Genetic And Molecular Mechanism Of Fruit And Plant Shape Traits In Watermelon

As an important cucurbit crop,watermelon(Citrullus lanatus)is rich:in diversity related to agronomic and quality traits that has concerned the breeders for crop improvement.Selection of watermelon varieties at early stages of growth with desired characters related to agronomic and fruit quality traits,has always been the breeding goal for breeders.At present,our understanding of these important agronomic traits of watermelon is scarce;there are fewer studies on the genetics and molecular mechanisms.In this study,we focused on the three characters namely fruit shape,yellow skin and plants with no-branches,and hope to clone the key genes controlling these traits through molecular biology to analyze their genetic and molecular mechanisms.Clear understanding of mechanism of genes controlling these traits along with marker assisted selection will provide an effective way for early selection and to develop new cultivars.We mainly did three aspects which are as follow:1.We identified a locus on chromosome 3 of watermelon genome controlling fruit shape.Segregation analysis of F2 and BC,populations derived from a cross between two inbred lines 'Duan125'(elongate fruit)and 'Zhengzhouzigua'(spherical fruit)suggested that fruit shape of watermelon might be controlled by a single locus and elongate fruit was incompletely dominant to spherical fruit with the heterozygotebeing oval fruit.GWAS of fruit trait using 315 accessions identified a major locus on watermelon chromosome 3,which was confirmed by BSA-seq mapping in the F2 population.The candidate gene was mapped to a region of 46 Kb on chromosome 3.There are only four genes present in the corresponding region in the reference genome.The four genes from this region were sequenced from both parents,and the CDS of Cla011257 showed a 159 bp deletion which resulted in 53 amino acids missing in elongate watermelon.An indel marker was developed based on the 159 bp deletion and used to test the F2 population and 105 watermelon accessions.The results showed that Cla011257 cosegregated with watermelon fruit shape.The Cla011257 has the highest expression at ovary formation stage.The predicted protein of the Cla011257 gene belongs to IQD protein family which was reported to be associatedn with cell arrays and Ca2+-CaM signaling modules.2.We identified a locus for watermelon yellow skin through BSA-seq and GWAS.Segregation analysis in F2 and BC1 populations derived from a cross of two inbred lines '94E1'(yellow skin)and 'Qingfeng'(green skin)suggested that skin color is controlled by a single dominant gene.Interestingly,the same locus also controls the color of veins and petioles.Both BSA-seq mapping and GWAS mapped the causal gene to chromosome 4.Several major markers were designed to delimit the region to 59.8 Kb on chromosome 4.Unfortunately,this region was not well assembled in the reference genome and only one gene(Cla002755)was found.However,no sequence and expression differences of this gene with unknown function were found between the two parents.Nevertheless,we designed a SNP marker highly linked with the yellow skin phenotype.This marker will be helpful for future breeding programs.3.We identified a locus controlling no-branch in watermelon.A segregation analysis in F2 and BC1 populations derived from a cross of two inbred lines '790010'(branch)and 'Wuchazao'(no-branch)suggested that no-branch is controlled by a single recessive gene.The candidate region was mapped to a 270 Kb interval on chromosome 4.There are 32 genes present in the corresponding region in the reference genome.The SNPs analysis and sequence alignment show that a point mutation in the no-branch parent in Cla018413 resulted in the chang of amino acids.It suggested that the Cla018413 gene might be the candidate gene controlling no-branch in watermelon.This hypothesis will be tested using a large segregating population and function analysis.