Exploring neutral and adaptive processes in expanding populations of gilthead sea bream, Sparus aurata L., in the North-East Atlantic

Abstract

Recent studies in empirical population genetics have highlighted the importance of taking into account both neutral and adaptive genetic variation in characterizing microevolutionary dynamics. Here, we explore the genetic population structure and the footprints of selection in four populations of the warm-temperate coastal fish, the gilthead sea bream (Sparus aurata), whose recent northward expansion has been linked to climate change. Samples were collected at four Atlantic locations, including Spain, Portugal, France and the South of Ireland, and genetically assayed using a suite of species-specific markers, including 15 putatively neutral microsatellites and 23 expressed sequence tag-linked markers, as well as a portion of the mitochondrial DNA (mtDNA) control region. Two of the putatively neutral markers, Bld-10 and Ad-10, bore signatures of strong directional selection, particularly in the newly established Irish population, although the potential 'surfing effect' of rare alleles at the edge of the expansion front was also considered. Analyses after the removal of these loci suggest low but significant population structure likely affected by some degree of gene flow counteracting random genetic drift. No signal of historic divergence was detected at mtDNA. BLAST searches conducted with all 38 markers used failed to identify specific genomic regions associated to adaptive functions. However, the availability of genomic resources for this commercially valuable species is rapidly increasing, bringing us closer to the understanding of the interplay between selective and neutral evolutionary forces, shaping population divergence of an expanding species in a heterogeneous milieu.

Candidate gene variation in gilthead sea bream reveals complex spatiotemporal selection patterns between marine and lagoon habitats

“…The aim of this study was to re-investigate patterns of genetic differentiation among sea bream populations collected in coastal lagoons of the Gulf of Lions using a larger array of microsatellite markers than in previous studies in this species (Alarcón et al 2004, De Innocentiis et al 2004, Chaoui et al 2009; but see Coscia et al 2012). A particular focus was made on microsatellite markers located close to 2 candidate genes, GH and Prl, that are functionally related to growth and osmoregulatory capacities in fish (Streelman & Kocher 2002, Almuly et al 2008, Quéré et al 2010) and have been previously showed to be involved in habitat-based genetic differentiation in sea bream (Chaoui et al 2012).…”

“…Protocols for PCR amplification and genotyping at these 2 loci are reported in Chaoui et al (2009). The remaining 21 microsatellite loci were developed by Franch et al (2006) and Coscia et al (2012) (Table S1) and were considered as putatively neutral. These loci were grouped in 2 multi plexes of 10 and 11 loci for GH and Prl genes, respectively.…”

Observations on the increase of wild gilthead seabream, Sparus aurata abundance, in the eastern Adriatic Sea: problems and opportunities

“…In the last decade, a significant increase in wild gilthead seabream populations has been documented and linked to climate change in some coastal zones such as the northeast Atlantic (Coscia et al 2011). In other Mediterranean locations such as Messolonghi lagoon in Greece, this phenomenon was related to the farming of large mature specimens in commercial aquaculture farms which spawned in the sea cages (Dimitriou et al 2007).…”

Sex change and effective population size: implications for population genetic studies in marine fish

“…We assumed a widely accepted rate μ = 11% per site per million year for the Sparid mtDNA control region (Bargelloni et al, 2003;Sala-Bozano et al, 2009;Coscia et al, 2012), equal to 0.055 substitutions per site per million years. The age at maximum egg production was estimated with L inf = 47 cm for C. puniceus and L inf = 75 cm for C. nufar (where L inf , a parameter of the von Bertalanffy growth equation, is defined as the length that an individual would reach if it grew indefinitely).…”