Genomic Variation between PRSV Resistant Transgenic SunUp and Its Progenitor Cultivar Sunset

Abstract

Background: The safety of genetically transformed plants remains a subject of scrutiny. Genomic variants in PRSV resistant transgenic papaya will provide evidence to rationally address such concerns. Results: In this study, a total of more than 74 million Illumina reads for progenitor ‘Sunset’ were mapped onto transgenic papaya ‘SunUp’ reference genome. 310,364 single nucleotide polymorphisms (SNPs), 34,071 small Inserts/deletions (InDels) and 1,200 large structural variations (SVs) were detected between ‘Sunset’ and ‘SunUp’. Those variations have an uneven distribution across nine chromosomes in papaya. Only 0.27% of mutations were predicted to be high-impact mutations. ATP-related categories were highly enriched among these high-impact genes. The SNP mutation rate was about 8.4×10-4 per site, comparable with the rate induced by spontaneous mutation over numerous generations. The transition-to-transversion ratio was 1.439 and the predominant mutations were C/G to T/A transitions. Spontaneous mutations were the leading cause of SNPs in transgenic papaya ‘SunUp’. A total of 3,430 nuclear plastid DNA (NUPT) and 2,764 nuclear mitochondrial DNA (NUMT) junction sites have been found in ‘SunUp’, which is proportionally higher than the predicted total NUPT and NUMT junction sites in ‘Sunset’ (3,346 and 2,745, respectively). Among all nuclear organelle DNA (norgDNA) junction sites, 96% of junction sites were shared by ‘SunUp’ and ‘Sunset’. The average identity between ‘SunUp’ specific norgDNA and corresponding organelle genomes was higher than that of norgDNA shared by ‘SunUp’ and ‘Sunset’. Six ‘SunUp’ organelle-like borders of transgenic insertions were nearly identical to corresponding sequences in organelle genomes (98.18~100%). None of the paired-end spans of mapped ‘Sunset’ reads were elongated by any ‘SunUp’ transformation plasmid derived inserts. Significant amounts of DNA were transferred from organelles to the nuclear genome during bombardment, including the six flanking sequences of the three transgenic insertions.Conclusions: Comparative whole-genome analyses between ‘SunUp’ and ‘Sunset’ provide a reliable estimate of genome-wide variations and evidence of organelle-to-nucleus transfer of DNA associated with biolistic transformation.