List, Description and Interactions of the Genes Affecting Fruit Color in Cucurbita pepo

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Cucurbit Genetics Cooperative Report 12:72-74 (article 31) 1989

Harry S. Paris
Department of Vegetable Crops, Agricultural Research Organization, Newe Ya’ar Experiment Station, P.O. Haifa, Israel

Cucurbita pepo L. contains a fascinating array of fruit colors. A few genes affecting fruit exterior color have been identified, and their preferred symbols and names were recently summarized (2) as B (Bicolor) fruit), D, (Dark green stem), l (light fruit color), l-2 (light pigmentation on fruit-2), St (Striped fruit), W (White fruit), and Y (Yellow fruit color). However, this list is not complete, nor does it contain a full description of the effects and interactions of these genes. Some new data would also indicate that modification of the list is also necessary. The goal here is to present a revised list of genes affecting fruit exterior color in C. pepo, including a description of the effect(s) and interactions of each, and to review some of the literature, especially with respect to its synonymies and anomalies.

Table 1. Revised gene list

Preferred gene symbol

Synonym

Name
B Bicolor fruit (13)
D R Dark peduncle, stem, and fruit (3, 7)
Ep-1 Extender of precocious yellow coloration-1 (15)
Ep-2 Extender of precocious yellow coloration-2 (15)
1-1, 1-1St C, St light fruit coloration-1 (7,13)
1-2 light fruit coloration-2 (7)
W White fruit coloration (16)
Y Yellow fruit coloration (16)

Description of effects and interactions

B. Preanthesis ovaries yellow or bicolor, yellow and green. Incompletely dominant to alternative allele represented as b in literature prior to 1981 and as B+ in literature since 1981. A third allele, represented as Bw (weak B) probably exists (13, 14) but proof with an allelism test has not been presented. When homozygous or when heterozygous in the presence of at least two Ep alleles (15), B is epistatic to Y (11). Interacts in complementary fashion with L-2 to condition orange fruit flesh color (5), with L-1 and L-2 to condition intense yellow color of young (summer squash) fruit (7), and with W to produce cream (instead of white) mature fruit color (13). Pleiotropic, affecting foliar as well as fruit characteristics, with differing degrees of expression of the various effects occurring in different genetic backgrounds (14).

D. Plant stems dark, fruits and their peduncles dark from two weeks past anthesis; thus pleiotropic, affecting foliage and fruit. Alternative allele d for light stems, peduncles and fruits (3, 7). the D allele is epistatic to both l-1 and l-2 when either or both l genes are in homozygous recessive state (7). In half-mature fruit, D also is epistatic to l-1st. Originally (3), D was considered to condition dark stems only, and reported to be tightly linked to a fruit-color gene, R (Reversal, or non-persistent color, or r for recessive white, refs. 2, 3). The R symbol was subsequently (8) adopted based on the contention that D and R are linked but separate loci. However, results of later studies (5, 7 and H.S. Paris unpublished data) cast serious doubt on the case for separate linked loci. Due to the lack of firm evidence, R should presently be considered synonymous with D. Other genes affecting stem color probably exist.

Ep-1, Ep-2. Extend the precocious yellow coloration conditioned by B (15). Incompletely dominant to alternative alleles ep-1 and ep-2 and additive in action: two doses of any combination of Ep alleles result in completely yellow fruit when B is heterozygous and in extension to the adjacent peduncle, calyx, and/or corolla when B is homozygous. No known effect in genotype b/b (B+/B+) .

l-1, l-1St, l-2. Young fruits lightly colored when either l is homozygous recessive. Complementary action of L-1 and L-2 results in fruit that are intensively colored throughout development (7). The gene designations c (3) and l (13) have been shown to be synonymous (8). Other results (H.S. Paris, unpublished) show that the originally designated l is in fact l-1 and not l-2. Allele l-1St results in striped fruit and is recessive to L-1 and dominant to l-1. Complementary action of l-1St and L-2 results in striped young (summer squash) and mature fruits. The symbol St was originally suggested for the gene conditioning striping of ‘Caserta’ (10). Striping was considered by Shifriss (13) to be conditioned by an allele of l (l-1) but he did not present evidence in support of this contention, and therefore the symbol St was accepted (9). In the progeny (approximately 100) plants of the three-way cross, l-1/l-1 L-1/L-2 x (L-1/L-1 l-2/l-2) x ‘Caserta’, L-2/L-2), only intense-colored and striped, and no light-colored individuals were obtained, showing that the striping of ‘Caserta’ is conditioned by an allele of l-1, or is very tightly linked to l-1 (6). Therefore the symbol l-1St is now to be preferred. However, other genes conditioned striping undoubtedly occur at separate loci.

