Genetic Mapping And Functional Analysis Of Genes Controlling Stem And Leaf Prickle In Lettuce

Lettuce（Lactuca sativa）was originated from the Mediterranean coast,and its wild progenitor was Lactuca serriola.Most wild lettuces have prickles on their stems and leaves to prevent animals and insects from feeding.Prickles were considerably reduced during the process of domestication.We found that prickles may exist in on leaf veins and stems.Some cultivars have prickles on both veins and stems,while others have prickles on veins only.In this study,several populations were constructed to genetically map the genes controlling stem prickles and leaf prickles.The locus controlling stem prickles was located in 306.31 Mb—307.31 Mb on chromosome 5 in a previous study in our lab.However,no annotated genes in this region have function known to be associated with stem prickles.To confirm the mapping results,we used a F_4:5:5 population derived from the cross between PI491245and W111 to genetically dissect this trait one more time.Using a population of 2000individuals,the gene controlling prickles on lettuce stems was mapped to the similar but slightly different region（305.22Mb—307.26Mb）as reported previously.We annotated all coding genes as well as LncRNAs from the candidate region,focusing on their variations on sequences and expressions.Interestingly,we found a gene encoding a cell division protein,which was not included in the candidate region in the previous study and showed expression difference between stems with prickes and stems without prickles.An overexpressing vector was constructed and transformed into prickleless lettuce C20.In the T0 generation,some of the transgenic plants showed prickle phenotype on the veins,and the expression of the gene increased to a certain extent.We speculated that the gene could produce prickle traits only under high expression.In another segregating population maintained in our lab,the R37 population,there was a segregation of prickles and non-prickles on leaf veins.We mixed 20prickly plants randomly chosen from the population to form the prickly extreme pool,and mixed 20 non-prickly to form the prickless extreme pool.RNA were extracted for each pool and sequenced using second generation sequencing technology.Using the BSR-seq script developed by the laboratory,we found that at least four loci controlled the formation of leaf prickles.One of them was consistent with the formation of stem prickles on chromosome 5.Screening using molecular markers confirmed that the locus associated with prickles on leaf prickles was the same as the the locus controlling prickles on stem.Besides the locus on chromosome 5,there are also loci on chromosomes 6,8 and 9 that control the formation of leaf prickles.This project provides an important step for future cloning of genes controlling the formation of prickles in lettuce and will shed light on the domestication of lettuce.