Population Genomics of Silver Birch

Silver birch (Betula pendula) is a prevalent pioneer tree species in boreal forests across Europe and Asia. It is an attractive model for modern genomics-based biotechnology, because flowering can be easily induced within one year of germination without transgenics approaches. We have assembled the genome sequence of a fourth generation self-pollinated individual and anchored it by genetic linkage to chromosomes. The genome size is 440 Mbp, and it has not undergone recent whole genome duplication events.

Evolution of the species was analyzed by partitioning duplicate genes based on their evolutionary origins. Duplicates from the pan-core-eudicot paleohexaploid event were enriched for transcription factor functions, whereas tandem duplicates were over-represented with environmental responses and secondary metabolism. Population genomic analyses with 80 resequenced individuals, representing most of the geographic species range, were carried out to study historical effective population size and patterns of selection. We found selective sweeps to be enriched for key developmental or physiological triggering functions, suggesting that at least in birch, selection has acted by tuning the timing and crosstalk between fundamental plant processes.