Interspecific introgression and widespread intraspecific gene flow in a
clade of tropical and subtropical seabirds
Abstract
The mechanisms that restrict gene flow between populations and
facilitate population differentiation and speciation vary across the
tree of life. In systems where physical barriers to gene flow are
dynamic over time and space, such as many marine species, introgression
may be a major factor in the speciation process. In sympatric species of
seabirds with no land barriers between them, hybridization has been
frequently observed but few studies have investigated patterns of
introgression. We used whole-genome sequence data to test for
interspecific introgression between five pairs of tropical and
subtropical seabirds and to test for gene flow within species across
major land masses and ocean basins. We found evidence for introgression
between blue-footed (Sula nebouxii) and Peruvian boobies (S. variegata);
masked (S. dactylatra) and Nazca boobies (S. granti); and blue-footed
and Nazca boobies. We found no evidence of introgression between
blue-footed and brown boobies (S. leucogaster) and masked and brown
boobies despite observed hybridization between these species. We also
found evidence for gene flow across several major land masses in three
pantropical species: red-footed (S. sula), brown, and masked boobies.
Finally, we report evidence for ancient introgression between brown
boobies and the ancestor of blue-footed, Peruvian, masked, and Nazca
boobies. Our work indicates (1) that interspecific introgression has
shaped contemporary booby diversity in the eastern Pacific, and (2) that
contemporary physical barriers to gene flow between booby colonies are
not impenetrable. Our findings contribute novel insights to the growing
body of evidence that suggests introgression is a widespread
evolutionary process.