pepo and the B of C. maxima by breeding experiments. This is because the two species are isolated by strong genetic barriers. The barriers were circumvented by transferring the B genes of these species to C. moschata. As a result, two different B lines of C. moschata were established: NJ-B and IL-B. NJ-B carries the B of C. pepo and IL-B carries the B of C. maxima. A preliminary study of inheritance involving NJ-B x IL-B was conducted at Rutgers University, New Brunswick, NJ, and the results raised some unexpected issues.

First, individual F1 plants differed in their capacity to manifest a “midrib pattern” of chlorophyll depletion in leaf blades (Fig. 1, ref. 2). Neither NJ-B nor IL-B exhibits this pattern. Second, apart from a small proportion of albino (lethal) seedlings, most F2 segregates were difficult to classify, casting some doubt on the validity of the data. The F2 plants were grown without supplementary light during winter months in a greenhouse that was not well insulated from outside temperature fluctuations.

The difficulties experiences in classification were largely due to the fact that precocious depletion of chlorophyll in this F2 can affect several or all aerial organs of a plant. Moreover, a particular organ may or may not be affected depending on the stage in plant development at which it is differentiated. The effect is also subject to variations in the environment. Under such circumstances each F2 plant must be observed over a long period of time in order to critically assess its complex phenotype, a laborious task.

Nevertheless, the preliminary results suggested that the analysis of this cross might shed some light not only on the relationship between the two B genes but also on the genetic control of chlorophyll during plant development. And this thought gave the impetus to the present investigation.

Breeding materials. Two clones were available from the previous study. Clone NOMP was obtained from an F1 plant (5356-1) that did not exhibit the midrib pattern, and clone MP was obtained from an F1 plant (5356-14) that exhibited this pattern. The two clones were propagated vegetatively and grown to maturity for five years. During this period they behaved in a consistent manner: NOMP did not exhibit the pattern and MP exhibited it in winter but not in summer. For the present study, new F1 seed was obtained from NJ-B x IL-B. But the BC1 and F2 seed was obtained through the use of NOMP and MP clones.

Environment. The seed of the parental inbreds, the new F1, the BC1 and the F2 was sown in a greenhouse in Naples, Florida, on 12 September 1988, and the seedlings were transplanted to the field on 22 September. The greenhouse temperature exceeded 30°C during germination and early seedling growth. Cultural practices were similar to those commonly used by commercial growers in the area,. Field observations of individual plants continued until the end of November.

Results and Interpretation (consult Table 1). The first significant observation was the absence of albino (lethal) seedlings in any one of the breeding materials.

In the BC1 (test 4), the proportion of plants with precociously pigmented fruits and precociously pigmented stems (phenotypic classes 4 + 5) to plants with precociously pigmented fruits and green stems (class 3) to plants with precociously pigmented bicolor fruits and green stems (class 2) does not disagree with a 2:1:1 ratio (97:46:49, the expected ratio being 96:48:48, P=0.90-0.95). In the F2 (test 7), the proportion of plants with precociously pigmented fruits and precociously pigmented stems (classes 4 + 5 + 6) to plants with precociously pigmented bicolor fruits and green stems (class 2) to plants with green fruits and green stems (class 1) does not disagree with a ratio of 12:1:2:1 (258:17:42:19, the expected ratio being 252:21:42:21, P =0.75-0.90).

The new results are compatible with the hypothesis that the two B genes are non-linked; that there exists a third gene; that the third gene is closely linked to the B of IL-B; that this linked gene activates the expression of B in stems; and that the bicolor fruited plants carried a single dose of B, donated exclusively by NJ-B, and three doses of B+ . This suggests that the effect of a single B of IL-B is stronger than that of a single B of NJ-B in extending precocious chlorophyll depletion over the entire fruit.

If the above hypothesis is basically correct, let B1 represent the B of C. pepo, B2 the B of C. maxima and Ac-B the activator of B. Then, the partial genotype of NJ-B is B1* Ac-B+ / B1 * Ac-B+ , B2+ * Ac-B+ / B2+ * Ac-B+ . And the partial genotype of the IL-B is B1+ * Ac-B+ / B1+ * Ac-B+, B2 * Ac-B / B2* Ac-B.

