Cucurbit Genetics Cooperative Report 10:93-99 (article 51) 1987
Notes on Squash Breeding
21 Walter Ave., Highland Park, NJ 08904
Interspecific hybridization (11). The cross C. pepo
x C. moschata is important from both theoretical and practical
points of view. The natural reproductive isolation between these species
is not as strong as that between other species of Cucurbita. Therefore,
it might be possible to identify some of the major genetic barriers which
keep these species apart. Furthermore, this cross can result in gene exchanges
of economic value. Of special significance is the fact that, unlike the
extreme susceptibility of C. pepo to cucurbit pests, C.
moschata is an excellent source of resistance. Unfortunately, my
notes on this cross are fragmentary.
In 1947 1 obtained hybrid seed from crossing a bush line of 'Table Queen'
(C. pepo) and 'Butternut' (C. moschata), using
the former as seed parent. The cross was made at Fordhook Farms (W. Atlee
Burpee Co.), Doylestown, PA, early in the morning (5-6 AM) of a cloudy fall
day. The 20 cross-pollinated flowers of 'Table Queen' developed into mature
fruits and all of them contained viable hybrid seed, ranging from 3 to 25.
Parenthetically, an earlier report (2) suggested that 'Table Queen' may
not be compatible with C. moschata. My results support the
conclusion of some other breeders that favorable environmental conditions
are essential for successful hybridization. The F1 plants were
large, semi-bush, and vigorous. The average fruit weight of the parents
was 700g (Table Queen) and 1000g (Butternut) and that of the F1,
5000g, a remarkable heterosis for fruit size in squash. The F1
fruit had a large central cavity and the "flesh" was of poor quality.
The F2 consisted of many male-female sterile segregates which
exhibited serious growth abnormalities.
Some of the consequences of a similar cross were observed during 1979-1983.
This cross was between 'Jersey Golden Acorn', BB, bearing golden,
acorn- shaped fruits (C. pepo), and 'Burpee Butterbush', B+B+,
bearing tan, bottle- shaped fruits with short thick "neck" (C.
moschata). The fruit of the F1, BB+, was bicolor
and intermediate in shape. The average fruit of the parents was 680g and
700g respectively, and that of the F1, 1041g. The F2
(n = 283) was highly variable in fruit characteristics, in degree of fertility,
and perhaps in proneness for parthenocarpy. With respect to fruit weight,
the F2 ranged from 146g to 2080g. But the most conspicuous features
of the F2 were severe growth abnormalities. Three other comments
are of interest. First, the BC1, F1, BB+, x 'Jersey Golden
Acorn', BB, consisted of a few BB plants which produced deeply
scalloped disk-shaped fruits (similar to those of 'White Bush Scallop' of
C. pepo) of intense golden color. Second, some F2
segregates produced fruits of exceptionally thick flesh and very small central
cavity. Third, some segregates were male-female sterile; others were male
sterile early in plant development and male fertile later on; and still
others were self-incompatible, but cross-compatible with 'Jersey Golden
Acorn' as seed parent.
There is little doubt that the cross C. pepo x C.
moschata deserves critical analysis as well as use in breeding. But
it is evident that the ease by which F1, F2, and backcross
seed can be obtained does not reflect the degree of 'disharmony" between
the two genomes.
Intraspecific hybridization. Each of the economically important
species consists of several distinct groups of cultivars. Among other things,
these groups differ from one another in fruit characteristics and duration
of their artificial or geographical isolation. The short-term concerns in
the squash seed industry are to perpetuate the existing groups of cultivars
and to develop F1 hybrids from intra-group crosses. In the long-term,
it would be advisable to utilize the advantages of inter-group crosses.
First the F1 hybrid of inter-group crosses often exhibit greater
heterosis than F1 hybrids of intra-group crosses. However, the
development of such hybrids will require additional breeding efforts. Second,
some inter-group crosses generate tremendous genetic diversity without sterility
and growth abnormalities. These crosses will provide the breeder an access
to a larger portion of the gene pool including untapped potentially useful
genetic elements. It is expected that most of this diversity will be inferior.
