Genome sequencing and population genomic analyses provide insights into the adaptive landscape of silver birch

Abstract

Silver birch ($\textit{Betula pendula}$) is a pioneer boreal tree that can be induced to flower within 1 year. Its rapid life cycle, small (440-Mb) genome, and advanced germplasm resources make birch an attractive model for forest biotechnology. We assembled and chromosomally anchored the nuclear genome of an inbred $\textit{B. pendula}$ individual. Gene duplicates from the paleohexaploid event were enriched for transcriptional regulation, whereas tandem duplicates were overrepresented by environmental responses. Population resequencing of 80 individuals showed effective population size crashes at major points of climatic upheaval. Selective sweeps were enriched among polyploid duplicates encoding key developmental and physiological triggering functions, suggesting that local adaptation has tuned the timing of and cross-talk between fundamental plant processes. Variation around the tightly-linked light response genes $\textit{PHYC}$ and $\textit{FRS10}$ correlated with latitude and longitude and temperature, and with precipitation for $\textit{PHYC}$. Similar associations characterized the growth-promoting cytokinin response regulator ARR1, and the wood development genes $\textit{KAK}$ and $\textit{MED5A}$.