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Molecular genetic diversity among natural populations of Populus Ismail, Mohamed


Genetic diversity is a key factor in species survival, evolution, and adaptation. It also reveals species genetic structure and provides insights into how different demographic forces shape species genetic variability. Although, black cottonwood (Populus trichocarpa Torr. & Gray) is the first tree to have its genome completely sequenced; however, information regarding its natural genetic diversity and population structure is lacking. I have investigated the extent of genetic diversity within and among 38 natural populations of P. trichocarpa sampled across British Columbia using 10 nuclear (nuSSR) and 12 chloroplast microsatellite (cpSSR) markers. CpSSR represents two haplotypes, clustering as northern and southern groups; however, a Bayesian population structure analysis suggested the presence of three highly admixed groups supported by low population differentiation (low FST and RST). Monmonier’s spatial analysis suggested the presence of one genetic discontinuity dividing the studied area into northern and southern regions. These findings indicated that P. trichocarpa might have originated from two, northern and southern, glacial refugia that have experienced moderate contact through extensive gene flow. Nucleotide diversity for 10 candidate-gene loci involved in adaptive, defence, and housekeeping functions was abundant and varied across loci, with the majority showing neutral variations. Linkage disequilibrium (LD), decays rapidly to r² ≈ 0.18 within 700 base pairs (bp). Comparing the nucleotide diversity between P. trichocarpa and P. balsamifera L. to the Eurasian P. tremula L. indicated that the two North American species had lower diversity (θw range 0.002 to 0.004) than the Eurasian poplar (θw = 0.005). The estimated time of divergence between the two North American and the Eurasian species indicated that the latter was five- to six-fold older compared to the two former species. The substitution rate was lower in North American species (0.4 x 10-⁸ per year) compared to the Eurasian poplar (2 x 10-⁸). Different association genetics models produced strikingly different results after the inclusion or exclusion of population structure, highlighting the importance of proper model construction.

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