But some genotypes will exhibit new useful traits; others will enhance the
economic value of known desirable traits; and still others will reduce the
expression of undesirable characteristics. This thought received its impetus
from studies of genes B and M as well as from studies of growth
habit, flowering, and sex expression.
Effects of B. This gene is widespread in C. maxima.
A gene of similar behavior was transferred from the bicolor ornamental gourds
to some of the major edible cultivars of C. pepo (7). In 1980,
the B genes of C. maxima and C. pepo
were transferred to C. moschata (9). And more recently, Paris
and his colleagues in Israel reported an additional B transfer, from
C. pepo and C. moschata.
Gene B conditions precocious chlorophyll depletion in fruit and
this leads to precocious yellow pigmentation. Furthermore, B can
bring about precocious depletion of chlorophylls in all other normally photosynthetic
organs. In a broader sense, B can exhibit many "secondary effects"
some of which are deleterious and others, beneficial. There is growing evidence
indicating that the secondary effects result from interactions between B
and other elements in the gene pool, and that the detrimental and beneficial
effects are separable in breeding operations. Some of these elements are
responsible for distinct phenotypes of their own, but most of them are invisible
phenotypically except through their interactions with B. These "invisibles"
are difficult to identify. Indeed, very few valuable invisibles (modifiers
or regulators) have been identified (8, 10). Therefore, in cases in which
the invisibles are not identified, the beneficial effects of B are
surmised from comparison between isogenic or near-isogenic lines, BB
and B+B+, of different backgrounds. But incisive tests are often
There exists a wide range of variation in the levels of fruit carotenoids
among standard, B+B+, cultivars of Cucurbita. Previous observations
(1956- 1962) of the B effect on fruit color in C. pepo
suggested that this gene can increase the carotenoid content, but that increases
of large magnitudes can be achieved through interactions between B
and genes for dark green fruit color such as L, and provided gene
inhibitors are not present (7). Subsequent chemical analyses by S. A. Garrison
(in reference 6) and others (Table 1; see also reference 5) essentially
confirmed this suggestion. Similar interactions exist in C. maxima
and C. moschata.
The mechanism governing the synthesis of different fruit carotenoids
is not understood. The specific role of environmental factors is not known.
And most of the genes (including enhancers and inhibitors) which control
pigmentation have not been identified. Weak or strong inhibitors appear
to act a few days after B. They affect either the external portions
of the fruit or both. They study of Schaffer et al (4) and other observations
pose a number of intriguing questions concerning the role of B on
plastid transformation in different tissues. It seems that a product of
B, perhaps a diffusible substance (7), is the signal that regulates
The first commercially available carotenoid-rich cultivar of summer squash
was Burpee Golden Zucchini. It was developed by T. C. Torrey and introduced
in 1973. This cultivar is similar to our PFZ (Precocious Fordhook Zucchini)
breeding line developed in 1963. Both lines originated from the same germplasm.
The use of cross IL-B x NJ-B (9) in breeding could lead to the development
of nutrient-rich BB cultivars adapted to mechanized harvesting. Except
for their green leaf blades, all other plant parts of these cultivars would
be precociously golden. Such cultivars might be valuable to the food processing
industry for various purposes, including dehydrated meal; stems for feed
supplement, and fruit and seeds for human consumption.
In summer squash, the fruits of several BB lines have firmer flesh
textures and exhibit new pleasing flavors to the extent not found in any
of the known standard B+B+ cultivars. Thus, gene B could be
valuable for the development of new cultivars that are better adapted for
use in freezing. 'Blondie' is the first BB+ hybrid designed specifically
for this purpose (David Groff, personal communication). The fruits of some
"precocious" winter squash, such as 'Jersey Golden Acorn', BB+
have a flavor reminiscent of yellow sweet corn. This is a new incipient
trait which might be greatly reinforced through incorporation of genes for
higher sugar content and absence of remnant of bitterness. I see the possibility
for the development of dual-purpose B cultivars of high quality in
C. pepo and C. maxima. These cultivars will
serve both as "summer" and "winter" squash. And among
them there will be cultivars used in food as water chestnut.
