Genetic diversity and population structure of two lancelets along the coast of China

The western Pacific lancelet, once recognized as a monospecies, Branchiostoma belcheri, is a frequently used model in evolutionary and developmental studies, and researchers usually collect samples from the field without consideration of species identification and genetic divergence. However, recent studies found divergence of the lancelets from different localities and divided this monospecies into two separate species (S. belcheri and B. japonicum). To further estimate the genetic diversity of lancelet populations and the cause of their formation, we sampled 70 individuals from four major distribution areas along the coast of China, using both mitochondrial DNA and microsatellite markers in this investigation. Our results demonstrate that the two species possess extremely high genetic diversity at both mtDNA sequence level (h approaches 1.0) and microsatellite loci (He is above 0.8). Further demographic analysis reveals that the lancelets B. japonicum and B. belcheri underwent a recent historical population expansion at approximately 117,000 and 73,000 years ago respectively. Analyses on the population genetic structure revealed weak differentiation among different local populations. No evident differentiation was found among different local populations of the same species using mtDNA sequence data, but certain divergences among them were identified based on the microsatellite data. We suggest that discontinuous habitats may be responsible for the phylogeographic structure of the lancelets along China coasts.