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Logo of bmcgenoBioMed Centralsearchsubmit a manuscriptregisterthis articleBMC Genomics
 
BMC Genomics. 2009; 10: 136.
Published online Mar 28, 2009. doi:  10.1186/1471-2164-10-136
PMCID: PMC2667193
A close phylogenetic relationship between Sipuncula and Annelida evidenced from the complete mitochondrial genome sequence of Phascolosoma esculenta
Xin Shen,corresponding author#1,2 Xiaoyin Ma,#3 Jianfeng Ren,2 and Fangqing Zhaocorresponding author4
1Jiangsu Key Laboratory of Marine Biotechnology/College of Marine Science, Huaihai Institute of Technology, Lianyungang 222005, PR China
2Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, PR China
3Wenzhou Medical College, Wenzhou 325035, PR China
4Department of Biochemistry and Molecular Biology, Pennsylvania State University, Pennsylvania 16802, USA
corresponding authorCorresponding author.
#Contributed equally.
Xin Shen: shenthin/at/163.com; Xiaoyin Ma: mxy082002/at/yahoo.com.cn; Jianfeng Ren: rjf98yy/at/yahoo.com.cn; Fangqing Zhao: fuz3/at/psu.edu
Received September 8, 2008; Accepted March 28, 2009.
Abstract
Background
There are many advantages to the application of complete mitochondrial (mt) genomes in the accurate reconstruction of phylogenetic relationships in Metazoa. Although over one thousand metazoan genomes have been sequenced, the taxonomic sampling is highly biased, left with many phyla without a single representative of complete mitochondrial genome. Sipuncula (peanut worms or star worms) is a small taxon of worm-like marine organisms with an uncertain phylogenetic position. In this report, we present the mitochondrial genome sequence of Phascolosoma esculenta, the first complete mitochondrial genome of the phylum.
Results
The mitochondrial genome of P.esculenta is 15,494 bp in length. The coding strand consists of 32.1% A, 21.5% C, 13.0% G, and 33.4% T bases (AT = 65.5%; AT skew = -0.019; GC skew = -0.248). It contains thirteen protein-coding genes (PCGs) with 3,709 codons in total, twenty-two transfer RNA genes, two ribosomal RNA genes and a non-coding AT-rich region (AT = 74.2%). All of the 37 identified genes are transcribed from the same DNA strand. Compared with the typical set of metazoan mt genomes, sipunculid lacks trnR but has an additional trnM. Maximum Likelihood and Bayesian analyses of the protein sequences show that Myzostomida, Sipuncula and Annelida (including echiurans and pogonophorans) form a monophyletic group, which supports a closer relationship between Sipuncula and Annelida than with Mollusca, Brachiopoda, and some other lophotrochozoan groups.
Conclusion
This is the first report of a complete mitochondrial genome as a representative within the phylum Sipuncula. It shares many more similar features with the four known annelid and one echiuran mtDNAs. Firstly, sipunculans and annelids share quite similar gene order in the mitochondrial genome, with all 37 genes located on the same strand; secondly, phylogenetic analyses based on the concatenated protein sequences also strongly support the sipunculan + annelid clade (including echiurans and pogonophorans). Hence annelid "key-characters" including segmentation may be more labile than previously assumed.
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