Environmental Correlates of Population Differentiation in Atlantic Herring

Abstract

The marine environment is characterized by few physical barriers, and pelagic fishes commonly show high migratory potential and low, albeit in some cases statistically significant, levels of genetic divergence in neutral genetic marker analyses. However, it is not clear whether low levels of differentiation reflect spatially separated populations experiencing gene flow or shallow population histories coupled with limited random genetic drift in large, demographically isolated populations undergoing independent evolutionary processes. Using information for nine microsatellite loci in a total of 1951 fish, we analyzed genetic differentiation among Atlantic herring from eleven spawning locations distributed along a longitudinal gradient from the North Sea to the Western Baltic. Overall genetic differentiation was low (theta = 0.008) but statistically significant. The area is characterized by a dramatic shift in hydrography from the highly saline and temperature stable North Sea to the brackish Baltic Sea, where temperatures show high annual variation. We used two different methods, a novel computational geometric approach and partial Mantel correlation analysis coupled with detailed environmental information from spawning locations to show that patterns of reproductive isolation covaried with salinity differences among spawning locations, independent of their geographical distance. We show that reproductive isolation can be maintained in marine fish populations exhibiting substantial mixing during larval and adult life stages. Analyses incorporating genetic, spatial, and environmental parameters indicated that isolating mechanisms are associated with the specific salinity conditions on spawning locations.

Detecting population structure in a high gene-flow species, Atlantic herring (Clupea harengus): direct, simultaneous evaluation of neutral vs putatively selected loci

“…Using the same fish for all analyses, thus excluding yearclass effects, we show that levels of population differentiation in herring were low for the presumed neutral genetic markers (allozymes, mtDNA and microsatellites), in agreement with earlier studies from the same region (Ryman et al, 1984;Hauser et al, 2001;Bekkevold et al 2005;J rgensen et al, 2005;Mariani et al, 2005;Larsson et al, 2007). For the putatively selected markers, the microsatellite locus Clha-DAA-INTR3 embedded in the MHC gene showed characteristics similar to those of the neutral microsatellite loci, and there was no indication of selection influencing this locus.…”

Inter-population differences in otolith morphology are genetically encoded in the killifish <i>Aphanius fasciatus</i> (Cyprinodontiformes)

“…We therefore suggest that the specific salinity and hydrological parameters of the Hamdoun site have triggered the genetic divergence in this population. This assumption concurs with results from previous studies, in which specific salinity parameters have been detected as the causative agents of strong selective pressure on organisms that eventually led to reproductive isolation in spite of possible intermingling during larval and adult life stages (Bekkevold et al 2005, Fuller 2008, Williams et al 2008.…”

Imprints from genetic drift and mutation imply relative divergence times across marine transition zones in a pan-European small pelagic fish (Sprattus sprattus)

“…Despite the usual lack of physical barriers in the sea it is now generally accepted that many marine organisms show population structures deviating from a pattern of panmixia and often distance may be the only factor restricting gene flow. Indeed, many species seem to display population structures reflecting barriers to gene flow over relatively small geographic scales (for example, Ruzzante et al, 1998;Bekkevold et al, 2005). Such genetic discontinuities are often referred to as phylogeographic breaks (Avise, 2000) and can arise and be maintained from a multitude of processes, including climatic and glacial cycles separating previously panmictic populations or connecting populations that diverged in allopatry (Barton and Hewitt, 1985).…”