Population genetic structure of the Japanese eel Anguilla japonica: panmixia at spatial and temporal scales

Abstract

Since the 1970s, the population of the Japanese eel Anguilla japonica has dramatically declined in East Asia. Consequently, conservation and resource management of this species are urgently required. However, the population genetic structure of this species, in temporal and spatial scales, is still poorly understood. We used 8 polymorphic microsatellite DNA loci to investigate its genetic composition. For cohort analysis, juvenile (glass) eels were collected yearly between 1986 and 2007 from the Danshui River, Taiwan; for arrival wave analysis, glass eels were collected monthly from Fulong Estuary, Taiwan; and for spatial analysis, glass eels were collected from Taiwan, China, Korea and Japan. Genetic differentiation among annual cohorts, arrival waves and spatial samples was very low; a significant difference was observed among annual cohorts and spatial samples, but not among arrival waves. However, specific temporal or spatial scale patterns were not seen in either pairwise genetic comparisons or the phylogenetic tree of all samples. Occasional genetic variations among samples occurred randomly, but a stable lasting genetic structure could not be formed. The isolation by distance (IBD) test showed no evidence of genetic structuring at the spatial scale, and the results of the isolation by time (IBT) test were insignificant among arrival waves. Genetic heterogeneity over a 21 yr time scale showed marginal significance, potentially reflecting a genetic drift in the Japanese eel. Our results suggest the existence of a single panmictic population of Japanese eel in East Asia. Therefore, the Japanese eel should be considered as a single management unit for conservation.

Genetic evidence for panmixia of Japanese eel (Anguilla japonica) populations in China

“…Significant global genetic differentiation was found in microsatellite data extracted from Japanese glass eel samples (F ST = 0.0026, P = 0.008), and these results were similar to those of other studies using one or more of these markers (F ST = 0.003, P < 0.001) (Han et al, 2010a). Nonetheless, the conclusion that the population genetic structure for Japanese eels shows spatial genetic differentiation is not easily drawn based on the present analysis.…”

“…For the microsatellite DNA, AMOVA for each locus indicated that the majority of total genetic variation was located within samples (99.38%) and a mere 0.49% of the variation was found between samples, which is similar to results found by Han et al (2010a). AMOVA for each locus indicated significant genetic variation within samples (>99.4%), which is much higher than the previously reported levels of 95.73% documented by Tseng et al (2006).…”

“…Although the genetic composition of the Japanese eel in a single location was found to be generally stable, global genetic differentiation for samples taken from cohorts over multiple years was significant (F ST = 0.002, P = 0.002) (Han et al, 2008). The seemingly conflicting results among previous studies of genetic structure in European and Japanese eel populations, ranging from no detectable heterogeneity to small but statistically significant differences, can be explained through isolation by distance or temporal patterns among eel specimen samples distributed across vast continental regions (Daemen et al, 2001;Wirth and Bernatchez, 2001;Maes and Volckaert, 2002;Dannewitz et al, 2005;Andrello et al, 2006;Maes et al, 2006;Pujola et al, 2006;Tseng et al, 2006;Chang et al, 2007;Palm et al, 2009;Han et al, 2010a). These differences may also be accounted for by variations in sampling design and choice of molecular markers, combined with a lack of power estimates or complex methods for effectively comparing existing results that may exhibit various margins of error (Palm et al, 2009).…”

“…A comparison of annual cohort (or "arrival wave") studies focusing on European eel populations revealed evidence of genetic patchiness Pujolar et al, 2006). In Japanese eel populations, Han et al (2010a) observed slight, but distinctly significant, genetic differentiation among glass eel annual cohorts collected from a single location in the Danshui River, Taiwan.…”

“…Some of these studies, including all studies cited above, were limited by research methodology, employing only one DNA marker (microsatellites) (Sang et al, 1994;Chan et al, 1997;Ishikawa et al, 2001;Tseng et al, 2006;Chang et al, 2007;Han et al, 2010a,b). Other results were skewed by the use of relatively small numbers of loci (6-8 microsatellite loci) (Tseng et al, 2006;Han et al, 2010a) or sampling bias due to specimen collection from a single-annual cohort, resulting in misleading conclusions when applied to an entire geographic population (Sang et al, 1994;Ishikawa et al, 2001;Tseng et al, 2006;Han et al, 2010a).…”