In 1966, 1 suggested that gene B can enhance female expression
of the background of one of the summer squash cultivars. Since then this
effect of B has been confirmed in repeated comparative tests of isogenic
lines, BB and B+B+, of Early Prolific straightneck background.
Results of one of these tests are presented in Table 2. The BB line
involved is known as PEP. It was distributed among many breeders and would
be helpful if some of them publish their own findings on this subject. PEP
is one of the parents of "multipik", a hybrid known for its strong
female expression. As far as I know, the most strongly female line among
the monecious cultivars of Cucurbita is inbred NJ260, BB.
Yet, the effect of B on female expression was not reported or has
not been clearly evident in other backgrounds.
The recently synthesized closely related gynoecious lines (U.S. Patent
pending), BB and B+B+, are late-flowering and 100% female
in some environments, but they differentiate varying proportions of male
flowers in other environments. Although we have not identified all the non-genetic
factors which may promote late-flowering and male expression, it is clear
that high temperatures favor both. The fact that B was not essential
for the synthesis of these female lines does not necessarily exclude the
possible role of B as an accelerator of pistillate flower differentiation
in some backgrounds.
One of the advantages of the new gynoecious lines is that they can be
used in strongly female monoecious cultivars which branch and differentiates
their pistillate flowers more or less simultaneously, a desirable trait
for both manual and mechanized harvesting.
Although gene B is a highly stable element in some backgrounds,
it originated through the consequences of nuclear instability and is prone
to instability in some other backgrounds. Its behavior recalls the behavior
of a mobile element. This gene can manifest at least 2 kinds of variegation:
one kind appears as a developmentally fleeting, imprecise, and unpredictable
pattern seemingly due to phenotypic plasticity; and the other kind appears
as a precise and predictable pattern due to mutation of B. Fruits,
leaves and other parts of the plant may be affected. As a result, B
is a potential source of variations some of which may be valuable in breeding
of both edible and ornamental cultivars.
Finally, according to my observations all the known deleterious effects
of gene B can be supressed without interference on the expression
of its beneficial effect. These observations support the following hypothesis.
A potentially deleterious gene can become beneficial if two requirements
are fulfilled. First, the gene must have one or more beneficial effects.
Second, the gene pool must carry elements which supress the deleterious
effect independently of the beneficial one.
Virus infection. Some late-maturing pumpkins of C. pepo
possess mild field tolerance to virus infection. In addition, the extremely
intense dark green foliage of some breeding lines is more tolerant to virus
infection than their fruit, particularly golden fruit.
The implication of gene B in fruit tolerance to the adverse effects
of virus infection is difficult to assess. The best documented case is that
of 'Multipik", introduced in 1981. According to T. H. Superak (personal
communication), 'Multipik', BB is near-isogenic to 'Golden Girl',
B+B+, but the former is less adversely affected by CMV-induced fruit
symptoms than the latter. In addition, Adlerz et al (1) reported that the
fruits of 'Multipik' are partially tolerant to WMV-2. And Paris et al (3)
noted fewer virus- induced fruit symptoms (presumably CMV and WMV-2) in
'Goldy', BB+, than in 'Gold Rush', BB+. Obviously, if B
is a contributing factor it alone cannot account for the difference in response
between these hybrids.
Another issue worthy of further exploration is the possible escape mechanism
of the silvery-leaf fruit trait against aphids (see communications on this
subject in previous issues of CGC Rpt.). It is known that silvery leaves
reflect more light than green leaves. And the inference is that greater
light reflection tends to repel aphids. Our first silvery line was NJ260,
BB. This line, however, proved to be be highly susceptible to wilting
under mild water stress conditions in the field. We transferred the genotype
for the silvery trait to 'Precocious Caserta', BB, and found that
the resulting silvery line, BB, is not more susceptible to wilting
than standard B+B+, green cultivars.
