When positive selection of neurotoxin genes is missing: The riddle of the sea anemone Nematostella vectensis

Rapid evolution driven by positive Darwinian selection appears in toxins of vipers, scorpions, and marine snails. Although the vast phylogenetic distances between these animals suggest that this phenomenon is common, the recent release of the genome of Nematostella vectensis (Starlet anemone) as a collection of contigs portrays another extreme. Besides potassium channel toxin domains, which resemble potassium channel blockers, embedded in various genes, only one gene family encoding for sodium channel neurotoxins has been found, and the putative mature product of 10 family members is identical. Whereas the existence of a single toxin encoded by multiple genes may be explained by the unique ecology of N. vectensis, the complete absence of substitutions including synonymous ones is surprising and suggests either that these genes have been duplicated recently, or that their total conservation was advantageous. A retro-element identified downstream to one of the genes offers a possible mechanism of enhanced toxin gene duplication. This assumption still awaits further verification as soon as the various contigs are assigned within larger genomic fragments.