Gene flow at major transitional areas in sea bass (Dicentrarchus labrax) and the possible emergence of a hybrid swarm

Abstract

The population genetic structure of sea bass (Dicentrarchus labrax) along a transect from the Atlantic Ocean (AO) to the Eastern Mediterranean (EM) Sea differs from that of most other marine taxa in this area. Three populations (AO, Western Mediterranean [WM], EM) are recognized today, which were originally two allopatric populations. How two ancestral genetic units have evolved into three distinct units has not been addressed yet. Therefore, to investigate mechanisms that lead to the emergence of the central WM population, its current status, and its connectivity with the two parental populations, we applied 20 nuclear loci that were either gene associated or gene independent. Results confirmed the existence of three distinct gene pools, with higher differentiation at two transitional areas, the Almeria-Oran Front (AOF) and of the Siculo-Tunisian Strait (STS), than within any population. Significant linkage disequilibrium and heterozygote excess indicated that the STS is probably another tension zone, as already described for the AOF. Neutrality tests fail to reveal marker loci that could be driven by selection within or among metapopulations, except for locus DLA0068. Collectively, results support that the central WM population arose by trapping two tensions zones at distinct geographic locations of limited connectivity. Population assignment further revealed that WM individuals were more introgressed than individuals from the other two metapopulations. This suggests that this population might result from hybrid swarming, and was or is still seeded by genes received through the filter of each tension zone.

Metapopulation patterns of additive and nonadditive genetic variance in the sea bass (Dicentrarchus labrax)

“…Wild sea bass used to produce the broodstock for this study was collected from five distinct origins in the Northeastern Atlantic or the Mediterranean Sea recognized as the main subpopulations of wild sea bass (Chatain & Chavanne, 2009; Quéré et al., 2012) (Table 1). Former population genetics studies reported that the English Channel and the Bay of Biscay belong to the same panmictic unit in sea bass (Naciri et al., 1999; Lemaire et al., 2005; Fritsch et al., 2007; Coscia et al., 2012; Souche et al., 2015).…”

“…Panmixia justified to not include further population subdivision (as dam and/or sire) to the data set. The sea bass population from the Gulf of Lions was also found to be panmictic (Garcia de León, Chikhi, & Bonhomme, 1997; Quéré et al., 2012), and, as for NAT, no further population subdivision as sire or dam was included in the analyses. The distinction between NAT and SAT is motivated by findings by Castilho and McAndrew (1998) and Souche et al.…”

“…As for many other species (Patarnello et al., 2007), populations of sea bass have been shown to be genetically differentiated between the Atlantic and the Mediterranean Sea, and within the Mediterranean Sea itself (Quéré et al., 2012; Souche et al., 2015; Tine et al., 2014) (Figure 1). In sea bass, nuclear and mitochondrial (mtDNA) markers provided different pictures of genetic differentiation.…”

“…A mismatch is then observed between nuclear and mitochondrial data regarding the number of major metapopulation units in sea bass. How two ancestral populations evolved in three genetically distinct units remains unsolved (Quéré et al., 2012). …”

“…The broken blue line indicates further differentiation of Eastern Mediterranean metapopulation in a northern and a southern component. Based on nuclear markers, this differentiation has been reported twice in the literature (Castilho & Ciftci, 2005; Quéré et al., 2012), but geographic boundaries are poorly known. It is also observed in other species, such like the grey mullet ( Mugil cephalus ; Durand et al., 2013).…”