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

Phylogenetic analysis conducted on a 784-bp fragment of 82 actin gene seque nces of 44 coleoid cephalopod taxa, along with results obtained from genomi c Southern blot analysis, confirmed the presence of at least three distinct actin loci in coleoids. Actin isoforms were characterized through phylogen etic 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 i n cephalopods was closely related to actin sequences expressed in the muscu lar tissues of other molluscs. A second isoform was most similar to cytopla smic-specific actin amino acid sequences. The muscle type actins of mollusc s were found to be distinct from those of arthropods, suggesting at least t wo independent derivations of muscle actins in the protostome lineage, alth ough statistical support for this conclusion was lacking. Parsimony and max imum-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 orde r Octopoda, the octopod suborder Incirrata, and the teuthoid suborder Myops ida. The monophyly of several taxonomic groups within the Decapodiformes wa s not supported, including the orders Teuthoidea and Sepioidea and the teut hoid suborder Oegopsida. Parametric bootstrap analysis conducted on the sim ulated cytoplasmic actin data set provided statistical support to reject th e 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 nonm onophyly.