Large‐scale asymmetric introgression of cytoplasmicDNAreveals Holocene range displacement in a North American boreal pine complex

Abstract

Jack pine (Pinus banksiana) and lodgepole pine (Pinus contorta var. latifolia) are two North American boreal hard pines that hybridize in their zone of contact in western Canada. The main objective of this study was to characterize their patterns of introgression resulting from past and recent gene flow, using cytoplasmic markers having maternal or paternal inheritance. Mitochondrial DNA (mtDNA) and chloroplast DNA (cpDNA) diversity was assessed in allopatric populations of each species and in stands from the current zone of contact containing morphological hybrids. Cluster analyses were used to identify genetic discontinuities among groups of populations. A canonical analysis was also conducted to detect putative associations among cytoplasmic DNA variation, tree morphology, and site ecological features. MtDNA introgression was extensive and asymmetric: it was detected in P. banksiana populations from the hybrid zone and from allopatric areas, but not in P. contorta populations. Very weak cpDNA introgression was observed, and only in P. banksiana populations. The mtDNA introgression pattern indicated that central Canada was first colonized by migrants from a P. contorta glacial population located west of the Rocky Mountains, before being replaced by P. banksiana migrating westward during the Holocene. In contrast, extensive pollen gene flow would have erased the cpDNA traces of this ancient presence of P. contorta. Additional evidence for this process was provided by the results of canonical analysis, which indicated that the current cpDNA background of trees reflected recent pollen gene flow from the surrounding dominant species rather than historical events that took place during the postglacial colonization.

Integrating phylogeography and paleoecology to investigate the origin and dynamics of hybrid zones: insights from two widespreadNorthAmerican firs

“…Within the current allopatric range of the invader species, populations showing evidence of mtDNA capture mark a historical occurrence of the resident species (see empirical example of Godbout et al . ; reporting this pattern for Pinus contorta × P. banksiana ). In such a case, the introgressed population located farther away from the current sympatric zone should approximately indicate the maximum extent of the resident species distribution ( i.e .…”

“…Hence, mtDNA capture was detected far within Pinus banksiana allopatric range and differentiation between allopatric regions was higher for cpDNA than mtDNA markers (Godbout et al . ). A pattern of asymmetric introgression was also reported in the moving hybrid zone between Picea engelmannii and Picea glauca : most introgressed individuals had a higher genetic contribution from the resident Picea engelmannii than the invader Picea glauca (De La Torre et al .…”

“…; Petit & Excoffier ; Godbout et al . ). Petit & Excoffier () reviewed relative rates of introgression at different markers in animal species with sex‐biased dispersal.…”

“…This colonization pattern and hybrid zone dynamics contrast with that reported for two North American species complexes: pines (Godbout et al . ) and spruces (De La Torre et al . ).…”

“…; Godbout et al . ). Indeed, by construction, first generation hybrids carry the cpDNA from the paternal species, the mtDNA from the maternal species and half of each parental species nuclear genome (ncDNA).…”