Phylogenetic relationships among the Decabrachia cephalopods inferred from mitochondrial DNA sequences.

ABSTRACT To clarify phylogenetic relationships among the decabrachia cephalopods, especially the family Sepiidae and Sepiolidae, mitochondrial cytochrome c oxidase subunit I (COI) gene and partial 16S rRNA gene were sequenced for 13 species. Phylogenetic analyses were performed by the distance and parsimony method. Coleoids were divided into 2 main lineages, Octobrachia and Decabrachia (including Sepiida, Sepiolida, and Teuthida). In all phylogenetic trees, the monophyly of the Sepiolidae and Sepiidae was supported well, but their rank and position within the Decabrachia were not clear. Based on partial COI rDNA and its amino acid sequences, Parsimony analyses showed Sepiolidae, Sepiidae, and the (Loligo chinensis, Chtenoteryx sicula) clade from Teuthida were in the same level. Compared with Sepiidae, Sepiolidae was more closely related to Teuthids using 16S rDNA sequences. We are inclined to support the current classification: Sepiolidae and Sepiidae belong to different Orders. According to the phylogenetic analysis, the 2 genera (Sepiella and Sepia) from the Sepiidae can be distinguished (78% neighbor-joining (NJ); 64% maximum parsimony (MP) in 16S rRNA gene), but do not have visible boundline using COI gene and its amino acid data. This suggests that COI gene may be much fitter to analyze cuttlefish phylogeny at a high taxonomic level (i.e., family), and 16S rRNA gene could be used as a precious tool to analyze taxonomic relationships at the genus level.

KEY WORDS: phylogeny, cephalopods, Decabrachia, COI, 16S rRNA gene

INTRODUCTION

According to Voss (1977), the Sepioidea includes Spirulidae, Sepiidae, Sepiolidae, Idiosepiidae, and Sepiadariidae. To date, the taxon system is still used in China. The position of the Sepiolidae as a sister group of the Sepiidae and the monophyly of the squids are, however, questioned by some scholars (e.g., Berthold & Engeser 1987, Clarke 1988, Boletzky 1999).

Molecular information from DNA data has been used to clarify the relationships among Cephalopods since the 1990s. The nucleotide divergence between sequences of one gene or a portion of gene from the mitochondrial or nuclear genome can be analyzed phylogenetically. Previous DNA sequence diversity and phylogenetic relationships of octopods have been investigated using the mitochondrial cytochrome c oxidase subunit I (mtCO I), mtCO II, mtCO III, 16S rRNA gene (Carlini & Graves1999, Carlini et al. 2001, Bonnaud et al. 1996, Bonnaud et al. 1997, Soller et al. 2000, Allcock & Piertney 2002, Piertney et al. 2003). Anderson (2000) sequenced 2 mitochondrial genes (16S rRNA and CO I) to clarify loliginid phylogeny. Sequence analyses from the 3' end of the mt 1-rRNA (16S) gene of decapod cephalopods have shown that this portion of gene was a useful tool for taxonomic relationships at the infrafarnilial level (Bonnaud et al. 1994). The COI gene for phylogenetic analysis of the coleoid cephalopods exhibited a high degree of nucleotide sequence variability, with one half of the sites varying in at least one taxon; COI amino acid sequences were highly conserved, but were useful in determining basal-level relationships among the Coleoidea (Carlini et al. 2001).

The cephalopods, especially decapods, are an important and valuable fishery resource in China, South Korea, and Japan (Nesis & Kir 1982, Okutani 1995). For example, Sepia esculenta, Sepiella maindroni and Loligo chinensis are all high commercial species (Dong 1991). A great deal of fundamental and applied researches on cephalopods, such as fauna, systematics, morphology, embryology, population genetics, and biodiversity have been completed since 1960s (e.g., Lee 1963, Lee 1983, Dong 1993, Lu 1998, Lu 2000, Zheng et al. 2001a, Zheng et al. 2001b, Zheng et al. 2004). There are, however, few documents referring to the molecular evolution and phylogenetics of cephalopods living in the coastal waters of China. In…