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 22,058bp 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 (4769bp), 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.

The phylogeny of Nereididae (Annelida) based on mitochondrial genomes

“…Aguado et al (), for example, used gene order variation to establish groups within Syllidae, while Zhang et al () used gene order changes in Aphroditiformia clades to reveal patterns of evolution in the group. The number of mitochondrial genomes sequenced in Annelida is still limited (Seixas, Russo, & Paiva, ; Zhang et al, ), but increasing.…”

Mitochondrial genome of Sabella spallanzanii (Gmelin, 1791) (Sabellida: Sabellidae)

“…spallanzanii mitogenome differs from that of Sp. giganteus where gene order is remarkably divergent from those of other annelid lineages (Seixas et al 2017).…”

Multiple introns in a deep-sea Annelid (Decemunciger: Ampharetidae) mitochondrial genome

“…1; Table 1). Other previously studied annelids have mtDNA sizes between 14,414 and 22,058 bp 14, 19, 20, 22, 30 . Although the mitochondrial genome size varied slightly among our three specimens, the intergenic region between nad2 and cox1 showed the greatest variation.…”

“…A recent analysis of Syllidae also showed marked variability on the order of protein enconding genes, with four distinct gene orders 14 . With only 89 complete mtDNAs sequenced from annelids 14, 15, 19, 20, 22 , more variation in gene orders will certainly be uncovered. Slight differences in the number of tRNAs were also revealed in Decemunciger sp., if compared to previous Terebelliformia mtDNA.…”

“…Recent descriptions of mitochondrial genomes from several annelid linneages revealed marked differences in gene order that are helping to resolve phylogenetic relationships, even though some inconsistencies between sequence data and phylogenies remain 14, 20, 21 . There are currently about 90 complete annelid mitochondrial DNA sequences (mtDNA) published 14, 22 , with many underrepresented linneages, making broad scale mitogenomic comparisons limited given the extremely high number of species in the deep-sea 13 . For instance, in the family Ampharetidae, there are only two incomplete mitochondrial genomes reported ( Eclysippe vanelli and Auchenoplax crinita ) 18 .…”