4. Discussion
Mitochondrial DNA was widely used to analyze the population genetic structure and phylogeny in different species. The mitochondrial D-Loop region, with the highest variation and mutation rate of mtDNA, has been thought as an appropriate marker to study the genetic diversity and genetic differentiation in fishes, such as common carps (Thai et al., 2005; Liang et al., 2009) and tilapia (Szitenberg et al., 2012; Zou et al., 2015). The aquaculture of swamp eel as an economically important fish has made rapid progress, however the genetic resources of wild swamp eel has suffered serious challenge because of overfishing (Lei et al., 2012). Thus, background knowledge of genetic diversity and population structure is necessary to effectively protect and sustainably utilize swamp eel. In this study, the genetic diversity of six wild populations from dominant culturing regions in China was analyzed. The results showed overall haplotype diversity was high in the six investigated populations. Among them, genetic diversity from CQ population of Monopterus albus was the lowest. Only two haplotypes with four variation sites were observed and uniquely existed in the CQ population. Cai et al. (2008) also analyzed the genetic diversity of swamp eel of Sichuan Basin and found that genetic diversity in Chongqing populations was low (Hd ¼ 0.404, p ¼ 0.0007). Lacking of genetic variation of swamp eel populations may result from the founder effects, intrinsic traits and/or genetic drift (Ray et al., 2015). The haplotype diversity (Hd) and nucleotide diversity is always used to measure the degree of polymorphism within a population. The haplotype diversity results showed that the genetic variation of swamp eel in the Anhui province were abundant (0.708)
