PLS221                                        Instructor: Carlos F. Quiros

Cucurbitacea: Cucurbita spp., Squash, pumpkins, gourds .

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(slides)



Species origin domestication and distribution

Cultivated species

Classification

Evolutionary relationships among species

Cultivated species

Interspecific hybridization and cytogenetics

Male sterility

Alien addition lines

Inheritance of important traits

Genetic markers: isozymes and RFLPs

Breeding objectives

Bridging species and utilization of wild species

Disease resistance

Biotechnology



Origin and domestication:

Genus Cucurbita is very rich in cultivated species, which contain the highest genomic number of all cultivated cucurbitaceae species

2n=2x=40

Most likely that polyploidy is involved in the evolution of these species.

This genus includes crops such as squash, pumpkins, and gourds.

These are indigenous of the Americas, mostly Mexico and Central America, branching out to North and South America

The Squash-Bean-Corn-Potato complex has a prominent place in agriculture, instrumental in the development of pre-Columbian civilizations

Archeological remains in a Oaxaca cave indicate that Cucurbita species were cultivated approximately 10,000 BC and in Ecuador 12,000 years ago.

Squash is an important source of provitamin A due to high carotene content. Cucurbitacins seem to have anti-inflammatory and anticancer activities.

27 species of Cucurbita have been described. These include immensely variable forms, fruits of different shapes, colors, and sizes.

Main cultivated species and their classification:

There are 5 cultivated species which are separated by hybrid sterility barriers. Cucurbita species  are monoecious. The cultivated species can be differentiated from each other by different morphological traits, which are very diagnostic such as trichomes, fruit peduncle and seed margin, the most important ones.
 

The cultivated species were domesticated at different times and in different areas.

Annual species

C. moschata: e.g: winter squash: Butternut.

C. pepo: pie pumpkins,summer squash: Zucchini,  scallop, crookneck, cocozelle. This is the most variable and economically most important species, especially in North America.

C. argyrosperma (syn. C. mixta): winter squash: cushaw type (long, curved neck)

C. maxima: pumpkins, baking squash, buttercup, hubbard

Summer types: bushy habit, Winter types: vine habit.

Perennial species

C. ficifolia: Fig leaf gourd, lacayote

The annual species are classified in two main types, squashes and pumpkins, however, this separation is often not clear-cut.

Squash: fruits with fine grain flesh, mild flavor. For example varieties of C. pepo consumed immature, all baking cvs. of C. maxima. Some varieties C. pepo (acorn) and C. moschata (buttercup) baked at mature stage.

Pumpkin: fruits with coarse flesh, round shape, strong flavor, used for pies, and seed consumption.

Oldest remains for these crops are recorded for C. pepo in Oaxaca, 9000 yr. old.

  Gene pools and Distribution:

The cultivated species and their wild relatives can be classified in two large classes, and each of these in groups:

A) Mesophytic: encompass all the cultivated species:

1) C. ficifolia

C. ficifolia Found in the mountains of No. Mexico to No. Argentina. It is the most distinctive of the five cultivated species, with peculiar fruits resembling oblong watermelons and broad black seeds. Generally grown at much higher altitudes that the other domesticated species. Popular also for many centuries in Asia. High altitude species.

The origin of this species is unknown, no ancestral wild species have been found. It is predicted that it may be somewhere in the Andes.

C. ficifolia is the most removed from the other cultivated species. It has the widest range of distribution, is a strong climber and tends to perennate. Also it shares some morphological traits of the xerophytic species.

2) C. andreana-C maxima group:

Found in So. America. Relatively recent domestication, approximately 1200 years ago. Includes winter squash types such as `hubbard', banana. buttercup, turban etc.

A diverse range of landraces occur in So. America

There are two wild species of importance related to C. maxima: C. andreana and C. ecuadorensis.

C. andreana is the probable ancestor of this cultivated species since it produces fertile hybrids with C. maxima.

3) C. moschata:

It includes butternut squash, golden cushaw and most of the common cultivars of low land Central America and So. America. Domesticated 5000-7000 years ago. Ancestral wild species to C. moschata are unknown, perhaps they may be found in northern Colombia . C. moschata has the widest range of distribution, also found in Central and So. America.

