After reading this article you will learn about:- 1. Origin of Cucurbita 2. Species of Cucurbita 3. Botany 4. Pollination 5. Cucurbita Gene Index 6. Breeding objectives 7. Breeding Methods 8. Molecular Markers 9. Seed Production 10. Varieties.
Origin of Cucurbita:
The genus Cucurbita is native to the Americas. The centre of diversity is in the tropics near the Mexico-Guatemala border. The archaeological evidences indicate that these were widely cultivated in Southwestern United States, Mexico, and Northern South America in pre-Colombian times i.e. prior to 1492 A.D. C. pepo is native to north America.
Squash and pumpkin (Cucurbita spp, 2n = 2x = 40) are the words quite often used interchangeably. However, the term ‘squash’ is more commonly used for C. pepo which is eaten as an immature fruit. The term ‘pumpkin’ is normally applied to the edible fruit of any species of Cucurbita utilized when ripe as a table vegetable or in pies.
The cultivated species of Cucurbita are widely distributed throughout tropical, subtropical and temperate regions of both the New and Old World. In these areas squash and pumpkin are important in agriculture, but are considered as minor crops. Details are available in Robinson and Decker-Walters (1997).
According to FAO-2006 statistics, over 1 million ha of pumpkin, squash and gourds are grown annually with total production of nearly 19 million tons. Summer squash accounts for a small portion of this area and production. Among the largest producers of summer squash are Turkey, Italy, Egypt, Spain, USA and Mexico. China and India lead world production for Cucurbita species.
Species of Cucurbita:
There are 26 species of Cucurbita, five of which are domesticated, but genetically isolated from each other. The wild species contain many ecospecies which are separated’ geographically but are genetically cross-compatible. The details on five domesticated species are given in Table 29.1. The fruits are shown in Fig. 29.1.
Botany of Cucurbita:
The four cultivated species of Cucurbita are herbaceous annuals with viny growth and several runners. However, C. pepo has short internodes and bush type appearance. The vine (stem/runner) may be prickly or spiny, rounded or angled. There are often roots at the nodes. The tendrils are long and branched. Leaves are large, alternate, shallow to deeply lobed and palmate.
The flowers are large, showy with yellow or creamy corolla. The plant is monoecious. Flowers are unisexual and occur singly in the axils of the leaves. The staminate flowers are near the centre of the plant and have long, slender pedicels. The pistillate flowers have short, ridged pedicels and are distal to the staminate flowers. Tap root system is strong, efficient and goes substantially down in the soil.
The morphology of the peduncle is distinct enough to be used in determining species limits. In C. pepo, the peduncle is deeply furrowed and five to eight-ridged. In C. moschata, the peduncle is five ridged and flared at the point of fruit attachment.
In C. maxima, the peduncle is cylindrical or claviform but never prominently ridged. In C. mixta, the peduncle is basically five-angled, rounded, but not at all or only slightly, enlarged at the fruit attachment. In C. ficifolia, the peduncle is hard, smoothly angled, slightly flaring, but comparatively much smaller than in other species of Cucurbita.
Calyx and corolla are campanulate. There are three anthers. Filaments are partly free. Pistil is oblong or discoid, unilocular with three to five placentae. Style is thick. The stigmas are three each two-lobed. Fruit is pepo.
Varietal Groups in C. Pepo:
Summer squash (C. pepo) exhibits perhaps highest level of polymorphism for fruit size, shape and colour. Mature fruits of C. pepo can range from several centimeters in diameter to over 20 kg in weight. Fruit shape can vary from round to disc to very long.
The fruits can be smooth or warted, with or without longitudinal ribs (10 rounded protrusions), longitudinal grooves (shallow, narrow depressions, usually 20 in number), furrows (usually 10 deep, broad, angular depressions alternating with angular ridges of the same number) or wavy lobes (scalloping, usually 10 broad, rounded projections alternating with the same number of indentations).
External colour may be green, orange or yellow but colour can range in shading and intensity from almost black to almost white and can appear in potterns of longitudinal striping which can be broad and contiguous, narrow and non-contiguous, irregular, and/or in longitudinal bi-colour patterns, all superimposed on barely discernible to obvious light colour speckling.
The colour of mature fruit is most often light yellow-orange but can range from greenish white to intense orange. Paris (2008) has given 8 varietal groups in summer squash (Fig. 29.2).
Pollination of Cucurbita:
The pistillate and staminate flower buds are located a day before anthesis and are protected by tying the tip of corolla tube. The following morning, as soon as the pollen sacs dehisce, the staminate flower is taken out and pollen is applied on the receptive stigma of the pistillate flower. This is easily done by rubbing the anthers against the stigma. Pollination can be done from morning till noon.
