A Case of Extremely Low Expressivity of Gene B2 in C. moschata.

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Cucurbit Genetics Cooperative Report 16:64-67 (article 23) 1993

Oved Shifriss
21 Walter Avenue, Highland Park, NJ 08904

An F2 population of the cross IL-B x NJ-B, C. moschata, was grown in the fall of 1989 in Naples, Florida. According to present interpretation (1, 2), IL-B is B1 + B1 + B2B2 and NJ-B, B1B1 B2 + B2 +. Each of the B genes conditions chlorophyll deficiency.

Both parents exhibit precocious depletion of chlorophyll in ovaries, prior to anthesis, and their fruits are uniformly pigmented. But the stems of IL-B are persistently golden, devoid of chlorophyll, and the stems of NJ-B are persistently green.

One of the F2 segregareds, plant 807-34, produced bicolor fruits and persistently green stems. althoughits ovaries were uniformly green, at pre-anthesis stages, they later on turned into bicolor fruits at some point during the post-anthesis stages. The fact that the bicolor design ws first visible sometime during the post-anthesis stages indicated, according to present interpretation, tht 807-34 carried gene B2.

An F3 progeny was obtained from 807-34 and thisprogeny ws grown in spring of 1990. It consisted of 110 plants, 106 of which were classified with a high degree of confidence. Ana analysis of this F3 progeny suggested that 807-34 was B1 + B1 + B2B2+ (see test 5, table 2, in reference 1).

Of the 106 F3 plants, 56 were bicolor. All the 56 plants produced fruits that turned bicolor sometime during the post-anthesis stages, and were therefore considered to be B2B2+. However, these bicolor individuals greatlyvaried in size of the chlorophyll deficient area in their fruits and in extent of chlorophyll deficiency in their stems.

A program of inbreeding was initiated in order to clarify the basis for the wide range of variation in the above F3 progeny. As a result, two phenotypically distinct lines evolved through selection and inbreeding two different bicolor-fruited individuals: (a) plants that produced fruits in which chlorophyll deficiency effected about one-half of their surface, and (b) plants that produced fruits in which chlorophyll deficiency was confined to a few golden spots or a relatively small area (about 1.0 cm2) at the proximal end. The data for case (b) are presented and interpreted here. Three phenotypic classes were identified and the symbols for these phenotypes are described in the following.

Symbols for three phenotypic classes (Table 1):

I. PDC-UP, PDC-SL = precocious depletion of chlorophyll uniformly affects ovaries prior to anthesis; fruits are uniformly golden; chlorophyll deficiency clearly affects the stems early in plant development, the basal portion of the plant being golden, but laer on the expressivity level of chlorophyll deficiency is low, fluctuating between golden and green.

Unlike the PDC-UO, PDC-S phenotype of IL-B whose petioles are golden (tests 10-11, Table 1, in reference 1), the petioles of PDC-UO, PDC-SL are green.

II. GOT-VL, GS = ovaries are uniformly green at pre-anthesis stages; chlorophyll deficiency becomes visible late in post-anthesis stages and is confined to a small area, often inconspicuous, at the proximal end of the fruit (late turning); stems are persistently green.

This phenotype represents a very low expressivity level of chlorophyll deficiency. and significantly, some fruits of a given plant may be uniformly green.

III. GO, GS = ovaries, fruits and stems are persistently green.

It should be emphasized that unless a large number of fruits is carefully examined in each plant, a potentially phenotype of class II may be mistakenly in class III. Furthermore, there is evidence suggesting that the environment can transform some or all plants of class II into class III (e.g., see the offspring of plant 3 in the present Table 1).

Interpretation

The analysis of the data in Table 2 does not disagree with either the 3:1 or the 13:3 hypothesis. However, the 3:1 hypothesis is favored with some elaboration. The present contention is (i) that the PDU-UO, PDC-SL phenotype of this line is B2B2; (ii) that the GOT-0VL, GS phenotype is B2B2+; (iii) that the GO, GS class consists of either B2+B2 exclusively or a combination of B2+B2+ and B2B2+ in various proportions; (iv) that the genetic background of this line is homozygous for certain regulators of B2 (v) that said regulators attenuate or partially suppress the action of B2; (vi) that the degree of suppression is especially high in the heterozygotes; and (vii) that the environment can further intensity this suppression. Consequently, in some environments, B2 can operate as recessive rather than as dominant or codominant gene.

If the 13:3 hypothesis were applicable, one would have expected (a) that the GOT-VL, GS phenotype was heterozygous for both B2 and its partial suppressor, and (b) that the offspring of such double heterozygote would have consisted of some bicolor individuals in which chlorophyll deficiency effected a large portion of the fruit. No such individuals were observed in the present experiment.

Table 1. Offspringof self-pollinated plants that manifested extremely low expressivity of gene B2 (see text)

Offspring

Parentsz

Phenotypic Class I

Phenotypic Class II

Phenotypic Class III
Plantsy
Pedigree
Growing Season
Growing Season
PDC-UO PDC-SL
GOT-VL
GS
GO
GS
 Total
1 807-34-110x Spring, ’90 Fall ’90 10 18 12 40
Fall ’92 8 11 13 32
2 807-34-110-2 Fall, ’90 Spring ’91 4 8 8 20
Fall, ’92 2 12 18 32
3 807-34-110-2-10 Spring, ’91 Fall, ’91 12 11 8 31
Spring, ’92 4 0 27 31
4 807-34-110-2-10-13 Fall, ’91 Spring ’92 7 0 25 32
Fall ’92 4 8 11 23
5 807-34-110-2-10-30 Fall ’91 Spring, ’92 4 0 28 32
Fall, ’92 3 0 4 7
Grand Total 58 68 154 280

z All parental plants exhibited extremely low expressivity (phenotype GOT-VL, GS) of gene B2 (see text).
y Two samples of each parental plant were tested. the two samples were drawn from the same seed packet.
x F3 segregate of the cross IL-B (phenotype PDC-UO, PDC-S) X NJ-B (phenotype PDC-UO, GS). See reference 1.

Table 2a. Analysis of the data presented in Table 1.

Offspring
Phenotypic Classes Hypothesis #1 Hypothesis #2
Parental Plants II & III I Total 3:1

X2

 13:3

X2
1 54 18 72 0.0000 1.8462
2 46 6 52 5.0256 1.7751
3 46 16 62 0.0699 2.0265
4 44 11 55 0.7333 0.0564
5 32 7 39 1.0341 0.0442
222 58 280 6.8629 5.7484
2.7428 0.7092
4.1201 5.0392

Table 2b. Analysis of the data presented in Table 1.

X²

df

P
Hypothesis #1 Deviation 2.74 1 0.05-0.10
Heterogeneity 4.12 4 0.30-0.50
Hypothesis #2 Deviation 0.71 1 0.30-0.50
Heterogeneity 5.04 4 0.20-0.30

Acknowledgement: I thank Rogers NK SEed Co. for enabling me to conduct this study.

Literature Cited

  1. Shifriss, O. 1991. Expression of the B Genes in Cucurbita. Cucurbit Genetics Coop. 14:113-115.
  2. Shifriss, ). 1991. The Two B Genes in Cucurbita are Unlinked. Cucurbit Genetics Coop. 14:116-122.