Introgressive hybridization in North American hakes after secondary contact.

High levels of bidirectional introgressive hybridization were found between the two Atlantic North American hakes Merluccius albidus and M. bilinearis in their overlapping distribution area between the 34 and the 42 degrees N parallels, employing mitochondrial and nuclear DNA markers. Absence of F(1) hybrids, and varied levels of bidirectional introgression, indicate long-time hybridization and backcrossing. Based on the evolutionary history of the genus Merluccius, originated in this area from the ancestor of the present M. bilinearis by the rise of the Panama Isthmus, secondary contact between the two species has probably been promoted by northwards displacement of M. albidus. Higher introgression rates in southern areas of M. albidus could be explained by restricted gene flow in that area which may allow long-term accumulation of introgressed genes.