Abstract
Population genetic studies on white sea bream Diplodus sargus have revealed different patterns in the subdivision of populations in the Mediterranean Sea. However, the stability of observed allele frequencies over time remains poorly tested. The aim of this study was to show that the genetic structure of D. sargus could significantly change over time by analysing temporal varia- tions in allozymes. In order to determine temporal variation in the genetic structure of 5 natural D. sargus populations in the SW Mediterranean, we screened 14 allozyme loci. Our main finding was the significant genotypic differentiation among cohorts (year-classes) in the Guardamar (FST = 0.012; p < 0.001) and Cape of Palos (FST = 0.008; p < 0.001) populations. The differentiation observed in the present study when considering pair-wise comparisons between cohorts is similar to that of all popu- lations throughout the Mediterranean Sea. Our results suggest that microgeographical variations, also known as 'chaotic genetic patchiness', could occur in D. sargus populations from the SW Mediterranean. The recruitment of genetically variable cohorts at 1 site each year may account for these variations. We also discussed alternative explanations for this genetic pattern. This study con- firms the importance of understanding the ecology, behaviour and environment of fish populations when investigating population genetic structure. Our results also highlight the importance of incor- porating temporal samples when conducting population structure studies.
Highlights
Many marine fishes exhibit high dispersal potential and a large population size and, as a consequence, one usually expects the degree of genetic differentiation among populations to be low (Bernal-Ramírez et al 2003). Such low genetic differentiation can account for the relationship between speciation and dispersal/population dynamics in marine species (Domingues et al 2006)
Five cohorts (95, 96, 97, 98 and 99) accounted for 69 to 81.6% of the total population; in the Tabarca population, these cohorts accounted for only 54.5%
In terms of pair-wise comparisons between cohorts, results from previous studies on the genetic differentiation of D. sargus throughout the Mediterranean Sea are similar to those obtained in the present study, which raises questions on the value and origin of genetic divergence observed between distant populations (Lenfant & Planes 1996, González-Wangüemert et al 2004, Pérez-Ruzafa et al 2006, GonzálezWangüemert et al 2006b)
Summary
Many marine fishes exhibit high dispersal potential and a large population size and, as a consequence, one usually expects the degree of genetic differentiation among populations to be low (Bernal-Ramírez et al 2003). Such low genetic differentiation can account for the relationship between speciation and dispersal/population dynamics in marine species (Domingues et al 2006). Most of the available research is limited in terms of its description and discussion of observed genetic patterns. Some studies have documented the relevance of long-term genetic stability research, in the context of understanding which factors
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