Date Awarded

1998

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

John E. Graves

Abstract

Although the fossil record of early cephalopods is rich and demonstrates the dominance of the group in Paleozoic times, the mainly soft-bodied coleoids (Cephalopoda: Coleoidea) are poorly represented. Therefore, little is known of the evolutionary history of coleoids through paleontology and current classifications of the subclass are based primarily on the morphology of extant representatives. A molecular phylogenetic analysis of the Coleoidea was therefore warranted. Phylogenetic relationships within the Coleoidea were constructed using molecular sequence data from one mitochondrial and two nuclear genes: cytochrome c oxidase I (COI) and two unlinked actin genes (Actin I and Actin II, respectively). A 657 base-pair portion of the COI gene was examined for 55 coleoid taxa encompassing a broad spectrum of diversity in the subclass. The COI gene exhibited the most rapid evolutionary rate among the three genes examined. Eighty-two sequences from a 784 base-pair portion of three paralogous actin genes were obtained from 44 terminal taxa. The Actin I gene was highly conserved and provided information for determining deep-level relationships. The Actin II gene was intermediately conserved and exhibited a broad range of sequence divergence than the COI and Actin I genes. The evolution of the actin gene family in cephalopods was compared to that in other molluscs, protostomes, and deuterostomes. Analyses of actin gene family evolution provided evidence that the Actin I gene encodes a muscle-type of actin, and that the Actin II gene encodes a cytoplasmic actin. These analyses also supported at least two independent derivations of muscle-type actins during the evolution of the protostome lineage. The following conclusions were drawn from the results of phylogenetic analyses: (1) the cephalopod subclass Coleoidea is monophyletic; (2) the order Octopoda is monophyletic and is sister group to the monotypic order Vampyromorpha; (3) the Decapodiformes, consisting of the orders Teuthoidea and Sepioidea, is monophyletic; (4) the orders Teuthoidea and Sepioidea are polyphyletic; (5) the teuthoid suborders Myopsida and Oegopsida are monophyletic and polyphyletic, respectively; (6) the Myopsida and the oegopsid families Chtenopterygidae and Bathyteuthidae are more closely related to the sepioid families Spirulidae, Sepiidae, and Sepiolidae, than they are to other teuthoid groups.

DOI

https://dx.doi.org/doi:10.25773/v5-3pyk-f023

Rights

© The Author

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