ORIGINAL PAPER Mapping species differences for adventitious rooting in a Corymbia torelliana × Corymbia citriodora subspecies variegata hybrid Mervyn Shepherd & Shabana Kasem & David J. Lee & Robert Henry Received: 15 August 2007 / Revised: 9 January 2008 / Accepted: 19 February 2008 / Published online: 19 March 2008 # Springer-Verlag 2008 Abstract Quantitative trait loci (QTL) detection was carried out for adventitious rooting and associated propagation traits in a second-generation outbred Corymbia torelliana × Corymbia citriodora subspecies variegata hybrid family (n =186). The parental species of this cross are divergent in their capacity to develop roots adventitiously on stem cuttings and their propensity to form lignotubers. For the ten traits studied, there was one or two QTL detected, with some QTL explaining large amounts of phenotypic varia- tion (e.g. 66% for one QTL for percentage rooting), suggesting that major effects influence rooting in this cross. Collocation of QTL for many strongly genetically correlat- ed rooting traits to a single region on linkage group 12 suggested pleiotropy. A three locus model was most parsimonious for linkage group 12, however, as differences in QTL position and lower genetic correlations suggested separate loci for each of the traits of shoot production and root initiation. Species differences were thought to be the major source of phenotypic variation for some rooting rate and root quality traits because of the major QTL effects and up to 59-fold larger homospecific deviations (attributed to species differences) relative to heterospecific deviations (attributed to standing variation within species) evident at some QTL for these traits. A large homospecific/hetero- specific ratio at major QTL suggested that the gene action evident in one cross may be indicative of gene action more broadly in hybrids between these species for some traits. Keywords QTL mapping . Propagation . Species effects Introduction Adaptation and speciation may give rise to differences in morphology or behaviour that separate species but which are not necessarily involved in species isolation and the block of gene flow (Orr 2001). Knowledge of the genetics of these species effects primarily comes from quantitative trait loci (QTL) studies in interspecific hybrids (Doebley and Lukens 1998; Orr 2001). When summarised across plants and animals and a range of traits under natural or sexual selection, not surprisingly, the genetic architecture of species differences is variable (Orr 2001). Nonetheless, species difference tends to involve modest numbers of QTL (1–20) and, when measured against standing variation within species, almost invariably involve genes of major effect (Orr 2001). The existence of such large effects segregating in interspecific hybrids, and the ability to associate them with genetic markers, has provided new approaches (i.e. deter- ministic variety development (Tanksley et al. 1989)) for improvement of domesticated crops (Lorz and Wenzel 2005; Paterson 1998). Studies of species differences between outcrossing tree species has also suggested Tree Genetics & Genomes (2008) 4:715–725 DOI 10.1007/s11295-008-0145-1 Communicated by D. Grattapaglia M. Shepherd (*) : S. Kasem : R. Henry Centre for Plant Conservation Genetics, Southern Cross University, P.O. Box 157, Lismore, NSW 2480, Australia e-mail: mervyn.shepherd@scu.edu.au M. Shepherd : R. Henry Cooperative Research Centre for Forestry, Southern Cross University, P.O. Box 157, Lismore, NSW 2480, Australia D. J. Lee Department of Primary Industries and Fisheries, Horticulture and Forestry Science, LB 16 Fraser Road, Gympie, QLD 4570, Australia