Genetics of Watermelon Domestication.

Modern watermelon (Citrullus lanatus ssp. lanatus) cultivars are products of intense breeding for increased sugar content, enlarged fruit, enhanced flavor, and decreased seed number.  Domestication, selection for horticulturally important traits, and elite parent recycling have produced bottlenecks and dramatically narrowed genetic diversity in the elite gene pool.  The wild gene pool, by contrast, is a rich source of novel alleles, both favorable and unfavorable.  The present study focused on gaining an understanding of the genetics of domestication traits in watermelon by comparative mapping of phenotypic and quantitative trait loci for diverse traits in elite watermelon x citron (C. lanatus ssp. citroides) and elite watermelon x Egusi melon F₂ populations.  Numerous simple sequence repeat (SSR) and single nucleotide polymorphism (SNP) markers were developed and genetically mapped.  Greatly reduced fertility and severe segregation distortion were observed in the watermelon x citron population, a hallmark of chromosomal rearrangements and reduced genome homology characteristic of speciation.  The phenotypic distributions for most traits were strongly skewed towards the exotic parent.  We scanned the genomes in both populations and identified quantitative trait loci affecting fruit and seed, sex-determination, male-fertility, and other horticulturally important traits.  Numerous loci appear to have been targeted by selection for domestication traits in watermelon.