Lack of variation in voltage-gated sodium channels of common bottlenose dolphins ( Tursiops truncatus ) exposed to neurotoxic algal blooms

In coastal marine ecosystems, neurotoxins produced by harmful algal blooms (HABs) often result in large-scale mortality events of many marine species. Historical and frequent exposure to HABs therefore may provide a strong selective pressure for adaptations that result in toxin resistance. Neurotoxin resistance has independently evolved in a variety of terrestrial and marine species via mutations in genes encoding the toxin binding sites within the voltage-gated sodium channel gene complex. Accordingly, we tested the hypothesis that genetic variation in the putative binding site of brevetoxins in common bottlenose dolphins ( Tursiops truncatus ) explains differences among individuals or populations in resistance to harmful Karenia brevis blooms in the Gulf of Mexico. We found very little variation in the sodium channel exons encoding the putative brevetoxin binding site among bottlenose dolphins from central-west Florida and the Florida Panhandle. Our study included samples from several bottlenose dolphin mortality events associated with HABs, but we found no association between genetic variation and survival. We observed a significant effect of geographic region on genetic variation for some sodium channel isoforms, but this can be primarily explained by rare private alleles and is more likely a reflection of regional genetic differentiation than the cause of different levels of HAB resistance between regions. In contrast to many other previously studied neurotoxin-resistant species, we conclude that bottlenose dolphins have not evolved resistance to HABs via mutations in genes encoding the brevetoxin binding site on the voltage-gated sodium channels.