Unidirectional gene flow from the Gulf of Mexico to the Atlantic Ocean
in bonnethead sharks (Sphyrna tiburo)
Abstract
Gene flow is important for maintaining the genetic diversity required
for adaptation to environmental disturbances, though gene flow may be
limited by site fidelity in small coastal sharks. Bonnethead sharks
(Sphyrna tiburo) - a small coastal hammerhead species - demonstrate site
fidelity, as females are philopatric while males migrate to mediate gene
flow. Consequently, bonnetheads demonstrate population divergence with
distance and Atlantic populations are genetically distinct from those of
the Gulf of Mexico. Indeed, Florida forms a vicariant zone between these
two bodies of water for many marine species, including some sharks.
However, while bonnetheads are expected to have limited dispersal, the
extent and rate of bonnethead migration remains uncertain. Thus, we
aimed to determine their dispersal capacity by evaluating connectivity
between disparate populations from the Gulf of Mexico and Atlantic
Ocean. Using 10,733 SNPs derived from 2bRAD sequences, we evaluated
genetic connectivity between Tampa Bay on the Gulf Coast of Florida and
Biscayne Bay on the Atlantic coast of Florida. While standard analyses
of genetic structure revealed little differentiation between Tampa Bay
and Biscayne Bay populations, demographic history inference based on the
site frequency spectrum favored the model with divergence between 2000 -
6000 years ago, with continuous unidirectional gene flow from Tampa Bay
to Biscayne Bay. Combined with prior findings of small-scale genetic
divergence in bonnethead mitochondrial (female-transmitted) loci, our
findings support the hypothesis that female bonnetheads are highly
philopatric while male bonnetheads often migrate over relatively large
distances (>300 miles) to find mates. Together, these
results provide optimism that under proper management, a small-bodied
globally Endangered shark can undergo long migrations to sustain genetic
diversity.