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Cucurbit Genetics Cooperative Report 11:23-24 (article 12) 1988

Interaction of Cucurbitacin Genes

R.W. Robinson, A. Jaworski, P.M. Gorski, and S. Shannon

Horticultural Sciences Department, New York State Agricultural Experiment Station, Geneva, NY 14456

The Bt gene of Cucumis sativus L. has been reported to produce bitter fruits (1). We found that the bitterness is due to a quantitative, but not a qualitative change in cucurbitacin content. Fruits of Bt plants are high in cucurbitacin C (Cc C), the same cucurbitacin compound present in normal cucumbers (Table 1). Plants heterozygous for Bt had fruits with an intermediate content of Cuc C.

The F2 progeny of 'Eversweet' x PI 173889 was analyzed for Cuc C content in cotyledons by thin layer chromatography (2) to study the interaction of Bt and bi. If Bt were epistatic to bi, a ratio of 15 with to 1 without Cuc C would be expected, while a 3:1 ratio would be expected if bi is epistatic to Bt. The observed segregation of 48 with Cuc C to 16 free of Cuc C conformed precisely to that expected on the basis of epistasis of bi. Bimodal segregation occurred among the F2 plants having Cuc C, with peaks at 0.25 and 0.40 mg Cuc C/g. Dividing the F2 population into three groups (0.3101.00, 0.10-0.30, and 0 mg Cuc C/g) resulted in a ratio of 34:14:16, in close agreement (p = 0.7-0.8) with the 9:3:4 ratio expected.

Additional evidence on the epistasis of bi to Bt was obtained when fruits of 'Spartan Salad' (+/+ bi/bi) x PI 173889 (Bt/Bt +/+) was analyzed for Cuc C. The ratio of 31 bitter (>0.01 mg Cuc c/g) : 11 normal (0.1-0.2) with the 9:3:4 ratio expected on the basis of epistasis of bi, and differed (p = < .001) with the 12:3:1 ratio expected if Bt were epistatic.

A single gene, cu, of Cucurbita pepo is known to govern cucurbitacin B content of cotyledons (4). We confirmed this, and found that cu determines cotyledon content of cucurbitacins D, E, and I as well as cucurbitacin B. Classification for cu can be made by tasting cotyledons. Cultivars such as 'Scallop' and 'Straightneck' that are recessive for cu have nonbitter cotyledons, whereas Zucchini' and other cultivars dominant for cu have bitter cotyledons. In this respect, the cu gene of Cucurbita pepo is similar to the bi gene of Cucumis sativus, since both produce a phenotype of nonbitter cotyledons. They differ, however, in gene action. Cucumber gene bi completely blocked cucurbitacin biosynthesis, while squash gene cu reduced but did not eliminate cucurbitacin formation. Another fundamental difference between Cucurbita gene cu and Cucumis gene bi is that cu is not epistatic to the dominant gene in the species for bitter fruits. The F2 of C. pepo cv. Early Prolific Straightneck x C. texana segregated 3 bitter fruits: 1 nonbitter, not in the 9:7 ratio that would be expected if cu were epistatic.

Table 1. Cucurbitacin content of cucumber fruits of different genotypes.

Line
Genotype
Phenotype
Cuc C content mg/g fr.wt.
Spartan Salad
+/+ bi/bi
nonbitter
0.00
Wisconsin SMR 18
+/+ +/+
normal
0.01
PI 173889
Bt/Bt +/+
very bitter
0.58
(Spartan Salad x PI 173889) F1
IBt/+ bi/+
bitter
0.26

Literature Cited

  1. Barham, W.S. 1951. The inheritance of a bitter principle in cucumbers. Proc. Amer. Hort. Sci. 62:441-442.
  2. Gorski, P.M., A. Jaworski, S. Shannon, and R.W. Robinson. 1986. Rapid TLS and HPLC quantification of cucurbitacin C in cucumber cotyledons. HortScience 21:1034-1036.
  3. Herrington, M.E. 1983. Intense bitterness in commercial zucchini. Cucurbit Genet. Coop. Rpt. 6:75-76.
  4. Sharma, G.C. and C.V. Hall. 1971. Cucurbitacin B and total sugar inheritance in Cucurbita pepo L. related to spotted cucumber beetle feeding. J. Amer. Soc. Hort. Sci. 96:750-754.
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