Sylleptic branching as an adaptive trait in Populus - QTL, candidate genes and association genetics
Résumé
Sylleptic branches, which grow from lateral buds during the same growing season, are known to play an important role in the generation of biomass in poplars through increased leaf area enabling the capture of more light and greater surface area for gaseous exchange. Sylleptic production may also be considered as an adaptive trait, being strongly influenced by environment. Control of sylleptic branch production has been hypothesised to be under the control of a number of genes, therefore a quantitative genetic approach is necessary to unravel the control of this trait. At present there are no definitive genes identified as being involved in initiation, growth and termination of sylleptic branches, and reasons for variation in their production between or within different species is largely unknown, although environment has been shown to have a role. This study aims to understand the control of sylleptic branching, and the traits adaptation to the environment using multiple approaches such as QTL mapping, hormone profiling and a candidate gene approach. As part of th POPYOMICS project, an F2 pedigree (Family 331; POP1) derived from a cross between Populus trichocarpa (93-968) and P. deltoides (111-129) was grown at three geographically different sites across Europe. These trials have established that the average number of sylleptics produced by P. trichocarpa was nearly six times (5.7) greater th an for P. deltoides. The F1 parents showed intermediate values and F2 shows normal distribution with evidence of transgressive segregation. Significant genotype x environment interactions were seen despite heritability values being moderate to high. Quantitative Trait Loci (QTL) were identified across the three sites. Additive genetic effects of QTL were seen to differ across sites for sylleptic branch trait QTL. The pedigree was also grown under elevated CO2 conditions which was seen to increase sylleptic branch production. QTL for response to CO2 concentration were identified. Hormone assessments showed levels to vary between the grand parent species which may go some way towards explaining the jack of control observed in P. trichocarpa on sylleptic branch production. Candidate genes seen to collocate to regions where QTL mapped were identified. The study 01 a natural population of P. nigra has shown variation in number of sylleptic branches to be highly significant across regions and may allow for a candidate gene approach to association mapping to be carried out.