Genetic analysis of the Picea sitchensis×glauca introgression zone in British Columbia

A genetic analysis of the introgression zone between Sitka spruce (Picea sitchensis) and white spruce (P. glauca) in northwestern British Columbia was conducted to (1) quantify the contributions of parental species to introgressed populations; (2) characterize genetic patterns of variation across the introgression zone; and (3) address practical issues of gene resource management in this region. Seeds were sampled and seedlings grown from 57 putative hybrid parent trees from 16 locations across the introgression zone. Foliage samples were also obtained for allopatric Sitka and white spruce reference populations. Based on allele frequencies for sequence-tagged-site (STS) molecular markers, a hybrid index was developed that reflects the relative contribution of Sitka spruce to hybrid populations (0=white spruce, 1=Sitka spruce). Mean STS hybrid index estimates ranged from 0.46 to 0.95 for locations within the zone of introgression. Significant differences in hybrid index were observed among sampling locations. Hybrid indices decreased from the coast to the interior within the introgression zone in spite of large among-family variation within locations. Variation among locations in hybrid index was better explained by geographic variables than by climatic or ecological variables. Physical drainage distance along river valleys to the nearest marine inlet explained 78% of the variation in STS hybrid index. Concordant clines were obtained when the same analytical techniques were applied to published allozyme frequencies for seedlots originating from a wider geographic area across the introgression zone. Allozyme hybrid index estimates ranged from 0.01 to 0.98. The relatively steep cline observed in hybrid index across the maritime/continental climate ecotone is best managed by limiting longitudinal seed transfer for reforestation in this region.