Phylogenetic analysis of cytochrome c oxidase I sequences to determine higher-level relationships within the coleoid cephalopods

Until recently, the higher-level phylogenetic relationships of coleoid ceph alopods have remained unclear. A thorough knowledge of the higher-level phy logeny of the group has been limited by the paucity of paleontological data for this poorly-fossilized group and the lack of cladistic analyses of dev elopmental, morphological, and molecular data applied to the coleoids. In t his study we analyzed a 657 base pair portion of the mitochondrial cytochro me c oxidase I (COI) gene from 48 cephalopod species representing a broad s pectrum of coleoid diversity to examine higher-level phylogenetic relations hips within the group. The COI gene exhibited a high degree of nucleotide s equence variability, with one half of the sites varying in at least one tax on. COI amino acid sequences were highly conserved, but were useful in dete rmining basal-level relationships among the Coleoidea. The evolutionary rat e of COI amino acid sequences differed significantly between the two main l ineages. The average amino acid sequence divergence within the Octopodiform es was over twice that of the average divergence within the decapods. In ad dition to analysis of the unweighted data set, phylogenetic analysis was co nducted on the data subjected to a single round of rescaled consistency ind ex (RCI) weighting to reduce the effect of homoplasious substitutions in de termining phylogenetic structure. To further reduce the influence of homopl astic change and to facilitate bootstrap analysis of the data, a nested ana lysis of the data was employed, beginning with an analysis of the entire da ta set to determine proper outgroups to be used in the more restrictive ana lysis of the decapods and octopods separately. We draw the following conclu sions from our analysis of cephalopod COI sequences: (I) the Coleoidea, Oct opoda, Vampyromorpha, and Decapoda are monophyletic groups; (2) the Vampyro morpha and Octopoda are sister groups; (3) the Sepioidea, as including the five families Spirulidae, Sepiolidae, Sepiidae, Sepiadariidae, and Idiosepi idae, is polyphyletic; (4) Spirula is more closely related to the Teuthoide a than it is to the remaining members of the Sepioidea; and (5) the Oegopsi da, as currently defined is polyphyletic.