Exome genotyping, linkage disequilibrium and population structure in loblolly pine (Pinus taeda L.).
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.



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 (r (2)) between pairs of SNPs linked within single scaffolds decaying to half maximum (r (2) = 0.22) within 55 bp, to r (2) = 0.1 within 192 bp, and to r (2) = 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.



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.
Department of Ecosystem Science and Management, Texas A&M University, 2138 TAMU, College Station, TX, 77843-2138, USA.|Molecular and Environmental Plant Sciences Program, Texas A&M University, 2474 TAMU, College Station, TX, 77843-2474, USA.
Mengmeng
M
Lu
Department of Ecosystem Science and Management, Texas A&M University, 2138 TAMU, College Station, TX, 77843-2138, USA. konstantin.krutovsky@forst.uni-goettingen.de.|Molecular and Environmental Plant Sciences Program, Texas A&M University, 2474 TAMU, College Station, TX, 77843-2474, USA. konstantin.krutovsky@forst.uni-goettingen.de.|Department of Forest Genetics and Forest Tree Breeding, Georg-August-University of Göttingen, Göttingen, 37077, Germany. konstantin.krutovsky@forst.uni-goettingen.de.|N. I. Vavilov Institute of General Genetics, Russian Academy of Sciences, Gubkina Str, Moscow, 119333, Russia. konstantin.krutovsky@forst.uni-goettingen.de.|Genome Research and Education Center, Siberian Federal University, 50a/2 Akademgorodok, Krasnoyarsk, 660036, Russia. konstantin.krutovsky@forst.uni-goettingen.de.
Konstantin V
KV
Krutovsky
USDA Forest Service, Southern Research Station, Southern Institute of Forest Genetics, 23332 Success Road, Saucier, MS, 39574, USA.|University of Kentucky, Forest Health Research and Education Center, 730 Rose Street, Lexington, KY, 40546, USA.
C Dana
CD
Nelson
Department of Ecosystem Science and Management, Texas A&M University, 2138 TAMU, College Station, TX, 77843-2138, USA.
Tomasz E
TE
Koralewski
Department of Ecosystem Science and Management, Texas A&M University, 2138 TAMU, College Station, TX, 77843-2138, USA.|Texas A&M Forest Service, 2585 TAMU, College Station, TX, 77843-2585, USA.
Thomas D
TD
Byram
Department of Ecosystem Science and Management, Texas A&M University, 2138 TAMU, College Station, TX, 77843-2138, USA.|Molecular and Environmental Plant Sciences Program, Texas A&M University, 2474 TAMU, College Station, TX, 77843-2474, USA.
Carol A
CA
Loopstra
D059472:Exome / Q000235:genetics*; D005838:Genotype; D060005:Genotyping Techniques; D015810:Linkage Disequilibrium; D041603:Pinus taeda / Q000235:genetics*; D020641:Polymorphism, Single Nucleotide
Exome sequence capture; Genotyping by sequencing; Linkage disequilibrium; Loblolly pine; Population structure; SNPs; Target enrichment