Exome genotyping, linkage disequilibrium and population structure in loblolly pine (Pinus taeda L.)
Background: Loblolly pine (Pinus taeda L.) is one of the most widely planted and commercially important forest
tree species in the USA and worldwide, and is an object of intense genomic research. However, whole genome
resequencing in loblolly pine is hampered by its large size and complexity and a lack of a good reference. As a
valid and more feasible alternative, entire exome sequencing was hence employed to identify the gene-associated
single nucleotide polymorphisms (SNPs) and to genotype the sampled trees.
Results: The exons were captured in the ADEPT2 association mapping population of 375 clonally-propagated loblolly
pine trees using NimbleGen oligonucleotide hybridization probes, and then exome-enriched genomic DNA fragments
were sequenced using the Illumina HiSeq 2500 platform. Oligonucleotide probes were designed based on 199,723
exons (˜49 Mbp) partitioned from the loblolly pine reference genome (PineRefSeq v. 1.01). The probes covered 90.2 %
of the target regions. Capture efficiency was high; on average, 67 % of the sequence reads generated for each tree
could be mapped to the capture target regions, and more than 70 % of the captured target bases had at least 10X
sequencing depth per tree. A total of 972,720 high quality SNPs were identified after filtering. Among them, 53 % were
located in coding regions (CDS), 5 % in 5’ or 3’ untranslated regions (UTRs) and 42 % in non-target and non-coding
regions, such as introns and adjacent intergenic regions collaterally captured. We found that linkage disequilibrium (LD)
decayed very rapidly, with the correlation coefficient (r2) between pairs of SNPs linked within single scaffolds decaying
to half maximum (r2 = 0.22) within 55 bp, to r2 = 0.1 within 192 bp, and to r2 = 0.05 within 451 bp. Population structure
analysis using unlinked SNPs demonstrated the presence of two main distinct clusters representing western and
eastern parts of the loblolly pine range included in our sample of trees.
Conclusions: The obtained results demonstrated the efficiency of exome capture for genotyping species such as
loblolly pine with a large and complex genome. The highly diverse genetic variation reported in this study will be a
valuable resource for future genetic and genomic research in loblolly pine.