PLS 221                                   Instructor: Carlos F. Quiros

Cucurbitaceae: Watermelon, Citrullus lanatus

List of references (if unable to open, download it from SmartSite)



Origin and domestication

Related species and distribution

Traits of importance: Fruit traits

Sex expression

Polyploidy and its applications: triploidy and seedless varieties

Mapping and markers

Biotechnology



Citrullus lanatus

2n=2x=22

The origin of this species is in tropical Southern Africa. Its cultivation started in Egypt and India. Now it is cultivated all over the world. It was introduced to North America in the 1600's.

In addition to the cultivated species, there are 5 other x=11 related species which are cross compatible. They are a good source for disease resistance and other important traits.

C. lanatus has two subspecies: ssp. lanatus and ssp. vulgaris, which includes var. citroides (citron), a white flesh, small fruit variety used for preserves.

The wild species from N. Africa C. colocynthis, has been postulated as the probable the ancestor of watermelon. It is highly polymorphic, with annual and  perennial forms and xerophytic. However, recent evidence based on sequences of rDNA spacers indicates that the annual species C. rehmii from S. Africa might be instead the immediate ancestor of cultivated watermelon (Jarret and Newman, 2000).

Natural hybridization between C. lanatus and C. colocynthis, have been reported. These species are separated by chloroplast DNA polymorphism (Levi et al 2005). Hybrids have normal chromosome pairing and high fertility (Sain et al 2002). Most mapping populations based on these hybrids.

Traits of importance:

A number of genes have been described in this crop (Robinson et al. 1975). Among these are the genes determining fruit traits. Guner et al (2004) published the latest list of genes reported in watermelon.
 

Rind color:  (locus g, 3 alleles)

g+ :dark green

g : light green

gs :stripped green

Flesh color: Complex gene interaction of 3 loci and possible modifiers

y :yellow

y+ red

Wf  white flesh

Wf+ colored fresh

C modifier, enhances yellow to canary yellow

C+ modifier, decreases red to pink

A total of 13 genes affect fruit color.

su eliminates fruit bitterness. Fruit shape OO elongate, Oo oval, oo round. f= furrowed fruit, e= explosive rind

Fruit size, varies from 1 kg to >100 kg. Polygenic and quantitative with strong environmental effects. At least 5 genes with additive effects (Gusmine and Wehner 2007).

Seed coat color: Determined by 3 genes, r, t and w. Seeds can be red, tan, and white with 6 phenotypic variations. A forth locus d  determine black-dotted seeds by interaction with the other loci.

Vine mutants: Four dwarf genes (dw)

Sex expression:

Two sex types are found for this crop, monoecious and andromonoecious. Andromonoecious type is recessive to monoecious. Trait determined by locus a.
Watermelon like other cucurbitacea is insect pollinated.

The use of hormones to manipulate sex expression in this crop is not well developed.

Male sterility: The gene gms determines glabrous foliage male sterile plants due to the lack of foliage trichomes. It is useful for F1 hybrid production, although most hybrids are produced manually by daily removal of male flowers in the "female" lines.

Polyploidy

Although polyploidy is rare in watermelon, polyploidy is readily induced by colchine treatment. Ploidy manipulation has been successfully implemented in this crop to produce seedless triploid varieties. No cases of parthenocarpy reported in watermelons, so triploidy has been a good alternative, although there are some problems associated to its application.

Triploidy and Seedless watermelons:

Kihara and Nishiyama (1951) proposed the following scheme for creating triploids.

Chromosome doubling by colchicine to obtain 4n plants.

Crossing 4n x 2n plants after testing for good combining ability of the parental lines.   Reciprocal crosses are not successful.

Grow 3n F1 plants, which produce mostly parthenocarpic fruit after pollination with a 2n pollen line. The fruits are almost seedless and of excellent quality.

Problems associated to seedless watermelon:

1) Low yield of the 3n F1 plant.