The full genotype of the silvery trait has not been identified. The hypothesis
is that it consists of gene M, for leaf mottling, and several modifiers.
The silvery phenotype greatly fluctuates, in response to non- genetic variations,
from different degrees of mottling (silvery spots) to uniformly silvery
leaves. Attempts should be made to stabilize this trait through crosses
between silvery lines which exhibit developmentally different modes of expressions.
Future direction. The long range goal in squash breeding is to
restructure the Cucurbita genome through interspecific and intraspecific
crosses for the purpose of transforming squash into a staple food crop of
Table 1. Levels of carotenes and xanthophylls in ripe fruits of B+B+
and BB inbreds of four cultivar backgrounds of Cucurbita.
Each determination was based on a composite fresh sample obtained from the
mesocarp of ten fruits. All inbreds were grown in a replicated test. From
field studies, New Brunswick, New Jersey, 1978.
z Key to cultivar backgrounds: EP ='Early Prolific Straightneck', FZ
- 'Fordbook Zucchini', TK = 'Table King', all three of Cucurbita
pepo; GD ='Golden Delicious' (Munger's strain) of Cucurbita
maxima. B+B+ and BB inbreds of GD may not be entirely
y Determinations were made by the New Jersey Feed Laboratory, 910 Pennsylvania
Avenue, Trenton, NJ 08603, based on AOAC procedure.
Table 2. Female expression in B+B+ and BB inbreds of 'Early
Prolific Straightneck' background. The data are based on a replicated test
of 10 plants per inbred. From field studies, New Brunswick, New Jersey,
a The difference in cumulative number of pistillate flowers between BB
and B+B+ plants is statistically significant at each of the six weeks
- Adlerz, W. C., G. W. Elmstrom. and D. E. Purcifull. 1985. Response
of 'Multipik' squash to mosaic virus infection. HortScience 20:892-893.
- Erwin, A. T. and E. S. Haber. 1929. Species and variety crosses in
cucurbits. Bulletin No. 263, Iowa Agricultural Experiment Station, Ames,
- Paris, H. S., Z. Karchi, H. Nerson and Y. Burger. 1983. Yield and
yield quality in precocious yellow zucchini cultivars. HortScience 18:724-726.
- Schaffer, A. A., C. D. Boyer and T. Gianfagna. 1984. Genetic control
of plastid carotenoids and transformation in the skin of Cucurbita pepo L. fruit. Theo. Appl. Genet. 68:493-501.
- Schaffer, A. A., H. S. Paris and I. M. Ascarelli. 1986. Carotenoid
and starch content of near-isogenic B+B+ and BB genotypes
of Cucurbita. J. Amer. Soc. Hort. Sci. 111:780-783.
- Shifriss, O. 1974. Manifestations and use of gene B in Cucurbita.
In the proceedings of the 19th International Horticultural Congress. Vol.
1, page 100. Warsaw.
- Shifriss, O. 1981. Origin, expression and significance of gene B in Cucurbita pepo L. J. Amer. Soc. Hort. Sci. 106:220-232.
- Shifriss, O. 1982. Identification of a selective suppressor gene in Cucurbita pepo L. HortScience 17:637-638.
- Shifriss, O. 1986. Relationship between the B genes of two Cucurbita species. Cucurbit Genet. Coop. Rpt. 9:97-99.
- Shifriss, O. and H. S. Paris. 1981. Identification of modifier genes
affecting the extent of precocious fruit pigmentation in Cucurbita pepo L. J. Amer. Soc. Hort. Sci. 106:653-660.
- Whitaker, T. W. and G. N. Davis. 1962. Cucurbits. Interscience
Publications, Inc., New York.
* Some of the above notes, including tables 1 and 2, were taken from a
six year old manuscript originally written for a cucurbit book whose publication
has been delayed.