The Central-Periphery Hypothesis Revisited: Implications for Long-Term Genetic Conservation

The aim of the present study was to test the central-periphery hypothesis (CPH) of evolutionary genetics in terms of genetic diversity and differentiation in <i>Alnus glutinosa</i> (L.) Gaertn., 1790 and <i>Picea abies</i> (L.) H. Karst. populations. A total of 18 nuclear SSR loci were used to evaluate genetic diversity and differentiation of two rear-edge populations of each tree species from the south-eastern edges of their distribution ranges in Greece, and two populations of each species from the core distribution area in Lithuania. Peripheral populations of <i>A. glutinosa</i> exhibited high genetic diversity (mean <i>Ar</i> = 7.99, mean <i>He</i> = 0.72) and low genetic differentiation (peripheral and core population <i>F_ST</i> were 0.031 and 0.008, respectively). The genetic diversity values were even higher in the peripheral populations of <i>P. abies</i> (mean <i>Ar</i> = 12.27, mean <i>He</i> = 0.78), while genetic differentiation was also low (peripheral and core population <i>F_ST</i> was 0.013 and 0.011, respectively). Genetic differentiation between the peripheral and core regions was also low (<i>F_ST</i> = 0.038 and <i>G</i>″<i>_ST</i> = 0.262 for <i>A. glutinosa</i> and <i>F_ST</i> = 0.023 and <i>G</i>″<i>_ST</i> = 0.172 for <i>P. abies</i>). Observed heterozygosity was found to be higher in peripheral populations (0.80 on average for alder and 0.84 for spruce) than in core ones (0.72 and 0.83 on average for <i>A. glutinosa</i> and <i>P. abies</i>, respectively). On the other hand, expected heterozygosity was higher in <i>A. glutinosa</i> core populations than in its peripheral ones (0.73 vs. 0.72 on average for core and peripheral alder populations), while spruce populations were less heterozygous in the core area (0.78 vs. 0.75 on average for peripheral and core spruce populations, respectively). These results indicate only partial agreement with CPH. Rear-edge populations showed higher genetic differentiation, while their lower genetic diversity was not significantly different from that of core populations. The investigated rear-edge populations of <i>A. glutinosa</i> and <i>P. abies</i> present valuable genetic reserves of European importance. They show local adaptation and present ample genetic variation, and their effective population size will likely be sufficient for adaptive evolution in the future. Their long-term conservation status should be prioritized.