2) The 4n plant will produce also 4n seed by random mating to other 4n plants, so 4n seed can contaminate 3n seed. This can be avoided by removing male flowers from  the 4n plants or doing hand pollinations beween 4n and 2n flowers.  Attempts have been made to transfer male sterility gene to tetraploid line, which is not easy since it must by nulliplex for ms.

3) Difficult germination of 3n seed. These two problems make the seed very expensive.

4) The 4n parental line is difficult to maintain, due to poor seed production.

5) Developmental defects, hollow heart, colored empty seeds. This depends on
which line is used as the 4n parent.

Henderson's followed up fruit quality trait in hybrids using reciprocal crops, where each parent was tried as diploid and tetraploid lines

Varieties: Sugar Baby (SB)and  Sweet Princess (SP)

He found the following trends:

1) Fruits were significantly rounder in polyploids

2) Fruit wt and ploidy level inversely correlated.

3) Polyploids had thicker rind, and larger seed

4) Reciprocal hybrids of SP(4X) x SB (2x) and  SB(4x) x SP(2x)

differed significantly for hollow heart, fruit wt, shape etc.

Other methods proposed for seedless watermelon production:

A few other methods have been proposed, but have not been very successful enough for commercial adaptation. These include pollen irradiation of male parent to pollinate female diploid line.

Translocation method: Proposed by Sakaguchi and Nishimura, consists in
induction of multiple translocations by seed irradiation based on the following assumption:

> number of translocations > number of aborted gametes < seeds

The scheme is complex and involves selection of translocation heterozygous and various cycles of irradiation, which is not very practical for wide application. They obtained translocations for 3 or 4 pairs of chromosomes, having the plants only 3% pollen fertility.

Pollen irradiation: Another scheme proposed is pollination of diploid lines with irradiated pollen to produce parthenocarpic fruit. The seed producer will need a source to irradiate the pollen before pollinations.

Mapping and markers:

Isozymes and protein markers have been reported by  Zamir et al. and Navot.

19 seed protein and isozymes loci were disclosed.  Little variation among cultivars for 13 enzyme systems were reported, however a higher level of polymorphism was detected in crosses of  C. lanatus x C. collocynthis. 4 linkage groups were established.

A few microsatellite markers are now available for this crop. Guerra-Sanz 2002 reported 19 loci useful for variety identification. Also AFLPs has been used for variety identification (Che et al. 2003).

Ok et al (2000) isolated over 700 expressed sequence tags (ESTs) from watermelon leaves. Approximately 60% of these correspond to known genes from other organisms.

Hashizume et al. (2003) constructed a linkage map of 554 loci in 11 linkage groups based on molecular markers and analyzed QTLs for hardness of rind, sweetness, flesh and rind color.

Map was constructed by crossing watermelon to watermelon x C. collocynthes, consisting of approximately 170 molecular markers (Levi et al. 2002). Most recent map by same author published in 2006, includes 360 AFLP, SRAP, RAPD, ISSR and SSR markers. ACC gene mapped.

Maps based on recombinant inbreds, of hybrid lanatus x citroides. Mostly RAPD and SCAR markers.

BAC library constructed by Joober et al (2006). 36 SSR developed from these BACs.

Biotechnology:

   Organogenesis and plant regeneration has been reported form cotyledon and hypocotyl explants. The main objective of this work is to mass propagate triploid plants for seedless fruit production. Transformation using cotyledon explants have been accomplished in watermelon and its wild relative C. colocynthis (Dabauza et al. 1997).

Han et al (2011) improved plant biomass and fruit quality by enhancing Ca+ substrate specificity and reduced transport capability of Mn+ after transformation of a bottle gourd used as a rootstock with an A. thaliana ion transporter. This was due to higher osmotic pressure and soluble solids.

Zucchini yellow mosaic and papaya ringspot virus resistant watermelon were developed by Yu et al (2011) by virus coat protein transformation.

 



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Last modified, May 12, 2011

Carlos F Quiros, 1998