Mitochondrial DNA sequence variation, demographic history, and population structure of Amur sturgeon Acipenser schrenckii Brandt, 1869

The variability of the mtDNA control region (D-loop) was examined in Amur sturgeon endemic to the Amur River. This species is also classified as critically endangered by the IUCN Red List of Threatened species. Sequencing of 796- to 812-bp fragments of the D-loop in 112 sturgeon collected in the Lower Amur revealed 73 different genotypes. The sample was characterized by a high level of haplotypic (0.976) and nucleotide (0.0194) diversity. The identified haplotypes split into two well-defined monophyletic groups, BG (n = 39) and SM (n = 34), differing (HKY distance) on average by 3.41% of nucleotide positions with an average level of intragroup differences of 0.54 and 1.23%, respectively. Moreover, the haplotypes of the SM groups differed by the presence of a 13-14 bp deletion. Most of the specimens (66 out of 112) carried BG haplotypes. Overall, the pattern of pairwise nucleotide differences and the results of neutrality tests, as well as the results of tests for compliance with the model of sudden demographic expansion or with the model of exponential growth pointed to a past significant increase in the number of Amur sturgeon, which was most clearly manifested in the analysis of data on the BG haplogroup. The constructed Bayesian skyline plots showed that this growth began about 18-16 thousand years ago. At present, the effective size of the strongly reduced (due to overfishing) population of Amur sturgeon may be equal to or even lower than it was before the beginning of this growth during the Last Glacial Maximum. The presence in the mitochondrial gene pool of Amur sturgeon of two haplogroups, their unequal evolutionary dynamics, and, judging by scanty data, their unequal representation in the Russian and Chinese parts of the Amur River basin point to the possible existence of at least two distinct populations of Amur sturgeon in the past.