Reinterpretation of Fruit Color Inheritance in Cucurbita pepo L.

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Cucurbit Genetics Cooperative Report 15:90-92 (article 35) 1992

Carl D. Clayberg
Department of Horticulture and Forestry, Kansas State University, Manhattan, KS 66506-4002, USA

Most of the literature describes fruit striping in this species as being due to a single dominant gene, St, (1, 2, 5), which has been found to be part of an allelic series and is more properly designated as l-1St , (6, 3). According to the review by Robinson et al. (4, p.564), “Sinnott and Durham [1922] however, concluded that striping is recessive.” The summary of this latter paper (7, p.186) actually said that “Plain (solid or self) color of fruit is dominant over stripping in some cases but appears to be recessive in others. A single factor difference is involved.” Their data are presented in Table 1.

If we ignore cross 21 for the moment, a heterogeneity chi square test on the F2 data of the other three families for only their striping vs. nonstriping segregation gives us a value for XT2 = 4.30 (p.0.20), providing an acceptable fit to the hypothesis that striping is segregating as a monogenic dominant trait in all three families, as found by subsequent researchers.

The simplest explanation of cross 21 in light of these results is that it is due to the independent segregation of a dominant inhibitor, I mf , which prevents the production of any color in mature fruit, unlike W in these data. The original cross would be i mfimf StSt WW yy X ImfImf stst WW yy; the F1 Imfimf Stst; and the F2 segregation ratio 13 white, unstriped (ImfSt, Imf st, and imf st) : 3 white, striped (imf St ). The data fit this ratio well (X2 = 0.071, p.0.70). The presumed recessive inheritance of striping in cross 21 has been variously attributed to situations where striping is masked in darkly pigmented fruit (6, p. 218) and where dark stripes occur over the main vascular tracts (3, p.493), neither explanation being supported by the original report (7).

If we look at the complete segregation for cross 6, we find a 3 : 1 segregation for white ys. yellow fruit, but W and St are not assorting independently (X2 = 11.30 for 9:3:3:1, p,0.02), as indeed Sinnott and Durham (7, p.181) suggested: “…the further possibility suggests itself that there may be linkage between the factor for white and the factor for stripping.” We can calculate a product-ratio value here of 19.0+7.63 crossover units.

Returning to cross 5, we note that the F2 segregation of white to yellow is an excellent fit to the 15:1 ratio expected if white is determined by two pairs of dominant, duplicate genes, again as the authors suggested, although not accepted by Robinson et al. (4). More over, if we compute the overall expected values for all phenotypes in this F2 , based on the above-calculated linkage of St to one of these putative loci for white, and test these against the observed values, we again get an excellent fit (X2 = 1.23, p.0.70).

Although the family sizes are small, they are consistent in supporting all of the conclusions drawn here: striping is due only to a single dominant gene, presumably St (i.e., l-1 St); there are two independently assorting dominant genes, W1 and W2 , for white mature fruit color, neither of which suppresses the expression of St , and one of which is linked to St ; and there is a dominant color inhibiting gene, I mf , which is epistatic to St and assorts independently of it.

Table 1. Inheritance of striping in fruit.z

Pedigree

Parents

F1

F2
21 White, green striped X Plain white Plain white Plain white 43; White, green stripes 9
5 White, yellow striped X Plain yellow White, pale yellow stripes White, pale yellow stripes 23; Plain white 8; Plain yellow 2
6 Plain yellow X White, yellow stripes White, yellow stripes White, yellow stripes 22; Plain white 3; Yellow, stripes darker 3; Plain yellow 6
7 White, yellow striped X Plain yellow White, pale yellow stripes White, pale yellow stripes 20; Plain white 13

z Data from Table 2 of Sinnott and Durham (1922)

Literature Cited

  1. Globerson, D. 1969. The inheritance of white fruit and stem color in summer squash, Cucurbita pepo L. Euphytica 18: 249-255.
  2. Nath, P. and C.V. Hall. 1963. Inheritance of fruit characteristics in Cucurbita pepo L. Ind. J. Hort. 20:215-221.
  3. Paris, H,S. and Y. Burger, 1989. Complementary genes for fruit striping in summer squash. J. Hered. 80:490-493.
  4. Robinson, R.W., H.M. Munger, T.W. Whitaker, and G.W. Bohn. 1976. Genes of the Cucurbitaceae. HortScience 11:554-568.
  5. Scarchuk, J. 1954. Fruit and leaf characters in summer squash. J. Hered. 45:295-297.
  6. Shifriss, O. 1955. Genetics and origin of the bicolor gourds. J. Hered. 46:213-222.
  7. Sinnott, E.W. and G. B. Durham. 1922. Inheritance in the summer squash. J. Hered. 13:177-186.