W and Y. W was originally (16) reported to be epistatic to Y. W is probably epistatic to some fruit color genes (8, 13) but not to Y (2).

In summary, eight genetic loci having an effect on fruit exterior color have been identified. four of these loci affect fruit hue exclusively, whereas the other four affect intensity as well as hue (7). There are at least several other genetic loci which affect fruit exterior color, and these await identification.

In many articles on cucurbit genetics, “+” (wild-type, normal) notation has been used, as proposed by Robinson et al. (9). Such use may be appropriate when the mutant allele would have a clear deleterious effect on wild forms, such as the effect of B on young, developing fruit (1). However, in most cases, genetic studies have been conducted in crosses among cultivars rather than among wild and cultivated forms. In these crosses, the defining of “normal” is difficult, and given the lack of knowledge of the genetics of the wild form, the assignment of the wild-type symbol is usually guesswork. This can and has resulted in the assignment of the “+” to the mutant form of a gene. For example, the use of the symbol l+ instead of L in the latest gene list (2) is premature at best and a mistake at worst: Wild C. pepo has alternating, brown, intense-colored and narrow, light colored stripes. Conceivably, l-1St or some other, as yet unidentified, allele and not L-1 might be the wild type. Another example has occurred with regard to the locus responsible for the presence or absence of lignification of the fruit rind (phenyl ammonia lasses activity or inactivity, ref. 12). all C. pepo gourds have hard, lignified rinds. Mains (4) found that the hard rind of gourds was conditioned by a single dominant gene, and soft (not lignified) rind by its recessive allele. Robinson et al. (9) assigned the symbol Hr to this gene. Perhaps inevitably, the symbol Hr+ was subsequently (14) used to designate the recessive allele for soft rind!! Therefore, in crosses among cultivars, it would seem to be more prudent to use upper and lower case symbols for alternative alleles of the various identified loci, at least until such time as the wild-type alleles have been clearly identified.

Literature Cited

  1. Balzac, F. A., R. W. Carl son, and J. L. Harper. 1979. Contribution to reproductive effort by photosynthesis of flowers and fruit. Nature 279:545-555
  2. Gene List Committee, Cucurbit Genetics Cooperative, 1988. Gene list for Cucurbita spp. Cucurbit Genet. Coop. Rep. 11:96-103
  3. Lobbers, D. 1969. The inheritance of white fruit and stem color in summer squash. Euphytica 18: 249-255.
  4. Mains, E. B. 1950. Inheritance in Cucurbita pepo. Papers Micah. Ac ad. Arts and Letters 36: 27-30.
  5. Paris, H.S. 1988. Complementary genes for orange fruit flesh color in Cucurbita pepo. HortScience 23: 601-602.
  6. Paris, H.S. and Y. Burger. Complementary genes for fruit striping in summer squash. Submitted for publication.
  7. Paris, H.S. and H. Nerson. 1986. Genes for intense fruit pigmentation of squash. J. Heredity 77:403-409.
  8. Paris, H.S., H. Nerson, Z. Karchi, and Y. Burger. 1985. Inheritance of light pigmentation in squash. J. Heredity 76: 305-306.
  9. Robinson, R.W., H.M. Munger, T.W. Whitaker and G.W. Bohn. 1976. Genes of the Cucurbitaceae. HortScience 11: 554-568.
  10. Scarchuk, J. 1954. Fruit and leaf characters in summer squash. J. Heredity 45: 295-297.
  11. Schaffer, A. A., C. D. Boyer, and T. Guiana. 1984. Genetic control of plastid arytenoids and transformation in the skin of Cucurbita pepo L. fruit. Theor. Appl. Genet. 68: 493-501.
  12. Schaffer, A.A., C.D. Boyer, and H.S. Paris. 1986. Inheritance of rind lignification and warts in Cucurbita pepo L. and a role for phenylalanine ammonia lyase in their control. Z. Pflanzenzucht 96: 147-153.
  13. Shifriss, O. 1981. Genetics and origin of the bicolor gourds. J. Heredity 46: 213-222.
  14. Shifriss, O. 1981. Origin, expression, and significance of gene B in Cucurbita pepo L. J. Amer. Soc. Hort. Sci. 106: 220-232.
  15. Shifriss, O. and H.S. Paris. 1981. Identification of modifier genes J. Amer. Soc. Hort. Sci. 106: 653-660.
  16. Sinnott, E.W. and G. B. Durham. 1922. Inheritance in the summer squash. J. Heredity 13: 177-186.

Contribution No. 2544-E, 1988 series, from the Agri. Research Organization, Bet Dagan, Israel