For better fruit setting it is desirable:
(i) To pollinate first few pistillate flowers and
(ii) To remove the previously set fruit by open-pollination. Crossed fruits should be properly tagged and a stake may be placed adjacent to the pollinated fruit to mark its location.
Cucurbita Gene Index of Cucurbita:
Simply inherited characters in Cucurbita and their gene symbols are given in Table 29.2.
Breeding Objectives of Cucurbita:
1. High fruit yield
2. Early fruiting
3. First pistillate flower at early node number
4. High female to male flower ratio
5. Yellow or mottled skin of fruit
6. Non-ridged fruit surface
7. High antioxidants specially carotenoids.
8. Thick fruit flesh and small seed cavity
9. Round/oblong/flat round fruit shape
10. Orange flesh colour, rich in β carotene, the precursor of vitamin A
11. Resistance to powdery mildew, downy mildew, zucchini yellow mosaic virus, red pumpkin beetle
12. Tolerance to low temperature and saline conditions
Breeding Methods/Selection Criteria of Cucurbita:
Inbreeding:
Numerous experiments have failed to demonstrate marked inbreeding depression in Cucurbita, even after prolonged selfing, therefore inbreeding along with selection could be adopted as one of the breeding methods as applicable to any other cucurbit.
Hybrid Breeding:
Heterosis has been demonstrated in summer squash (C. pepo), hence hybrid breeding is also becoming popular in squash and pumpkin.
The common methods of commercial F1hybrid seed production are as follows:
(I) Manual Pollination/Use of Insects:
In this system female and male parents are planted in a ratio of several rows of female alternated with 1 row of male parent. Since Cucurbita flowers are large, male buds are detected and removed several days before anlhesis from the female rows. The female flowers of the female rows are hand pollinated by the male flowers of the male parent row or the female flowers are left for pollination by insects in nature.
Most Cucurbita types are monoecious. The blossoms open in the morning and are pollinated primarily by specially-adapted solitary bees. The large, connated corollas can be tied up both in male and female flowers to prevent pollinator entry the day before anthesis when the colour of the petals begins to turn to yellowish-orange. Male flowers produce big pollen grains which can be directly deposited on the fleshy stigmas the next morning.
Petals of the female flowers can then be tied up again until fruits are developed. Usually, manual pollinations are carried out with fresh pollen, although pollen from pre-anthesis flowers that are kept for a few days at low temperatures and high humidity can also be used.
(II) Use of Chemicals:
Robinson et al. (1970) carried out an experiment using 2-chloroethylphosphonic acid (ethephon) as a sex regulator in Cucurbita and this provided a basis for the economical production of hybrid seed. Production of male flowers on monoecious plants of Cucurbita pepo can be temporarily suppressed for 2-3 weeks by repeated sprays of ethephon at 250 ppm on young plants at the first true-leaf, third true leaf and fifth true leaf stage.
The application of 250 ppm of ethephon prevents the development of staminate flowers for extended periods, but does not affect the development of pistillate flowers. By the time two-three fertile fruits have developed on each mother plant the ethrel effect has gone. Further sprays would not be effective and at this stage, development of lateral male flowers is stopped by cutting off the plants growing point with a knife.
Thus, by arranging the seed field into alternate rows of treated and untreated plants, and harvesting only fruits from treated rows, hybrid seed can be produced in abundance with little hand labour. Shanon and Robinson (1979) used 2 applications of ethephon at 600 ppm at the 2 and 4 leaf stage and reported complete male suppression during the fruiting stage. Ethephon is being used for commercial production of hybrid seeds in squash.
Interspecific Hybridization:
Whitaker and Robinson (1986) have summarized the information as follows on the basis of crosses reported between 4 annual species of Cucurbita.
(i) F1 hybrids can be obtained with difficulty between any two species.
(i) Such hybrids are normally sterile.
(iii) There is no evidence for spontaneous crossing among these four cultivated species.
(iv) Some of the amphidiploid lines of C. maxima x C. moschata have been found to be reasonably fertile and produce fruits of quality comparable to that of baking squash.
(v) Seeds of interspecific hybrids of C. moschata X C. maxima are on sale by Sakata Seed Company, Japan.
(vi) Interspecific aneuploids can be produced as follows:
where M = C. moschata
P = C. palmata
This procedure has promise as a method for transferring genes for desirable traits from one chromosome of wild species to cultivated ones without simultaneously transferring undesirable genes on other chromosomes.
