WATERMELON (Citrullus lanatus) BREEDING HANDBOOKGabriele GusminiRaleigh NC, 2003AbstractThe watermelon (Citrullus lanatus) is one of the most important horticultural crop for fruitproduction in the U.S. The watermelon is a monoecious species and therefore across–pollinated crop of the family Cucurbitaceae and does not manifest important levels ofinbreeding depression or heterosis.2Public and private breeders are developing leading varieties since the 1,800's (Wehner et al.,2001; Whitaker and Jagger, 1937) and lot of progress has been done particularly forqualitative traits. However, there are not very detailed informations available about thegenetics of this plant even though several genes have been identified. Priorities for the futureshould be the understanding of the mechanisms of inheritance and the variance componentsfor several important traits, along with the development of new breeding strategies that canhold the gain obtained in selection for qualitative traits broadening at the same time geneticvariability in the cultivated watermelons to allow the breeders to improve the quantitativetraits of this crop.Biotechnology has not played a central role yet for the improvement or the geneticalunderstanding of the watermelon but should be better used as a tool to short-cut the tipicallylong timing of every breeding program (gene sequencing, gene overexpression and/orsilencing, transformation techniques, molecular markers assisted selection, etc.).This report summarizes some important informations on the genetics and the breeding of thewatermelons and presents some new possibilities for its improvement.3IntroductionThe watermelon (Citrullus lanatus) is a major vegetable crop in the U.S. with an averageproduction in the period 1999-2001 of about 39 million cwt/year. The major center ofproduction was Florida (in 1999-2001 about 8.9 million cwt/year), even though the state withthe major acreage in the same triennium was Texas (about 37,000 acres/year) (U.S.D.A.,2002).The watermelon has high lycopene content in the red-fleshed varieties: 60% more thantomatoes. Lycopene has been classified as a useful component of human diet for preventionof heart-attacks and certain types of cancer (Perkins-Veazie et al., 2001).In the U.S. the market is more oriented now toward smaller watermelons, either round orelongate, preferentially seedless. The watermelon can have flesh of different colors, eventhough red is the most common. The market recently has welcome yellow and orangevarieties, particularly in processed watermelons. In other countries , i.e. China, small andyellow watermelons are preferred to the red ones; in Europe, however, the comsumers stillwant red and big fruits.The american market is nowadays divided equally between seeded and seedless types but therequest for seedless on the market either for fresh and processed products is increasing.4Taxonomy (G.R.I.N.)Kingdom: PlantaeDivision: MagnoliophytaClass: MagnoliopsidaOrder: CucurbitalesFamily: CucurbitaceaeSubfamily: CucurbitoideaeTribe: BenincaseaeSubtribe: BenincasinaeGenus: CitrullusSpecies: lanatusVarieties: lanatuscitroidesTable 1. Related species of agricultural interest and their capability of intercrossing withwatermelon Species Chromosome Level of Capability ofNumber (2n) Relationship Intercrossing Citrullus lanatus 22 - -Cucurbita pepo 40 Family NoCucurbita maxima 40 Family NoCucurbita mixta 40 Family NoCucurbita moschata 40 Family NoCucumis sativus 14 Family NoCucumis melo 24 Family NoLuffa spp. 26 Family No5Table 2. Intercrossablewild related species of watermelon (Mohr, 1986) Species Chromosome Life SexNumber (2n) Cycle Type Citrullus lanatus 22 Annual MonoeciusCitrullus colocynthis 22 Perennial MonoeciusCitrullus naudinianus 22 Perennial DioeciousCitrullus ecirrhosus 22 Perennial MonoeciusCitrullus rehmii 22 Perennial Monoecius The species listed in table 2 are all intercrossable and seeds have been recovered andsuccessfully germinated in all possible crosses (Mohr, 1986). So far only Citrulluscolocynthis has received attention as a source of valuable germplasm and has been includedgenetic studies (Levi et al., 2001a).6BiogeographyThe watermelon is thought to be native from Africa (Maynard et al., 2001; Mohr, 1986;Whitaker and Jagger, 1937) but the distribution of the PI accessions present in the U.S.D.A.collection after the recent updates of the past 10 years shows that it is possible to identify atlist other three important centers of germplasm diversity: South-Asia, Turkie, and Middle-East.Table 3. Distribution of hypothetic centers of diversity for watermelon based on the origin ofthe PI Accessions present in the U.S.D.A. germplasm collection at Griffin, GA Until now the germplasm has been collected mainly in Africa, China, Middle-East, South-Europe, and slightly in America. The major problem of the collection of watermelongermplasm is the political instability of the area where it is widely present in the wild:political reasons, for instance, limited exploration of most countries in Africa and Asia7(Vietnam, Laos, and Cambogia); protectionistic politics adopted from the middle-easterncountries limited also the collection of germplasm in that area. For sure future collectionshould concentrate in the asian countries since now they are politically stable and much moreopen toward international cohoperation and exchange of germplasm. Africa is becomingeven more dangerous to explore and the most stable countries are generally not willing toallow export of their germplasm.Breeders can find germplasm directly in the U.S. by request at the U.S.D.A. repository inGriffin, GA.Table 4. Watermelon germplasm held at the U.S.D.A. repository in Griffin, GA Species Variety PI Accessions Citrullus lanatus lanatus 1,435citroides 134Citrullus colocynthis 23Citrullus