The Extent And Direction Of Introgression Between Three Spruce Species

Introgression frequently occurs between closely related species when there is incomplete reproductive isolation. However, few studies have been designed to examine the extent and direction of introgression from all three genomes, especially when they have different rates of gene flow. Conifers, in which chloroplast(cp), mitochondrial(mt) and nuclear(nr) genomes show contrasting rates of gene flow, provide a good model in which to carry out such a case study. Here we use population genetic data on sequence variation in 18 DNAs from three genomes for 311 individuals of 31 populations in order to investigate the extent and direction of introgressions between three spruce species(Picea wilsonii, P. meyeri and P. neoveitchii) in areas where they are sympatrically distributed. We found that one species(P. wilsonii) seem to have introgressions derived from the other two species(P. meyeri and P. neoveitchii) in areas of sympatric distribution;there were fewer introgressions from the paternally inherited cp DNA, which has the highest rate of gene flow, and more introgressions from the maternally inherited mtDNA, which has the lowest rate of gene flow, while moderate introgressions from the bi-parentally inherited nrDNA; Further coalescent analysis of population genetic data suggested that the species containing more introgressions had experienced range expansion in the recent past. Our results support the existing hypotheses that introgressions occurred more frequently at genetic markers with lower rates of gene flow and that they usually took place from local species towards invading species that had undergone recent demographic expansion.