Genetic evidence reveals a unique lineage of Bryde’s whales in the northern Gulf of Mexico

Abstract

Bryde’s whales Balaenoptera edeni are the only year-round resident baleen whale species in the northern Gulf of Mexico (GOMx). The current population abundance estimate is 33 (CV 1.07) and the population is severely restricted in range. We characterized genetic diversity and phylogenetic relationships of these whales to other members of the Bryde’s whale complex. We analyzed DNA sequence data from 3 mitochondrial DNA (mtDNA) and 9 nuclear genes, and examined 42 nuclear microsatellite loci for 21 Bryde’s whale samples collected in the GOMx and 2 from the western North Atlantic. mtDNA diversity was extremely low; only 2 haplotypes were found in the first 375 bp of the control region and no variability in cytb or cox1 genes was seen. Twenty-five microsatellite loci were monomorphic, 16 had 2 or 3 alleles, and 1 had 4 alleles. Most nuclear genes exhibited shared alleles across balaenopterid species. Phylogenetic reconstruction using the control region and all published Bryde’s whale sequences revealed that GOMx Bryde’s whale haplotypes are evolutionarily distinct from other members of the Bryde’s whale complex examined to date. Within the first 375 bp of the control region, we found 25-26 fixed differences between GOMx haplotypes and those from sei whales and the 2 recognized Bryde’s whale subspecies. The GOMx whales are as divergent as these subspecies and as species are from each other. The level of divergence suggests a unique evolutionary trajectory worthy of its own taxonomic standing. The small population size and markedly low genetic diversity raise conservation concern for this unique group of whales.