Transcriptome Analysis Of Different Floral Colors In Xanthoceras Sorbifolium And Its Phylogenetic Analysis

Xanthoceras sorbifolium Bunge belongs to the family Sapindaceae and is endemic to northern China.The original species having white flowers was used to produce the purple flowered Xanthoceras sorbifolium Bge.’Purpurea’ after artificial cultivation,which is used as a garden ornamental tree.In this study,we used next-generation sequencing(NGS)to sequence the petal tissues from different developmental stages of four varieties.The candidate structural genes involved in anthocyanin biosynthesis were inferred from gene function annotation and the calculated fragments per kilobase of transcript per million mapped reads(FPKM).Simultaneously,the bHLH transcription factor family of the regulatory genes were identified.Transcriptome data was used to obtain the gene datasets to explore the phylogenetic position of Xanthoceras.The aim of this study was to identify candidate genes underlying the anthocyanin synthetic pathway in X.sorbifolium,and enrich the corresponding genomic data.The main results of this study are as follows:1.Transcriptome sequencing of X.sorbifolium petal tissues at different developmental stages was earried out,and 357.219 Gbp data,807,309 transcripts,and 41,672 unigenes were obtained.More than 94%of the data had a base error rate less than 0.01%and the data was compared with the reference genome,yielding map ratios more than 90%which indicated reliable sequencing quality.Our data had high correlation with the reference genome,which can enrich the existing transcriptome data of X.sorbifolium and can be used for subsequent analysis.2.We performed an enrichment analysis of the differentially expressed genes based on gene function annotation.The 11 candidate genes involved in the anthocyanin synthetic pathway were selected by comparing the differences in expression levels of genes involved in this pathway.Allost all of the candidate genes were upregulated in the purple flower than in the white flower,with the highest expression level observed in the color changed stage(S2).3.We identified 106 bHLH genes in X.sorbifoliim using the conserved sequence of bHLH and named them XsbHLH1 to XsbHLH106.We analyzed gene structure and conserved motifs of these identified genes.Based on transeriptole data,the expression levels of and functional pathways associated with bHLH genes at different developmental stages were analyzed.It was found that XsbHLH6,XsbHLH55,XsbHLH74,and XsbHLH93 were upregulated in purple flowers and all of them were involved in the biosynthesis process of anthocyanin-containing compounds.Therefore,we conclude that these four genes may be important regulators in anthocyanin biosynthesis.4.The phylogenetic location of Xanthoceras in Sapindaceae was investigated using high-throughput RNA-sequencing data of 24 species combined with the transcriptome sequencing data obtained in this study by screening low-copy nuclear genes,anthocyanin gene,and chloroplast gene datasets.The results showed that X.sorbifolium was more closely related to Aceroideae,based on anthocyanin candidate genes.Some species in Aceroideae,such as Aesculus × carnea and Handeliodendron bodinieri have bright flowers,which are different from the other species in Sapindales.The pigment genes can be considered as foreign genes transferred into X.sorbifolium to obtain new horticultural traits.The low-copy number homologous nuclear gene showed that the closest group to X.sorbifolium was the group of Aesculus chinensis and Aceroideae.This result supports the phylogenetic evolutionary relationship obtained using the anthocyanin gene.The reason for the conflict between the genetic tree constructed in the study and the final species tree can not be explained by simple incomplete lineage sorting.It is speculated that the conflict between the gene tree and the species tree may be due to interspecific hybridization.