The origin and evolution of the cultigens C.argyrosperma and C. moschata may be closely intertwined. These two species functions as a connecting link between wild and the rest of the cultivated species.

4) C. fraterna-texana- pepo group:

These are found in Central and No. America.

C. pepo includes the best known squashes in the US, with phenomenal range of fruits types, such as orange jack o lantern pumpkins, zucchinis, spaghetti and many ornamental gourds. This species is the oldest, with a domestication date of at least 8000 years.

5) C. sororia-C. argyrosperma group:

It includes Japanese pie pumpkin, white cushaw and various Mexican and Central America cultivars. Some are grown for their seeds more than flesh. C. sororia seems to be the ancestral species. It  spreads from Mexico to Nicaragua.

Gene flow by natural hybridization in Mexico have been reported for wild C. fraterna and cultivated C. argyrosperma. Cucurbitacins were followed as markers. Isozymes indicated also gene flow from from C. pepo to C. argyrosperma which explains high identity observed between these two cultivated species.

Species in the Sororia group seem to have an important role in the evolution of Cucurbita species, originating C. moschata and C. argyrosperma. C. argyrosperma can be crossed with all the species in the group as well as with C. moschata.

6) C. lundelliana group:

Two wild species are the most important in this group: C. lundelliana and C. martinezii (fruit used as detergent).

B) Xerophytic

7) C. digitata group:

It includes five wild species of which  C. palmata is the most important one.

8) C. foetidissima group:

It includes three wild species, of which C. foetidissima (Buffalo gourd) is the most important.

Recently this species was attempted to domesticate because of its abundant seeds rich in proteins, roots rich in carbohydrates and  xerophytic nature.

Except for C. andreana and C. ecuadorensis, all wild Cucurbita spp. occur north of El Salvador to Southern United States. The rest of the species were perhaps introduced by man to So. America.

C. maxima

C. andreana

 
Species domesticated independently and C. pepo likely domesticated more than once. Mitochondrial genes used for phylogenetic determination of domesticated and wild species (Sanjur et al. 2002).

 

 

 


C. moschata

 


 

 

 

 

 

 

 

 

 

 


Cytogenetics and interspecific hybridization:

Most of the cytogenetic research in Cucurbita species derive from interspecific hybridization  studies. These have resulted in the synthesis of a few amphiploids and alien addition lines. On the basis of chromosome pairing, different genomes have been tentatively assigned.

Alien addition lines:  

  MMMM (C. moschata)x PPPP (C. palmata)

          F1   MMPP

(Colchicine treatment of diploid MP is avoided, because it was too weak to tolerate colchicine).

          MMPP x MM

                  MMP x MM

Interspecific aneuploids (addition lines moschata/palmata) MM + P1....P20

Three alien addition lines recovered each with a different phenotype. These were used by Weeden to locate two groups:  fumarase and hard ring (Hr gene) ; Pgi and Got. These two were found linked also in C. pepo, C. maxima and C. ecuadorensis demonstrating colinearity for this segment in all 4 species.

Other hybrids:

Hybrids of C. maxima x C. ecuadorensis (virus resist):

Relatively fertile hybrid, but both F2 and BC display reduced fertility. This is attributed to hybrid breakdown or dysgenesis.

Important traits:

Sex determination: There is less variation on sex types in Cucurbita species than in other cucurbits. Most of the species are monoecious, with a few rare andromonoecious mutants due to recessive genes. The genus might be comparatively recent, compared to other cucurbits, because the presence of monoecious plants only indicates that Cucurbita species have not have enough time to evolve alternate sex as other cucurbits.

A gynoecious mutant has been reported for C. foetidissima, but in none of the cultivated species. It could be very useful for hybrid seed product.

Commercial hybrid seed is produced by application of 500 ppm of ethephon to seedlings,  preventing the development of staminate flowers in the line used as female.

Important traits

Fruit color: very complex, it changes with fruit maturity, it can go from light green to dark green ending in bright yellow. Dark fruit, allele D, light fruit color d.