Rapid Propagation of C. pepo:
Tissue culture has been used to produce rooted plantlets of cucumber from floral buds, axillary buds and hypocotyl explants. Plants have also been regenerated from shoot buds of watermelon and from hypocotyl and cotyledon explants of pumpkin as reviewed by Pink and Walkey (1984). These scientists have reported a tissue culture technique for the rapid multiplication of C. pepo clones.
Meristem-tips from seedlings of cultivar ‘Cinderella’ were grown initially on MS medium containing 2.56 mg/litre Kinetin and 8 mg/litre IAA, and then transferred to experimental media. Maximum shoot proliferation occurred on MS medium containing 1 mg/litre BA and no auxin. Cultures were rooted after 2-3 weeks on MS medium containing 8 mg/ litre IAA and no cytokinin.
Molecular Markers of Cucurbita:
The development of molecular tools for use in breeding Cucurbita species is in early stages. All Cucurbita species have 20 pairs of relatively short chromosomes as compared to seven pairs in cucumber, 11 in water-melon and 12 in melon.
Most morphological traits in Cucurbita appear to be unlinked and many markers are required to adequately map the genome. Weeden and Robinson (1986) used isozyme markers to build the first map of Cururbita.
It was based on the F2 of cross C. maxima X C. ecuadorensis and contained 11 isozyme loci in five linkage groups. Lee et al., (1995) developed a RAPD map of an F2 population of C. pepo x C. moschata interspecific hybrid. This map was small consisting of 28 markers in five linkage groups.
No morphological trait or other types of markers were included on the map. Brown and Myers (2002) used RAPD markers to construct a partial map of the Cucurbita genome.
The mapping population was the BCt progeny of the C. pepo L. yellow straight-neck inbred A0449 and the tropical Cucurbita moschata landrace Nigerian local. A 0449 was the recurrent parent. This cross was chosen because summer squash is considered to be economically important and C. moschata contains the valuable traits to be introgressed into summer squash. Further, the interspecific cross is expected to show greater variation.
The map thus generated contained 148 RAPD markers in 28 linkage groups. Loci controlling five morphological traits were placed on the map. The map covers 1954 cM which is estimated to be 75% of the Cucurbita genome.
The qualitative traits placed on the map include the B gene for fruit which turns yellow before anthesis, the M gene for silver mottling of leaves, and a locus controlling the intensity of rind colour on maturity.
Quantitative trait loci (QTL) associated with fruit shape and the depth of the indentation between primary leaf veins were identified. Cucurbita DNA is extracted using the modified CTAB (hexadecyl trimethyl ammonium bromide) technique as described in Brown et al. (1998).
The PCR mixture includes:
1. 30 ng of plant genomic DNA
2. 1.5 mM MgC12
3. 0.1 mM dNTPs
4. 3 p mole of primer (Operon or Univ. of British Columbia – UBC, etc.)
5. 0.5 units of Taq DNA polymerase
6. 1 X Taq buffer
7. In a total volume of 15 p./.
The amplification programme usually followed is:
1. 2 min at 94°C
2. 5 cycles of 5s at 94°C
3. 1 min at 37°C
4. 30 s at 54°C
5. 7 min ramp to 72°C
6. 2 min at 72°C
7. 30 cycles of 5 s at 94°C
8. 1 min at 37°C
9. 30 s at 54°C
10. 2 min at 72°C
11. 15 min at 72°C
12. Separation of PCR product on 1.5% agarose in 0.5% TBE buffer
Only strong repeatable bands are scored. For map construction, Mapmaker version 3.0 using the Group, Compare, Try, and Ripple commands is used. The map order is double-checked using the order command. A LOD score of 5.0 and a maximum recombination frequency of 30% are used to determine linkage groups. The relationship of quantitative traits to the mapped loci is analyzed using QTL Cartographer.
Molecular markers have also been used to describe intraspecific genetic diversity. In C. pepo, studies with isozymes allowed the grouping of wild and cultivated taxa in subspecies and varieties. Similar groupings were found with DNA markers, like RAPD, SSR, ISSR, AFLP and SRAP as summarized by Ferriol and Pico (2008).
In C. moschata, RAPDs, SRAPs, and AFLPs have been used to analyse the diversity of landraces in different centres of diversity. Fewer studies have been performed in C. maxima. More recently RAPDs. AFLPs and SRAPs have been used to study the diversity of Spanish C. maxima landraces.
