Range expansion and adaptation of Sitka spruce
Temperate tree species of the northern hemisphere share a history landscape recolonization after the last ice age, which ended about 18,000 years ago. Sitka spruce (Picea sitchensis) is one of them: this conifer has a long and narrow distribution that spans the Pacific coast of North America from California to Alaska. Its ongoing expansion in South-central Alaska is a prime natural laboratory to study the evolutionary outcome of climate-driven range shifts.
Taking advantage of the species’ remarkable longevity, this project combined neutral genetic markers and tree ring data to assess the pace of neutral evolution of Sitka spruce across five centuries of colonization on the Kodiak Archipelago. With this original combination of historical and genetic data, Joane found that allelic richness was efficiently recovered at the expansion front by early, open-grown colonizers, but heterozygosity remains low compared with the nearest mainland populations.
Range expansion often results in secondary contact with sister species: This is the case of Sitka and white spruce in South-central Alaska. Local hybridization of P. sitchensis colonizers with foreign pollen from white spruce (Picea glauca) populations occurred repeatedly during the early colonization period. However, introgression was suppressed in subsequent generations growing under a closed canopy, a likely result of pollen competition and selection.
This empirical project showed that understanding the demographic history of populations is an essential prerequisite to predicting their evolution. Joane explored the power and limitations of approximate Bayesian computation, one of the most widely used statistical genetics method used in demographic inference in natural populations. Specifically, she assessed the extent to which demographic parameters of range expansion can be estimated from genomic data and tested the effect of the type and quality of genomic resources used on the precision of the inference.
Funding: NSERC Discovery Grant, UBC Strategic Recruitment Fellowship