Genetic Analysis of Forking Defects in Loblolly Pine

Forking defects, including stem forking and ramicorn branching, are serious stem-quality problems in loblolly pine (Pinus taeda L.). The presence of forked stems and ramicorn branches greatly reduces wood quality, quantity, and the economic value of the wood. Assessing forking traits may enable breeders to successfully breed and deploy non-forked phenotypes in breeding populations. The objectives of this study were to assess the genetic basis of forking in loblolly pine by assessing trees in a large number of full-sib seedling tests throughout the Southeast and in a clonal test in South Carolina.;In the full-sib tests, the percentage of forking averaged 18%, and ranged from 4% to 80% across different test series. The individual-tree heritability was low (0.06), but family-mean heritabilities were moderately high (half-sib=0.76, narrow-sense full-sib=0.59, broad-sense full-sib =0.71), indicating that forking is partially under genetic control at the family level. By using half-sib family selection with a selection differential of 20%, genetic gain could be achieved with 12% to 23% reduction of forking across different regions. A weak unfavorable genetic correlation (0.18) was found between forking and height, suggesting that selection for growth alone will negatively impact forking in loblolly pine. A favorable genetic correlation (0.33) was found between forking and straightness.;In the clonal test, stem forking and ramicorn branching were serious problems in some clones. Forking averaged 17% with clone means ranging from 0% to 73%, and ramicorn branching averaged 24%, with clone means ranging from 3% to 50%. The estimated clone-mean repeatabilities were 0.86 and 0.67 for forking and ramicorn branching, respectively. Unfavorable genetic correlations between growth traits and forking defects were also found in this clonal test, suggesting that selection for either trait alone will negatively affect the genetic response for the other. A moderate positive genetic correlation between stem forking and ramicorn branching indicates that both traits may be partially controlled by the same genes and could be improved simultaneously.;Co-segregation analysis of markers and QTL for forking was conducted using the clonally replicated progeny of the outbred full-sib family from the clonal test. The linkage map was constructed with three types of markers containing various segregation patterns (F2, BC1 and BC2). A linkage map was achieved with18 linkage groups defined by 409 SNP markers. A single-marker analysis of 1257 loci identified 11 and 9 markers that were significantly associated with stem forking and ramicorn branching, respectively. Those SNP markers explained 7% to 10% of the total phenotypic variation for forking defects. By using the interval mapping method, two QTL were identified for forking, and three QTL were identified for ramicorn branching. Two QTL were detected for both traits using multiple traits analysis, suggesting there are pleiotropic effects on both traits. The result from this study further confirmed that there is genetic control for forking defects, and genetic gain can be achieved through family and clonal selection. With the identified QTL influencing forking defects, marker-assisted selection can be considered in decreasing forking defect in the breeding population of loblolly pine.