Mitochondrial genome of the Christmas tree worm Spirobranchus giganteus (Annelida: Serpulidae) reveals a high substitution rate among annelids

Abstract Here we describe, for the first time, the mitochondrial genome of Spirobranchus giganteus (Annelida: Serpulidae) and compare it with all available annelid mitogenomes. The entire mitogenome has 22,058 bp in length and bears 12 protein-coding genes (the ATP8 gene is missing), two rRNA, and 24 tRNA genes. The nucleotide composition and GC-skew are surprisingly different from those reported for other annelids. In addition, the pairwise genetic distances between the mitogenomes of S . giganteus and other annelids are higher than the distances for all annelid taxa analyzed. This result is consistent with a faster rate of mitochondrial sequence evolution in S . giganteus , which may explain the difficulty in obtaining PCR products with the available primers. The mitochondrial gene order of S . giganteus was remarkably different not only from that of the Sedentaria lineage, which includes S . giganteus , but also from the mitochondrial gene order of other major annelid lineages. The mitogenome of S . giganteus has no repetitive motifs despite its long control region (4769 bp), but genes are shorter and have a lower AT content than other members of Annelida. Finally, we show that mitochondrial gene order rearrangements can directly correlate to variations in gene length.