QTL Mapping And Candidate Gene Screening Of Parthenocarpy In Cucumber

Cucumber is one of the most important vegetables in melon plants and plays an important role in agricultural production and life.Cultivated cucumbers have a unique parthenogenetic ability to form seedless fruits in the absence of pollination or other stimulation.This characteristic is particularly important in the common facility cultivation conditions,where there is no pollinator or in the unsuitable environment such as high humidity,low light and high temperature.However,there is little research and mapping work on the germplasm resources of unisexual fruity cucumber.The study of QTL loci of cucumber unisexual fruiting traits is helpful to further understand the genetic mechanism of unisexual fruiting,promote the innovation of unisexual fruiting germplasm resources,and improve the unisexual fruiting efficiency of modern cucumber varieties.In this study,two female cucumber lines"19839"and"20266"were constructed to map the F₂population of self-crossing progenies.The phenotypic data of fruity phenotypes in autumn 2020 and spring 2021 were investigated.Genetic mapping was carried out using 34SSR markers and Indel markers that showed polymorphism between parents.A genetic linkage map of cucumber was constructed,and QTL mapping was carried out based on the results of QTL-Seq sequencing.The main QTL regions of cucumber parallel fruit set were preliminarily predicted.Strong parthenogenetic parents were used as recurrent parents to cross with the recombinant plants and construct a backcross generation population.The differential expression genes and possible candidate genes related to parathylocy fruit were predicted by analyzing the microstructures of the parents,combining RNA-Seq and expression analysis to screen the genes in the main effect QTL interval.The main results are as follows:1.Through the investigation of 50 cucumber germplasm resources,two European cucumber all-female inbred lines with remarkable difference in the fruit set were screened,among which inbred line"19839"had poor fruit set ability,while inbred line"20266"had strong fruit set ability.The F₁of the two fruits showed that the fruit at low node and middle and high node occasionally bore parthenotypic fruit,but the middle fruit at high node showed intermediate phenotype.Trait separation occurred in F₂population.2.Fruit sections of parents at-3d,0d,3d,6d and 9d showed that the phenomenon of fruit enlargement was probably related to cell division and amplification.Transcriptomic sequencing analysis of ovaries of parents at-3d,0d and 3d showed that the development process of fruit fruiting mainly involves plant hormones,cell cycle and carbohydrate metabolism,compared with fruit fruiting at-3d,0d and 3d.3.The F₂population was used for genetic mapping,and 380 SSR markers were obtained that could show polymorphic differences between parents.Based on chromosome location distribution,34 markers were selected and F₂was used as the target population to construct a genetic map containing seven linkage groups.QTL analysis predicted that there might be major or minor effect loci associated with parophytic fruiting on chromosomes 1,2and 7.Mixed pool sequencing of BSA extreme phenotype showed that the main effect locus was located on chromosome 7.In addition,there may be microeffect loci on chromosomes 2,3 and 5.At the same time,a QTL locus Parth7.1,located between SSR7020 and Indel5,was detected on chromosome 7 for the F₂population in autumn 2020.The backcross populations of different genotypes and 20266 were obtained by recombinant single plant screening.4.Transcriptomic data of parents in-3d,0d and 3d were analyzed for differential expression.During fruit development of parent 19839,10,789 differentially expressed genes were obtained.During fruit development of parent 20266,4318 differentially expressed genes were obtained,and GO analysis showed that they were mainly involved in coenzyme binding and glycokinase transport activities,fatty acid biosynthesis and microtubule movement.5.In transcriptome data,there were 405 genes between SSR7020 and Indel5 on chromosome 7.Combined with transcriptome data of parental fruits and transcriptome data of each tissue of 9930,15 genes with dominant or specific expression in fruits were found to be significantly differentially expressed in parental fruits.It was speculated that Csa7G167120,Csa7G073410,Csa7G073430,Csa7G073440 and Csa7G108300 might be candidate genes for cucumber parthyogenetic fruiting traits.At present,knockout and overexpression studies related to Csa7G073410 gene are under way.