The first map of Cucurbita was constructed from 11 isozyme loci in five linkage groups based on the F2 of the cross C. maxima x C. ecuadorensis. Since then, RAPD markers have been the most used for developing Cucurbita molecular maps, with the inconvenience that these markers are dominant and population specific.
Lee et al (1995) mapped 28 RAPD markers into five linkage groups from an F2 of C. pepo x C. moschata. The largest map, covering about 75% of the Cucurbita genome, was constructed by Brown and Myers (2002) as explained above.
Marker-assisted selection can only be useful for a few genes responsible for the expression of disease resistances, such as isozymes for WMV in crosses between C. maxima and C. ecuadorensis. Markers linked to ZYMV are only at a preliminary stage.
On the other hand, a high number of morphological traits have been linked to molecular markers, like isozymes. RAPDs, and SSRs. The genus Cucurbita was one of those in which genetic engineering was first applied. In fact, one of the first transgenic crops released was C. pepo with resistance to ZYMV.
Seed Production of Cucurbita:
Cultivar Description:
There are open-pollinated and hybrid cultivars. The following characteristics are based on UPOV (1991) which considers vegetable marrow, squash and courgette together, and as described by George (1999).
1. Plant:
Shape of cotyledon: elliptic, broad elliptic, or obovate
Growth habit: bush, semi-trailing or trailing
Branching: absent or present
Degree of branching: weak, medium or strong
Bush cultivars only: attitude of petiole (excluding lower external leaves) : erect, semi- erect or prostrate
2. Stem:
Green colour: light alone, dark alone or light and dark
Tendrils: absent or rudimentary, or well developed
3. Leaf blade:
Size: small, medium or large
Incisions: absent or very weak, weak, medium, strong or very strong
Green colour of upper surface: light, medium or dark
Marbling: absent or present
Area covered by marbling: very small, small, medium, large, very large
4. Petiole:
Green colour: light, medium or dark
Length: short, medium or long
Thickness (at base): thin, medium or thick
Shape of cross-section: round or angular
Number of prickles: few, medium or many
5. Female flower:
Ring at inner side of corolla: absent or present
Colour of ring at inner side of corolla: green, yellow or green and yellow
Intensity of ring colour inner side of corolla: weak, medium, strong
Length of sepal: short, medium or long
Colour of pistil (before opening): yellow or orange
6. Male flower:
Expression of coloured ring at inner side of corolla: absent or very weak, weak, medium, strong or very strong
Colour of ring at inner side of corolla base: green, yellow or green and yellow
Length of pedicel: short, medium or long
Diameter of pedicel: small, medium or large
Colour of pedicel: light green, medium green or dark green
Grooving of pedicel: weak, medium or strong
Hariness of pedicel: weak, medium or strong
Length of sepal: short, medium or long
7. Young fruit:
Length: short, medium or long
Maximum diameter: small, medium or large
Ratio length/maximum diameter: small, medium or large
Neck: absent or present
Curving of neck (on normally set fruits): absent or present
Number of colours: one, two or three
Major colour: whitish, yellow, orange, green, whitish and yellow, whitish and green or green and yellow
Intensity of major colour: light, medium or dark
Shape of cross-section: round, ribbed, angular or lobed
Warts: absent or present Glossiness: weak, medium or strong
Colour of ribs compared to colour of rest of fruit: same or different
Colour of linear bands on ribs: whitish, yellow, orange or green
Mottling: absent or present
Type of mottling: only diffuse; diffuse and in patches; diffuse and in linear bands; or diffuse, in patches and in linear bands
Colour of mottling of skin: whitish, yellow, orange or green
Conspicuousness of mottling of skin: weak, medium or strong
Size of flower scar: small, medium or large
Protrusion of flower scar: absent or present
Length of peduncle: short, medium or long
Main colour of peduncle: only green, only yellow, or green and yellow
Intensity of main colour of peduncle: light, medium or dark
Mottling of peduncle: absent or present
Conspicuousness of mottling of peduncle: weak, medium or strong
8. Fruit:
9. General shape: scallop-shaped, transverse elliptical, globular, top-shaped, ovoid, elliptical, cylindrical, club-shaped, pear-shaped or crook-neck
Fully developed in size, major colour: whitish, cream, yellow, orange or green
Ripe, major colour: whitish, cream, yellow, orange or green
Fully developed in size, intensity of major colour: light, medium or dark
10. Ripe fruit: intensity of major colour: light, medium or dark
11. Fruit fully developed in size: whitish, yellow, orange or green
12. Ripe fruit: colour of stripes on ribs: whitish, yellow, orange or green
13. Fruit fully developed in size: colour of mottling of skin: whitish, yellow, orange or green
14. Ripe fruit : colour of mottling of skin: whitish, yellow, orange or green
15. Fruit: maximum length: very short, short, medium, long or very long
16. Fruit: maximum diameter: small, medium or large
Ratio length/maximum diameter: very low, low, medium, high or very high
17. Seed:
Size: small, medium or large
Shape: narrow elliptic, elliptic or broad elliptic
Colour: whitish or yellowish
Isolation Distance:
1. Breeder/foundation seed – 800 m
2. Certified seed – 400 m
Seed Yield:
1. 500-800 kg/ha
2. 1000 seed weight – About 200 g
Varieties of Cucurbita:
Punjab Chappan Kaddu 1:
It is an inbred selection from the segregating local material of Punjab and recommended for cultivation in 1982. Plants are bush type, foliage thick and erect, leaves non-lobed and green without white specks, petiole and leaves hairy, fruits green, disc shaped, mildly ribbed with flat stem-end and attractive, average fruit about 800 g and a plant bears about ten fruits, early maturing and is ready for first harvest in about 60 days from sowing.
