Actin gene family evolution and the phylogeny of coleoid cephalopods (Mollusca: Cephalopoda)

Phylogenetic analysis conducted on a 784-bp fragment of 82 actin gene sequences of 44 coleoid cephalopod taxa, along with results obtained from genomic Southern blot analysis, confirmed the presence of at least three distinct actin loci in coleoids. Actin isoforms were characterized through phylogenetic analysis of representative cephalopod sequences from each of the three isoforms, along with translated actin cDNA sequences from a diverse array of metazoan taxa downloaded from GenBank. One of the three isoforms found in cephalopods was closely related to actin sequences expressed in the muscular tissues of other molluscs. A second isoform was most similar to cytoplasmic-specific actin amino acid sequences. The muscle type actins of molluscs were found to be distinct from those of arthropods, suggesting at least two independent derivations of muscle actins in the protostome lineage, although statistical support for this conclusion was lacking. Parsimony and maximum-likelihood analyses of two of the isoforms from which >30 orthologous coleoid sequences had been obtained (one of the cytoplasmic actins and the muscle actin) supported the monophyly of several higher-level coleoid taxa. These included the superorders Octopodiformes and Decapodiformes, the order Octopoda, the octopod suborder Incirrata, and the teuthoid suborder Myopsida. The monophyly of several taxonomic groups within the Decapodiformes was not supported, including the orders Teuthoidea and Sepioidea and the teuthoid suborder Oegopsida. Parametric bootstrap analysis conducted on the simulated cytoplasmic actin data set provided statistical support to reject the monophyly of the Sepioidea. Although parametric bootstrap analysis of the muscle actin isoform did not reject sepioid monophyly at the 5% level, the results (rejection at P = 0.068) were certainly suggestive of sepioid nonmonophyly.