Conifer Evolution, from Demography and Local Adaptation to Evolutionary Rates Examples from the Picea genus

Abstract: Evolutionary process can be inferred at three different levels: the species level, the population level and the molecular level. In this thesis, I applied approaches at these three levels and aimed to get a comprehensive picture of conifer evolution, from speciation and demography to geographic variation and local adaptation, and then to the molecular evolution of proteins and small regulatory RNAs.Spruce species have been observed to possess a large number of trans-species shared polymorphisms. Using an “Isolation with migration” model, we found that the large effective population size of spruce retained these shared polymorphisms, inheriting them from the common ancestor. Post-divergence gene flow only existed between Picea abies and P. glauca, and between P. wilsonii and P. schrenkiana. The combination of Tajima’s D and Fay & Wu’s H at most of loci suggested an ancient and severe bottleneck for most species except P. breweriana.Furthermore, I investigated the effect of local selection in two parallel clines, which is one of the major forces that can cause divergence or even speciation. The timing of bud set and growth cessation was found correlated with latitude in populations of P. abies and P. obovata. Using allele frequency spectrum analyses we identified three genes under local selection in both species including two circadian-clock genes GI and PRR7, and one photoperiodic gene FTL2. This indicated that parallel evolution could occur through groups of genes within related pathways. Clinal variation at expression level provided stronger evidence of selection in FTL2, which has previously been associated with bud set in P. abies.Finally we focused on the molecular evolution of mRNA and small regulatory RNAs in P. abies. With the help of Next-Generation sequencing, we have achieved in spruce the first de novel assembly of the needle transcriptome and a preliminary characterization of sRNA populations. Along with features common in plants, spruce also exhibited novelties in many aspects including lower substitution rate and protein evolutionary rate, dominance of 21-nt sRNA, and a large proportion of TIR-NBS-LRR genes as sRNA sources and targets.