Phylogenetic structure of Zacco platypus (Teleostei, Cyprinidae) populations on the upper and middle Chang Jiang (=Yangtze) drainage inferred from cytochrome b sequences.

We examined the genetic structure and phylogenetic relationships of some Chinese populations from the Chang Jiang (=Yangtze) drainage of the cyprinid Zacco platypus. We sequenced the complete mitochondrial cytochrome b gene of 64 individuals from 6 upper and middle tributaries of the Sichuan and Hunan Provinces to assess their population structure and systematics. The combined analyses of the phylogenetic information and the population structure suggested that Chinese Z. platypus consist of four distinct mtDNA lineages which exhibit high genetic variation and haplotypic diversity (Zacco A-D). The high molecular divergence observed among Zacco A-D mtDNA lineages (TrN+I (0.76) distance, mean 8.9%+/-1.7%) and their phylogeographic structure indicate that all four lineages have evolved independently. Analysis of molecular variance (AMOVA) indicates that most of the genetic variation observed is found among the four Zacco mtDNA lineages (thetaCT = 0.94) suggesting restricted gene flow among the Chang Jiang populations. Long-term interruption of gene flow was also evidenced by thetaST values higher than 0.9 that could be favoured by the discontinuous distributions of the lineages inhabiting upper (Sichuan Province) and middle (Hunan Province) Chang Jiang tributaries. The significant correlation between the geographic and genetic distances provide support for the importance of geographic discontinuity in shaping the Zacco genetic structure. Nested clade analysis (NCA) results were congruent with phylogenetic relationships recovered and confirm the genetic distinctiveness of four independent Zacco groups. These four groups correspond to the four Zacco A-D mtDNA lineages recovered in the phylogeny and were defined by nucleotide synapomorphies permitting bootstrapped and Bayesian confidence of 95% or greater. The high level of mitochondrial sequence divergence separating all Zacco mtDNA lineages suggested that the Z. platypus populations from the Chang Jiang drainage probably correspond to four different species.