Abstract
BackgroundAnnelids are one the most speciose and ecologically diverse groups of metazoans. Although a significant effort has been recently invested in sequencing genomes of a wide array of metazoans, many orders and families within the phylum Annelida are still represented by a single specimen of a single species. The genus of interstitial annelids Ophryotrocha (Dorvilleidae, Errantia, Annelida) is among these neglected groups, despite its extensive use as model organism in numerous studies on the evolution of life history, physiological and ecological traits. To compensate for the paucity of genomic information in this genus, we here obtained novel complete mitochondrial genomes of six Ophryotrocha species using next generation sequencing. In addition, we investigated the evolution of the reproductive mode in the Ophryotrocha genus using a phylogeny based on two mitochondrial markers (COXI and 16S rDNA) and one nuclear fragment (Histone H3).ResultsSurprisingly, gene order was not conserved among the six Ophryotrocha species investigated, and varied greatly as compared to those found in other annelid species within the class Errantia. The mitogenome phylogeny for the six Ophryotrocha species displayed a separation of gonochoric and hermaphroditic species. However, this separation was not observed in the phylogeny based on the COX1, 16S rDNA, and H3 genes. Parsimony and Bayesian ancestral trait reconstruction indicated that gonochorism was the most parsimonious ancestral reproductive mode in Ophryotrocha spp.ConclusionsOur results highlight the remarkably high level of gene order variation among congeneric species, even in annelids. This encourages the need for additional mitogenome sequencing of annelid taxa in order to properly understand its mtDNA evolution, high biodiversity and phylogenetic relationships.
Highlights
Annelids are one the most speciose and ecologically diverse groups of metazoans
Ten codons were highly used in all species (Relative Synonymous Codon Usage (RSCU) > 1.25) and most of them were the type of NNU: UAU, GUU, ACU, AAU, UUU, AUA, CUC, AUU, GAU, GCU
All protein coding genes (PCG) in O. adherens, O. diadema and O. puerilis had negative values of GC-skew suggesting a bias in C, whereas positive GC-skew was found for all PCG of O. japonica, O. labronica and O. robusta
Summary
Annelids are one the most speciose and ecologically diverse groups of metazoans. a significant effort has been recently invested in sequencing genomes of a wide array of metazoans, many orders and families within the phylum Annelida are still represented by a single specimen of a single species. The genus of interstitial annelids Ophryotrocha (Dorvilleidae, Errantia, Annelida) is among these neglected groups, despite its extensive use as model organism in numerous studies on the evolution of life history, physiological and ecological traits. The mitochondrial gene content is almost invariant among species, but the gene order has been found to vary considerably across Metazoans (such as flatworms, molluscs and tunicates [7]), generating interest in using mitochondrial DNA (mtDNA) gene order for phylogenetic inference [2]. The advent of generation sequencing has made it easier to obtain mitochondrial genomes even for classical nonmodel organisms. This enables the detection of gene rearrangements, as well as phylogenetic relationships among and within diverse phyla [8]. Gene order is known to vary extensively within the phyla Mollusca [9], Arthropoda [10] and Annelida [11, 12]
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