The effect of chlorophyll depletion on whole plants was more extensive and more severe in progenies obtained from the MP clone than in progenies obtained from the NOMP clone. This was particularly striking in the F2. The difference between the two BC1 progenies (test 3 vs test 2) was hardly perceptive to the observer in the field, and might not be biologically significant. On the other hand, the data in Table 1 do not reflect adequately the true magnitude of the difference between the two F2 progenies (test 6 vs test 5). The reason for this is that class 6 consisted of a wide spectrum of phenotypes. At one end of the spectrum were essentially class 5 plants that exhibited the midrib pattern late in the season. At the other end of the spectrum were highly variegated, almost completely yellow, plants that were essentially semi-lethal. In test 5, the 6 plants of class 6 were initially recorded as class 5 individuals, but at the end of November their new leaves exhibited the midrib pattern and therefore these plants were reclassified under class 6. In test 6, at least 35 of the 55 class 6 plants were recorded as variegated, and 8 of the 35 were almost completely yellow or essentially semi-lethal. Genotypes of such individuals might appear as albino (lethal) seedlings under conditions of low temperature and low light intensity.

Variegated plants similar to those of class 6 were observed in C. maxima about 10 years ago (Shifriss, unpublished). These variegated plants were F2 segregates of crosses between PI 165558, a B/B cultivar from India, and several North American cultivars, B/B and B+/B+. The stem of PI 165558 is precociously pigmented (indicating the presence of B2 * Ac-B), whereas the stems of most North American B/B cultivars are green (indicating the presence of B2 * Ac-B+). Since PI 165558 was the donor of B to IL-B (2), it must have actually donated B2 * Ac-B. Perhaps the gene pool of Cucurbita carries some elements that extend the effect of B2 * Ac-B over the entire plant.

Finally, two of the nine unclassified plants (test 7) were tentatively described as having precociously pigmented stems, green ovaries and green leaves. If the function of the linked gene, presently designated by symbol Ac-B, is not related to the effect of B, then this linked gene should be designated by a different symbol, e.g., Cds, for chlorophyll depletion in stems.

Table 1. Inheritance of precocious depletion of chlorophyll in a cross between two special lines of C. moschata. 1988 field data, Naples, Florida.

Test	Breeding materials	Phenotypic classesz						Number of classified plants	% plants of class 6	Number of unclassified plants
		1	2	3	4	5	6
		GF GS GP GB	PDC-BiF GS GP GB	PDC-UF GS GP GB	PDC-UF PDC-S GP GB	PDC-UF PDC-S PDC-P GB	PDC-UF PDC-S PDC-P PDC-B
–	P1* NJ-B	0	0	12	0	0	0	12	0	0
–	B2* IL-B	0	0	0	0	12	0	12	0	0
1y	F1* P1x P2	0	0	0	0	18	0	18	0	0
2x	BC1* F1x P1	0	20	23	49	1	0	102	0	0
3w	BC1*F1x P1	0	20	23	38	9	0	90
4	Total for BC1	0	40	46	87	10	0	192	0	0
5x	F2	1	16	9	54	36	6	122	4.9	3
6w	F2	18	26	8	54	53	55	214	25.7	6
7	Total for F2	19	42	17	108	89	61	336	18.2	9

^z Key to phenotypic symbols: B = leaf blade; Bi = bicolor; F = fruit; G = green; P = petiole; PDC = precocious depletion of chlorophyll; U = uniformly pigmented, referring specifically to fruit. PDC may be associated with either white, tan, yellow or golden pigmentation.
^y The F₁ hybrids of reciprocal crosses were indistinguishable phenotypically. None of the 18 plants exhibited the “midrib pattern” (see text). The petioles of the F₁ plants were less intensely pigmented and more variable than the petioles of P₂.
^x This test was made through the use of an old F₁ clone (NOMP) that did not exhibit the “midrib pattern”.
^w This test was made through the use of an old F₁ (MP) that manifested the “midrib pattern”.

Literature Cited

1. Shifriss, O. 1981. Origin, expression and significance of gene B in Cucurbita pepo. L. J. Amer. Soc. Hort. Sci. 106:220-232.
2. Shifriss, O. 1986. Relationship between the B genes of two Cucurbita Species. Cucurbit Genetics Coop. Rpt. 9:97-99.