Watermelon is an economically important crop of the cucurbitaceae family, which comprises two subfamilies, eight tribes, and ≈118 genera and 825 species (Jeffrey, 1990). China is the greatest producer and consumer of watermelon in the world, producing significantly more fruit than the next leading producer, Turkey (Food and Agriculture Organization of the United Nations, 2010). The genus Citrullus consists of four known species, including C. lanatus, C. colocynthis, C. eccirrhosus, and C. rehmii (Jarret et al., 1997; Robinson and Decker Walters, 1997).
Watermelon has a long history of cultivation in China and the Middle East. By the 10th century, the crop was grown in China and southern Russia. This crop was introduced to the New World by the Spaniards in the 16th century and rapidly became popular with Native Americans (Gusmani and Wehner, 2007). In the United States, commercial watermelon is produced mainly in the southern states and Indiana.
Random amplified polymorphic DNA (RAPD) markers were used to estimate genetic diversity among watermelon cultivars (Lee et al., 1996; Solmaz et al., 2010) and to construct an initial genetic linkage map (Hashizume et al., 1993). Levi et al. (2005) identified RAPD markers for distinguishing between American watermelon cultivars of C. lanatus var. lanatus, C. lanatus var. citroides, and C. colocynthis. Levi et al. (2001) also demonstrated low genetic diversity among 46 heirloom cultivars of watermelon and concluded that American watermelon cultivars share a narrow genetic base. Restriction fragment length polymorphisms were also used to analyze Citrullus species chloroplast variability in wild and cultivated plants (Dane et al., 2004). Additional markers were detected using the RAPD procedure (Hashizume et al., 1993; Zhang et al., 1994). Jarret et al. (1997) determined genetic variation among PI accessions of C. lanatus var. lanatus, C. lanatus var. citroides, and C. colocynthis using simple sequence repeat length polymorphisms (SSRs). Subsequently, codominant simple sequence repeat markers were used to detect genetic diversity in watermelon (Patcharin et al., 2011). Recently, two types of molecular markers (RAPD and SSR) were used to estimate diversity in 81 seedlings from eight accessions of watermelon collected in Zimbabwe; five accessions were citron type (C. lanatus var. citroides) and three C. lanatus var. lanatus (Mujaju et al., 2010). However, most of the previous work was performed on PI accessions from the U.S. Department of Agriculture germplasm collection, which only contains ≈20 Chinese cultigens. A thorough understanding of the diversity of Chinese ecotypes has not been published.
Studying the process of domestication and colonization on watermelon will explain this crop’s evolutionary diversification. In evolutionary ecology, an ecotype, sometimes called ecospecies, describes a genetically distinct geographic variety, population, or race within a species (or among closely related species), which is adapted to specific environmental conditions. Typically, diverse ecotypes exhibit phenotypic differences in morphology or physiology. Southern China includes Hubei, Fujian, Anhui, GuangDong, GuangXi, and Hainan provinces as well as islands of the South China Sea. The average temperature is 10 °C and average precipitation is 1400 to 2400 mm per year. Northern China includes Heilongjiang, Henan, Gansu, Hebei, and Shanxi provinces including Beijing and Tianjing city. The area is a subhumid, warm-temperate, continental monsoon climate with dry cold winters and rainy summers. Northwest China is an arid zone, which includes Neimengo, Xinjiang, Ninxia, and Gansu provinces; the average precipitation is lower than 200 mm per year. Recognizing the geographical distribution of watermelon lineages will benefit future watermelon genetic exchanges and provide insight into different factors that shape watermelon genetic diversity. Although watermelon is distributed and cultivated worldwide, the genetic diversity and genetic relationship using DNA analysis are poorly described in China. In this study, open-pollinated, hybrid and inbred lines were included for most of the ecotypes and are hereafter collectively referred to as cultigens unless an ecotype group is being discussed. The objective of this study was to evaluate morphological variation and assess genetic diversity and genetic relationships among Chinese watermelon cultigens collected in different geographical regions compared with watermelon from other countries.
AndersonJ.A.ChurchillG.A.AutriqueJ.E.TanksleyS.D.SorrellsM.E.1992Optimizing parental selection for genetic linkage mapsGenome36181186
DaneF.LangP.BakhtiyarovaR.2004Comparative analysis of chloroplast DNA variability in wild and cultivated Citrullus speciesTheor. Appl. Genet.108958966
Food and Agriculture Organization of the United Nations2010Top production quantity in the world Title 17. Top production on watermelons in 2010. 18 Apr. 2012. <http://faostat.fao.org/site/339/default.aspx>
HashizumeT.SatoT.HiraiM.1993Determination of genetic purity of hybrid seed in watermelon (Citrullus lanatus) and tomato (Lycopersicon esculentum) using random amplified polymorphic DNA (RAPD)Jpn. J. Breeding43367375
JarretR.L.MerrickL.C.HolmsT.EvansJ.AradhyaM.K.1997Simple sequence repeats in watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai]Genome40433441
JeffreyC.1990Systematics of the cucurbitaceae: An overview p. 2–9. In: Bates D.M. R.W. Robinson and C. Jeffrey (eds.). Biology and utilization of the cucurbitaceae. Cornell University Press Ithaca NY
JoobeurT.GusmaniG.ZhangX.LeviA.XuY.WehnerT.C.OliverM.DeanR.A.2006Construction of a watermelon BAC library and identification of SSRs anchored to melon or Arabidopsis genomesTheor. Appl. Genet.112553562
LeeS.J.ShinJ.S.ParkK.W.HongY.P.1996Detection of genetic diversity using RAPD-PCR and sugar analysis in watermelon [Citrullus lanatus (Thunb.) Mansf.] germplasmTheor. Appl. Genet.92719725
LeviA.ThomasC.E.KeinathA.P.WehnerT.C.2000Estimation of genetic diversity among Citrullus accessions using RAPD markersActa Hort.510385390
LeviA.ThomasC.E.KeinathA.P.WehnerT.C.2001Genetic diversity among watermelon (Citrullus lanatus and Citrullus colocynthis) accessionsGenet. Resources Crop Evol.48559566
LeviA.ThomasC.E.SimmonsA.M.ThiesJ.A.2005Analysis based on RAPD and ISSR markers reveals closer similarities among Citrullus and Cucumis species than with Praecitrullus fistulosus (Stocks)Pangalo. Genet. Resources Crop Evol.52465472
MujajuC.SehicJ.WerlemarkG.Garkava-GustavssonL.FatihM.NybomH.2010Genetic diversity in watermelon (Citrullus lanatus) landraces from Zimbabwe revealed by RAPD and SSR markersHereditas147142153
PatcharinT.TaeprayoonP.HadkamY.SrinivesP.2011Genetic diversity among Jatropha and Jatropha-related species based on ISSR markersPlant Mol. Biol. Rpt.29252264
RobinsonR.W.Decker-WaltersD.S.1997Cucurbits. CAB International New York NY
SolmazI.SariN.Aka-KacarY.Yesim Yalcin-MendiN.2010The genetic characterization of Turkish watermelon (Citrullus lanatus) accessions using RAPD markersGenet. Resources Crop Evol.57763771
YiK.2002Construction of genetic linkage map and localization of main agronomic characters in watermelon [Citrullus lanatus (Thunb.) Matsum & Nadai]. PhD diss. Hunan Agr. Univ. Chang Sha China