Eucalyptus is the main hardwood genus used in intensively managed forest tree plantations. Because of its high productivity on marginal sites, high fibre content and usefulness in producing a wide range of forest products, Eucalyptus has been widely planted in subtropical, tropical and temperate regions of the world, accounting for 8% of the planted forest area . Intensive silvicultural practices and traditional genetic improvement have both contributed to improve the productivity of wood biomass . Wood properties (WPs) have also become mandatory traits in breeding programmes for the development of improved varieties for pulp and paper, energy wood or timber production.
WPs vary greatly between species, within species and within a tree (reviewed by Plomion et al. ) and change with age (reviewed by Raymond ). They can be classified in five categories: i) mechanical properties in response to applied forces (e.g. longitudinal growth strain, modulus of elasticity, strength), ii) technological characteristics, which are the consequences of the mechanical state of the tree (e.g. splitting index), iii) physical characteristics, corresponding to the natural characteristics of wood which affect mechanical properties (e.g. wood density, shrinkage), iv) anatomical characteristics (e.g. fibre length, fibre thickness, coarseness), and v) traits related to chemical composition (e.g. cellulose and lignin content, lignin composition). A key constraint to higher-value uses of some Eucalyptus species used for timber production is their high internal stresses in the stem [5–7]. The release of growth stresses during cross-cutting may cause radial cracks at the log end. This "end-splitting" phenomenon increases with time according to an asymptotic curve. The extent of this degrade can be explained by the high level of growth stresses, low wood transverse strength and gradients of mechanical properties . Previous studies indicated that growth stresses were not correlated with growth or other WPs [9–11]. The chemical composition (cellulose, hemi-celluloses, lignins, flavonoids, tannins, etc.) of wood also greatly influences the properties and performance of pulp, paper and charcoal. Different authors reported strong correlations between pulp yield and cellulose content in E. urophylla  or lignin content in E. globulus . Another study suggested that the lower kraft pulp yield of E. viminalis can, to a large extent, be attributed to the higher soluble and insoluble tannin contents of the wood . Interestingly, lignin content has been reported to be negatively correlated with longitudinal growth strain in Eucalyptus hybrids , suggesting a relationship between micro (chemical composition) and macro (mechanical properties) characteristics of wood.
The genetic determinism of WPs has been fairly well studied in Eucalyptus . In E. globulus, Apiolaza et al.  reported moderate to high heritability for several WPs without any significant genetic correlation with growth, except between microfibril angle and trunk diameter (-0.86). In E. grandis, no significant correlation was found between wood density and growth . In E. urophylla, a narrow sense heritability of 0.5 was estimated for cellulose content with no correlation with growth . Silva et al.  found a significant genetic correlation between wood density (a highly heritable trait) and growth diameter in E. globulus. Other authors reported a significant correlation between WPs, e.g. microfibril angle and basic density (-0.70) and pulp yield and cellulose content (0.82) . Together, these results suggest: i) a higher heritability for WPs than growth-related traits, ii) some form of pleiotropic relationships between different WPs or close linkage between the genes controlling these traits, and iii) an inconsistency regarding the extent of genetic correlations between WPs and growth. Despite attempts to describe the number, location, effects and nature of the genes underlying these quantitative traits, up to now, very little effort has been made toward a more comprehensive analysis of the genetic architecture of WPs taking into account in a single experiment mechanical, physical, technological, anatomical and chemical properties.
Investigating the genetic basis of multi-scale WPs has become a major issue while the sequence of the Eucalyptus genome is nearing completion [2, 18]. Dissecting the genetic architecture of such complex traits can be addressed by QTL mapping . In Eucalyptus, several QTL studies have focused on wood density [20–22]. Barros et al.  identified molecular markers linked to wood splitting in open-pollinated E. grandis. Thamarus et al.  published the first significant results on the genetic architecture of WPs using high throughput phenotyping methods to detect QTLs for fibre length, cellulose, pulp yield and microfibril angle (MFA) in E. globulus. Three QTLs (wood-fibre, wood density and pulp yield) were detected across two half-sib pedigrees indicating a stable effect of the genetic control across different genetic backgrounds. Rocha et al.  found co-located QTLs for pulp yield and lignin content in an interspecific cross E. grandis × E. urophylla. Similar linkages were reported between pulp yield and cellulose content in E. globulus . More recently, Freeman et al.  discovered several genomic regions of E. globulus affecting physical (density) and chemical wood properties, e.g. co-located QTLs for lignin content, cellulose content and pulp yield. Finally, Thumma et al.  identified QTLs for density, MFA and wood chemical components in E. nitens.
The availability of Eucalyptus sequences in public databases (reviewed in Myburg et al. ) makes it possible to propose functional candidate genes a priori involved in the genetic control of the traits concerned, e.g. structural and regulatory genes involved in lignification for traits related to lignin content and lignin quality. Differentiating secondary xylem libraries  and high-throughput transcriptome sequencing  are also useful sources of functional molecular markers to decipher the nature of QTLs for WPs. Some co-locations between QTLs for wood quality related traits and functional candidate genes have already been reported: e.g. between a transcription factor (EgMYB2) and a QTL for lignin content in E. grandis , between a vascular-expressed Rac-like small GTPase (EgROP1) and lignin composition and fibre morphology in E. urophylla , between the gene encoding a cinnamoyl-CoA reductase (CCR) and a QTL for cellulose content in E. globulus , and between CCR and a QTL for MFA in E. nitens .
In this context, the objectives of this study were two-fold: i) to perform an extensive QTL analysis for a large range of WPs in an interspecific cross of Eucalyptus in order to dissect the genetic architecture of these complex multi-scale traits, ii) to identify positional candidate genes co-locating with WP-QTLs, as the basis of future positional cloning of major QTLs.