Evolutionary developmental genetics of Norway spruce variety Picea abies 'Acrocona'
Final Report Abstract
The evolutionary origin of the angiosperm flower is still a scientific mystery. To solve it may require that the genetic basis of reproductive organ formation in the closest relatives of the angiosperms, gymnosperms, is explored. Unfortunately, all extant gymnosperms, such as spruce, pine, cycads and ginkgo (to mention but a few), are woody plants that usually require many years to enter the reproductive phase and hence are not easy to investigate. We used the unique opportunity to study three segregating populations of a natural variety of Norway spruce (Picea abies), termed acrocona. These populations had been generated by crossing an acrocona plant with itself or with wild-type plants. They thus contained closely related mutant and wild-type plants whose phenotype could hence be easily compared under almost identical and natural growth conditions over many years in the field. The acrocona variety is characterized by the development of female reproductive cones at wrong positions, that is at the top of the plant and of primary branches. Our detailed analysis of many plants of the different populations demonstrates, however, that the acrocona phenotype goes far beyond this trait. Rather, it comprises a complex syndrome of features that develops under growth conditions in the field over years, including also very early (precocious), frequent and proliferous female cone formation, the development of indeterminate and imperfect structures termed pseudocones, the formation of perfoliate cones, and the frequent development of whorls of vegetative shoots at the top of branches. Also typical traits of acrocona mutants are a bushy plant habit lacking monopodial growth, and dwarfism. However, even though the acrocona phenotype represents a complex syndrome of features its genetics appears fairly simple. By determining the positions of the gene loci causing the different traits on a genetic linkage map of Norway spruce we could show that they are all very likely caused by a semidominant mutation of a single gene, ACROCONA. We have chosen more than a dozen putative developmental control genes of Norway spruce that are probably involved in the transition from vegetative to reproductive growth, the specification of meristem identity and the determination of female reproductive organ identity, and located them on the same genetic linkage map. We found all of these genes on linkage groups different from that of ACROCONA, so that we can almost exclude that they represent the same gene. Many gymnosperms, especially conifers such as Norway spruce, are of enormous economic importance, e.g. for the production of timber and lumber. Due to their extremely long generation time little has been done so far to improve gymnosperms by breeding, however. The fact that acrocona develops cones not only ectopically at terminal branches, but also much earlier, more frequently and in much higher numbers than the wild type, provides a chance to develop the mutant into a system for accelerated breeding. Our detailed study of the phenotype and genetics of acrocona may have helped to path the way for a rapid breeding of Norway spruce, concerning important traits such as wood quality and environmental stress resistance.
