Molecular identification of hybrids between the two largest whale species, the blue whale (Balaenoptera musculus) and the fin whale (B. physalus).

Three anomalous balaenopterid whales, one pregnant female and two sterile males, were investigated by applying molecular approaches in order to establish their identity. The analysis showed that the whales were species hybrids between the blue and the fin whales. The female and one of the males had a blue whale mother and a fin whale father. The other male had a fin whale mother and a blue whale father. The difference between the mitochondrial cytochrome b gene of the two species suggests that they separated greater than or equal to 3.5 million years ago. The sequences of the mitochondrial control region of the blue and the fin whales differ by 7%. The difference in the mtDNA control region between three blue whale mtDNA haplotypes was less than or equal to 1%, about one tenth of the difference between the two species.