A chromosome-scale genome sequence of pitaya (Hylocereus undatus) provides novel insights into the genome evolution and regulation of betalain biosynthesis

Pitaya(Hylocereus)is the most economically important fleshy-fruited tree of the Cactaceae family that is grown worldwide,and it has attracted significant attention because of its betalain-abundant fruits.Nonetheless,the lack of a pitaya reference genome significantly hinders studies focused on its evolution,as well as the potential for genetic improvement of this crop.Herein,we employed various sequencing approaches,namely,PacBio-SMRT,Illumina HiSeq paired-end,10×Genomics,and Hi-C(high-throughput chromosome conformation capture)to provide a chromosome-level genomic assembly of‘GHB’pitaya(H.undatus,2n=2x=22 chromosomes).The size of the assembled pitaya genome was 1.41 Gb,with a scaffold N50 of~127.15 Mb.In total,27,753 protein-coding genes and 896.31Mb of repetitive sequences in the H.undatus genome were annotated.Pitaya has undergone a WGT(whole-genome triplication),and a recent WGD(whole-genome duplication)occurred after the gamma event,which is common to the other species in Cactaceae.A total of 29,328 intact LTR-RTs(~696.45Mb)were obtained in H.undatus,of which two significantly expanded lineages,Ty1/copia and Ty3/gypsy,were the main drivers of the expanded genome.A high-density genetic map of F1 hybrid populations of‘GHB’בDahong’pitayas(H.monacanthus)and their parents were constructed,and a total of 20,872 bin markers were identified(56,380 SNPs)for 11 linkage groups.More importantly,through transcriptomic and WGCNA(weighted gene coexpression network analysis),a global view of the gene regulatory network,including structural genes and the transcription factors involved in pitaya fruit betalain biosynthesis,was presented.Our data present a valuable resource for facilitating molecular breeding programs of pitaya and shed novel light on its genomic evolution,as well as the modulation of betalain biosynthesis in edible fruits.