It has a predominant female tendency, field resistance to downy mildew and tolerance to CMV, powdery mildew and red pumpkin beetle. It has high yield (about 200-225 q/ha). It has been released by Punjab Agricultural University, Ludhiana. It belongs to C. pepo.
Patty Pan:
It is an introduction from United States and recommended for cultivation during 1972. Fruits disc shaped, chalky white, tender and very attractive at edible stage. It is a short duration variety with yield of 200 q/ha in 85 to 90 days. It is C. pepo.
Early Yellow Prolific:
It is an early variety with bush type plant and medium size fruits. Fruits are warted, tapering, orange-yellow on maturity. It was introduced by IARI regional station, Katrain. It is C. pepo.
Australian Green:
An introduction, very early, bush type, fruits dark green with longitudinal stripes of white colour all over. Fruits are 25 to 30 cm long. There are 15 to 20 fruits/plant. Fruits are tender at edible stage. Yield is 150 to 165 q/ha. It is C. pepo and was introduced by IARI regional station, Katrain.
Pusa Alankar:
It is an F1 hybrid between EC 27050 and Sel-lPI-8 (a derivative from cross between Chappan Kaddu and Early Yellow Prolific), early maturing, uniform dark green fruits with light coloured stripes, slightly tapering towards the stem end, tender delicious and matures in 45 to 50 days. It has been bred at IARI regional station, Katrain.
Arka Chandan:
It is an improvement of a collection from Rajasthan (IIHR-105) done at IIHR, Bangalore. It was identified in 1987 for zone VIII. Fruits are round with flat blossom end and medium sized (2-3 kg). Rind colour is light brown when mature. Flesh is thick, firm, sweet (TSS 8-10%), bright orange and rich in carotene (3331 IU/100 g). It has pleasant aroma. Cooking and keeping qualities are good. Yield is 300 q/ha, in 120 days. It belongs to C. moschata.
Cm-14:
It was developed from a local collection at KAU, Vellanikkara and identified in 1987 for zone VI where it gave 21% increased yield over control. It has spreading plant habit, fruits flat round in shape and weigh about 6 kg each, green in colour with shallow furrowed surface. Flesh thickness is about 4.30 cm. Average fruit yield/plant is 15 kg. It is C. moschata.
Pusa Biswas:
It is a local selection of line SM-107 at IARI, New Delhi. It was identified in 1987 for zones V and VII. It has vigorous vegetative growth, dark green leaves with white spots including veins. Fruits are light brown with thick, golden yellow flesh and are spherical. Average weight is about 5.0 kg. It matures in 120 days. Average yield is 200 q/ha. It is C. moschata.
Arka Suryamukhi:
It is an improvement done at IIHR Bangalore over a local collection (IIHR-79) from Mangalore. It was identified in 1987 for zone VIII. Fruits are small, round with flat ends, deep orange with creamy white streaks on the rind, each weighing 1-1.5 kg. Flesh is orange yellow. Keeping and transport qualities are good. It is tolerant to fruit fly. It yields 300 q/ha in about 100 days. It is C. maxima.
Narendra Amrit:
It is C. moschata developed and released at NDUAT, Kumarganj, Faizabad. Fruits are flat- round with uniform cream colour skin. It is rich in vit. A and has excellent flavour. Private seed companies in India have not gone for vigorous efforts in breeding and seed production of hybrids in pumpkin due to low market volume in total seed as such in pumpkin.