Gene flow from Mediterranean Sea towards the Slope Sea revealed by
interspecific introgression
To understand why genetic differentiation is maintained despite
presumable ongoing gene flow, we studied the effect of interspecific
introgression. Mitochondrial introgression from albacore (Thunnus
alalunga ) into ABFT has been previously reported, with all introgressed
individuals detected so far found in the Mediterranean Sea and the Slope
Sea (Alvarado Bremer et al. 2005,
ViƱas et al. 2011), but not in the Gulf
of Mexico. According to three diagnostic positions for mitochondrial
ancestry (Table S2), we found albacore origin introgressed mitochondria
in individuals of all age classes in both the Mediterranean Sea (4%)
and the Slope Sea (6%), but also to a lower extent in Gulf of Mexico
adults (1%) (Figure 2A and Table S1). These results were confirmed at
the nuclear level by a tree-based analysis of population splits and
admixture using allele frequency data (TreeMix), which supported an
introgression event from albacore into the Mediterranean Sea ABFT
(Figure 2A and S7). In accordance with this deviation from a strict
bifurcating evolutionary history, we also found an excess of derived
allele sharing between albacore and both the Slope Sea and the
Mediterranean Sea with respect to the Gulf of Mexico (ABBA/BABA test,
Figure 2B). However, the contribution of albacore introgression to
genetic differentiation between Mediterranean Sea and Gulf of Mexico
ABFT populations appears to be limited at best (Figure S8) and therefore
it could contribute but not explain the maintenance of genetic
differentiation between MED-like and GOM-like individuals.
Our results confirm the introgression of mitochondrial DNA and show for
the first-time traces of introgression at the nuclear level from
albacore to the ABFT. The gradient of albacore ancestry further suggests
that this introgression occurred (or has been more intense) in the
Mediterranean Sea where the signal is strongest, and then diffused
towards the Slope Sea and, to a lesser extent the Gulf of Mexico through
multigenerational gene flow. These different introgression signal
intensities from east to west support gene flow between the
Mediterranean Sea and Slope Sea spawning components, which is in
accordance with the admixed nature of Slope Sea individuals. On the
other hand, the differences in the introgression signal intensities also
imply reduced gene flow from the Mediterranean Sea to the Gulf of
Mexico, which contrasts with the frequency of MED-like spawners observed
in the Gulf of Mexico. This, together with the fact that introgression
is not the main contributor to genetic differentiation between spawning
components, strongly suggests that other mechanisms, such as local
adaptation, maintain genetic differentiation in the presence of gene
flow, or that migration towards the Gulf of Mexico has increased
recently.