The expression of three-finger toxins was extremely biased in Micrurus fulvius . Expression of three-finger toxin (3FTx) sequences was dominated by several multitranscript cluster members, suggesting a significant role for recent gene duplication within the gene family. A homologous toxin sequence from the mangrove snake (Boiga dendrophila) was used as an outgroup to root the phylogeny. Bayesian posterior probability values exceeding 50% are shown. Adjacent bars indicate expression levels relative to the most highly expressed member of the class, with a completely filled bar indicating the most highly expressed transcript of the class. (A) A maximum-likelihood phylogeny of M. fulvius 3FTx clusters under the HKY+G model. 3FTx expression levels were dominated by a handful of sequences, all representatives of multitranscript clusters (as indicated by the letter following the cluster number), suggesting a relationship between gene duplication and expression. (B) A maximum- likelihood phylogeny of M. fulvius transcripts identified in this study as well as orthologous 3FTx transcripts from M. corallinus and M. altirostris under the HKY+G model. Tips are color-coded by species. The subclade containing nine M. altirostris transcripts is sister to a sub-clade containing a single M. altirostris sequence and 3FTx-13 from M. fulvius. 3FTx-13 is expressed at a relatively low level, suggesting that the divergence of toxins occurs following speciation, at least for this one 3FTx paralog.