Rind striping: Different types of stripes determined by multiple alleles at L-1 and L-2 loci. L-1 and L-2 are complementary resulting in intense fruit color, the recessive ones l-1 and l-2 (light coloration) produce light color over the entire fruit, a series of alleles dominant or co-dominant to l-1 and l-2 produce different types of stripping (l-1 alleles Bst, iSt, l-2 allele R produces reverse stripping (light color stripped over dark background). (Paris H, 2009)

Locus B bicolored fruits

Large array of colors, at least 9 genes involved.

Fruit shape: Disc shape due to complementary action of two dominant alleles D1 and D2, spherical, single dominant at either locus, elongated fruit results from double recessive.

Neck dominant to neckless

Naked seed, lack seed coat, inhibition of lignin synthesis in cell wall, determine by single recessive n.

Bushy mutant type determined by single gene Bu.  In C. pepo and C. maxima this trait is important to produce more food in less space

Isozymes: Most of this markers have been developed for C. pepo .

Work by Ignart and Weeden (1987), at least 7 loci studied in various types of  C. pepo

Some fruit types might be linked to specific enzyme loci, for example:

Spaguetti squash has unique Got-5 allozyme.

Two acorn squahes tested, also had unique pattern for the same locus.

Scallop squash has specific Got-2 allozyme.

Yellow straight neck has specific Mdh-2 allozyme.

Kirkpatric et al (1988) reported inheritance of isozymes on crosses between C. pepo and its ancestral wild species C. texana demonstrating gene flow between these species.

In squash it is possible to run electrophoresis in single pollen grains, since they are large enough to be crushed with tweezers.

Interspecific cross C. maxima x C. ecuadorensis used to identify 5 linkage groups. Extensive gene duplication for isozymes support polyploidy in Cucurbita species.

DNA-based markers:

rDNA genes used for separating varieties of C. pepo by polymorphism of intergenic spacer.

Intergenic spacer in C. maxima is unusual in the sense that it is longer than in other plants species (5.5Kb). It has 5 repetitive domains and 3 unique regions.

Genetic variation was determined by RAPD markers (Gwanma et al. 2000) in landraces of C. moschata from Africa.

Linkage map with RAPD and AFLP markers in pepo now expanded with new SSRs and genes h and B and approximately 650 markers in 20 linkage groups (Zraidi et al 2007, Gong et al 2008)

Breeding:

Flesh of better quality C. maxima has been used as a source to improve flesh quality of C. moschata.

It is possible to hybridize both species but the sterility of the hybrid is high.

Sakata Seed from Japan sells F1 hybrid of better quality.

Bridging species:

Widely used in cucurbita breeding.

1) C. lundelliana is an ideal bridge species since it crosses to all cultivated ones.

By crossing it to these, interbreeding populations can be established.

2) C. moschata also is useful as a bridge species. It has been used to transfer  transfer disease resistance from C. martinezzi to C. pepo.

C. moschata crosses readily to C. martinezii, and the resulting hybrid crosses to C. pepo. C. martinezii does not cross directly to C. pepo. This three way cross was used to transfer powdery mildew resistance to C. pepo.

Gametic diversity helps hybridization, For example, F1  C. pepo hybrids are more
successful than inbred lines of C. pepo when crossed with C. moschata.

C. pepo scallop squash  crosses easier with C. moschata than any other C. pepo form.

C. pepo x C. ecuadorensis has been used to to transfer multiple disease resistances, but embryo culture is necessary.
 

Disease resistance:

Not much work done in the past, compared to cucumbers and muskmelons.

Absence of good sources of resistance in cultivated germplasm. It is necessary to use wild species.  Wild species as a rule are virus resistant.

C. martinezii is resistant to CMV.

C. ecuadorensis and C. foetidissima, are resistant to watermelon mosaics 1 and 2. Also squash mosaic virus tolerance found in above species.

Silvered leaves seems to deter virus transmitting aphids, determined by gene M.

Biotechnology: This area needs to be developed in squash and pumpkins. Transformation techniques by Agrobacterium rhizogenes is available. Transgenic virus resistant zucchini is one of the few transgenic crops commercially available.

Haploids obtained by pollen irradiation with gamma rays in C. pepo, the most effective doses were 25 to 50 Gy.



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Last modified, May 5th, 2010

Carlos F Quiros, 1998