Inheritance of Bitterness in Cucurbita pepo L.

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Cucurbit Genetics Cooperative Report 10:76-77 (article 39) 1987

Fenny Dane, A. G. Hunter, and O. L. Chambliss
Department of Horticulture and Alabama Agricultural Experiment Station, Auburn University, AL 36849

Cucurbitacins are very bitter and toxic secondary plant substances which can be found in roots, cotyledons and fruits of many cucurbit species. Synthesis of the bitter principles is initiated with the onset of seed germination. It increases rapidly in roots and cotyledons of up to 6 day old seedlings after which it decreases (Rehm, 1960). Jaworski et al. (1985) found a decrease in the cotyledons but not in the roots of Cucurbita pepo ‘Blackjack’. The inheritance of bitterness in C. pepo was ought to be controlled by one major dominant gene (Grebenscikov 1954). Nonbitter (NB) fruits were found on bitter (B) seedlings, however, which pointed to the action of a suppressor gene (Rehm, 1960). In later studies Rehm (1968) found the gene for fruit bitterness to be independent from the gene for seedling bitterness.

The occurrence of bitter fruit on ‘Zucchini’-type summer squash in commercial plantings prompted our genetic studies. Our aim was to determine the interrelationships of genes controlling bitterness in roots, cotyledons and fruits and to develop effective seedling screening techniques for the selection of NB genotypes. Bitterness of the roots, cotyledons and fruits was evaluated organoleptically; roots 4 days or more and cotyledons 6 days or more after germination. Thin layer chromatography techniques (Gorski et al., 1968) could not be used to distinguish B from NB roots and cotyledons on an individual plant basis.

Bitterness was detected in the seedlings of several C. pepo cultivars. The following combinations of B and/or NB roots and cotyledons were found:

Table 1. Combinations of B and/or NB roots and cotyledons found
Cultivar Root Cot Fruit No. plants tested
Early Summer Crook Neck NB NB NB 10
Yellow Summer Crookneck Improved NB NB NB 15
Early Prolific Straight Neck B NB NB 20
Zucchini Dark Green B B NB 10
Black Zucchini B B NB 10
Cocozelle B B NB 13

With the exception of ‘Cocozelle’, a decline in root and cotyledon bitterness could be detected in these cultivars and in bitter fruited ‘Zucchini’-type squash. Crosses between ‘ESCN’ and ‘Black Zucchini’ or ‘Cocozelle’ and backcrosses to ‘ESCN’ showed the expected monogenic B:NB ratios in the roots and cotyledons of ‘ESCN’ x ‘Cocozelle’, but not in ‘ESCN’ x ‘Black Zucchini’. The lower than expected degree of bitterness found in ‘ESCN’ x ‘Black Zucchini’ may be attributed to the low quantity of cucurbitacins present in the B parent.

Crosses between ‘Zucchini’-type squash and ‘EPSN’ differing in cotyledon bitterness only showed the expected 3B:lNB ratio for cotyledon bitterness.

Our genetic studies with F4 and F5 selections of bitter fruited ‘Zucchini’-type squash were in general agreement with Rehm’s (1968) studies. Fruit bitterness was found to be controlled by one dominant gene (Table 1). It segregated independently from genes controlling bitterness in other plant parts. The low degree of bitterness found in the roots and cotyledon points to a low concentration of cucurbitacins in these plant parts.

The results indicate that bitterness in the root, cotyledons and fruit of Cucurbita pepo appears to be controlled in each plant part by a monofactorial dominant gene. Seedlings with nonbitter roots generally had nonbitter cotyledons, while seedlings with bitter roots had non-bitter or bitter cotyledons. The gene for cotyledon bitterness may be dependent on the gene for root bitterness. Since fruit bitterness segregates independently from seedling bitterness, selection against bitterness of the fruit can not be done in the seedling stage.

Table 2. Chi-square test results of F2 and BC data from crosses between NB and B ‘Zucchini’ type summer squash.
Plant Part Generation Observed
B:NB ratio		 Expected
B:NB ratio
 x² P
root F2 124:67 3:1 10.53 <.005
root BC 31:52 1:1 5.32 <.05
cotyledon F2 65:126 3:1 170.99 <.005
cotyledon BC 15:68 1:1 33.84 <.005
fruit F2 140:50 3:1 0.18 >.50
fruit BC 37:44 1:1 0.30 >.50

 

Literature Cited

  1. Grebenscikov, I. 1954. (2) Notulae cucurbitologicae. I. Zur Vererbung der Bitterkeit und Kurztriebigkeit bei Cucurbita pepo L. Kulturpflanze 2:145-154.
  2. Gorski, P.M., A. Jaworski, S. Shannon, and R. W. Robinson. 1986. Rapid TLC and HPLC quantification of cucurbitacin C in cucumber cotyledons. HortScience 21:1034-1036.
  3. Jaworski, A., P.M. Gorski, S. Shannon, and R. W. Robinson. 1985. Cucurbitacin concentrations in different plant parts of Cucurbita species as a function of age. Cucurbit Genetics Coop. Rpt. 8:71-73.
  4. Rehm, S. 1960. Die Bitterstoffe der Cucurbitaceen. Ergebnisse der Biology 22:108-136.
  5. Rehm, S. 1968. Neuere Untersuchungen uber Cucurbitacine. Deutsche Apotheker Zeitung 108:878.