Evidence for isolation by time in the European eel (Anguilla anguilla L.)

Abstract

Life history traits of highly vagile marine species, such as adult reproductive success and larval dispersal, are strongly determined by oceanographic and climatic forces. Nevertheless, marine organisms may show restricted dispersal in time and space. Patterns of isolation by distance (IBD) have been repeatedly observed in marine species. If spawning time is a function of geographical location, temporal and spatial isolation, can easily be confounded or misinterpreted. In this study, we aimed at discriminating between various forces shaping the genetic composition of recruiting juveniles of the European eel (Anguilla anguilla L.). By controlling for geographical variation, we assessed temporal variation and tested for possible isolation by time (IBT) between recruitment waves within and between years. Using 12 polymorphic allozyme and six variable microsatellite loci, we show that genetic differentiation was low (F(ST) = 0.01-0.002) and significant between temporal samples. Regression analysis between genetic and temporal distance, was consistent with a subtle interannual pattern of IBT. Our data suggest that the population dynamics of the European eel may be governed by a double pattern of temporal variance in genetic composition: (i) a broad-scale IBT of spawning cohorts, possibly as a consequence of the large migration loop in anguillids and strong variance in annual adult reproductive contribution; and (ii) a smaller-scale variance in reproductive success (genetic patchiness) within cohorts among seasonally separated spawning groups, most likely originating from fluctuating oceanic and climatic forces. The consistency of both mechanisms remains to be verified with fine-scale analyses of both spawning/migrating aged adults and their offspring to confirm the stochastic/deterministic nature of the IBT pattern in eel.

Panmixia in European eel revisited: no genetic difference between maturing adults from southern and northern Europe

“…When concentrating only on cohorts with larger sample sizes we also observed a weak (albeit not significant) tendency for increased differences over time, which is in accordance with Maes et al (2006) who observed such a pattern between glass eel samples separated by a few consecutive years. The amount of random genetic change over time at neutral loci (genetic drift) is expected to be inversely proportional to the current effective population size (N e ), which is the rationale for estimating this parameter from molecular data using the so-called temporal method.…”

“…Hence, we looked for indications of a temporal trend (compare Maes et al, 2006) through testing for a correlation between linearized genetic (F ST /(1ÀF ST )) and temporal distance, measured as the number of years lapsing between cohorts. Thus we treated the cohorts from Italy and the Baltic Sea as if belonging to the same population, which appeared justified considering the lack of any detectable spatial genetic divergence.…”

Genetic patchiness in European eel adults evidenced by molecular genetics and population dynamics modelling

“…Pearson's correlation coefficient r, degrees of freedom (df) and p-values result from Mantel's (1967) Similarly, a microsatellite survey in rice, wheat and barley showed nonrandom distribution and frequencies of genomic and EST-derived microsatellites (La Rota et al, 2005), suggesting that genomic microsatellites have the potential for a larger polymorphism than those derived from coding regions, which are length-constrained by purifying selection. While our study detected no increased differences over time, suggesting that heterogeneity among adult samples was not attributable to an IBT pattern, Maes et al (2006) detected a subtle IBT pattern among glass eels. A marginally significant correlation was observed between samples differing by 2-3 years between microsatellite genetic distance and daily distance (r = 0.184, p = 0.043) but not yearly distance (r = 0.095, p = 0.134) or using allozyme genetic distance (daily distance: r = 0.005, p = 0.450; yearly distance: r = 0.077, p = 0.166).…”

“…value of NSE and consequent NBE) could determine the F-statistic values obtained with the 22 genetic markers in this study. In addition, the model was run using F-statistic values obtained from literature including the study of Pujolar et al (2009a) conducted using the same markers but on glass eels, plus the studies of Wirth and Bernatchez (2001), Dannewitz et al (2005) and Maes et al (2006) on glass eels using genomic microsatellites. Simulations were run for 200 generations until convergence to stable F-statistic values.…”

“…First, the low number of genetic markers used, with all recent studies being based on 6-7 microsatellite loci (Wirth and Bernatchez, 2001;Dannewitz et al, 2005;Maes et al, 2006;Pujolar et al, 2006Pujolar et al, , 2007. Second, neutral loci (microsatellites) might fail to detect differences at loci affected by selection, which could lead to underestimating adaptive variation between populations (Conover et al, 2006).…”