| The genus Sillago Cuvier 1817, which belongs to family Sillaginidae, suborder Percoidei, order Perciformes, is widely distributed from the Indo-West Pacific Ocean, Genus Sillago are easily misidentified due to their similarity of shape and body color. In recent years, many new species and cryptic species of Sillago have been demonstrated with molecular studies. In the present study, mitochondrial genome (mitogenome) sequences of six Sillago species, including Sillago aeolus, Sillago asiatica, Sillago sinica, Sillago japonica, Sillago sihama and Sillago indica, were determined with the genomic organization and structure being analyzed and compared. The phylogenetic relationship among six Sillago species was probed based on mitochondrial genome sequences. The main results are presented as follows:1. Characterisation of the mitogenomes of six Sillago speciesThe length of complete mitogenomes of the six Sillago species were ranged from 16493bp to 17119bp. The longtest mitogenome sequence of all was S. japonica and the shortest was S. asiatica. There were two non-coding genes between tRNALeu(UUR) and ND1 (103bp) as well as tRNAGln and tRNAMet (235bp), respectively. The length of all protein-coding genes of the six Sillago species were identical, excluding the ND2 gene (9bp indels) and ATP6 gene (6bp indels) of S. japonica. The control regions were variable and ranged from 826bp of S. sinica to 1274bp of S. sihama. There were tandem repeats in the control region of S. sihama, it was a 36bp unit and repeated 8 times. The six mitogenomes carry set of 22 tRNAs,2 rRNAs,13 protein-coding genes as well as a putative control region and OL, the gene order of which is identical to those of canonical vertebrate mitogenomes except for S. sinica whoes tRNAAla and tRNAAsn swapped. The nucleotide composition of the six mitogenomes was characterized by a strong excess of C relative to G but a slight excess of A relative to T on the H-strand. Anti-G bias and anti-C bias were found at the third codon positions of H- and L-strand, respectively. The protein-coding genes in all six mitogenomes employed the same start codon ATG except for COI gene as well as ATP6 of S. sihama being started with GTG, and ATP6 of S. japonica started with CTG. Four stop codons (TAA, TAG, TA-, and T--) were used in the six mitogenomes. All tRNA genes could putatively form typical cloverleaf secondary structures with the exception of tRNASer(AGY). Apart from the conserved blocks TAS and CSB-1,-2, and -3, all of the six putative central conserved sequences blocks (CSB-A to CSB-F) could be recognized.2. Phylogenetic analysis of Sillago species based on the complete mitogenomesAll rRNA genes, tRNA genes, the first and the second codon positions of 12 H-strand mitochondrial protein-coding genes were used for phylogenetic analysis after substitution saturation analysis in DAMBE. The phylogenetic relationships using NJ algorithm and partitioned Bayesian analysis were consistent:S. sihama represented the sister taxon with S. indica and they clustered with the sister lineages of S. japonica and S. asiatica; then they clustered together with S. sincia, S. aeolus showed the most distant relationships with others. The Kimura 2-parameters genetic distances revealed that S. aeolus and S. japonica showed the farthest distance (D= 10.6%) and S. sihama and S. indica showed the nearest distance (D= 6%). |
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