Abstract
Wood falls provide episodic fluxes of energy to the sea floor that are degraded by a species-rich benthic fauna. Part of this rich diversity includes annelid polychaetes but unfortunately, our understanding of such fauna is limited and their genetic variability and evolutionary origins remain poorly known. In this study, we sequenced complete mitochondrial genomes from three congeneric Decemunciger (Ampharetidae) individuals that had colonized multiple wood falls in the deep (~1600 m) NE Pacific Ocean. Mitochondrial gene order within Decemunciger was similar to the three other available Terebellomorpha genomes, consistent with the relatively conserved nature of mitochondrial genomes within annelids. Unexpectedly, we found introns within the cox1, nad1 and nad4 genes of all three genomes assembled. This is the greatest number of introns observed in annelid mtDNA genomes, and possibly in bilaterians. Interestingly, the introns were of variable sizes suggesting possible evolutionary differences in the age and origins of introns. The sequence of the introns within cox1 is similar to Group II introns previously identified, suggesting that introns in the mitochondrial genome of annelids may be more widespread then realized. Phylogenetically, Decemunciger appears to be a sister clade among current vent and seep deep-sea Ampharetinae.
Highlights
Wood falls provide episodic fluxes of energy to the sea floor that are degraded by a species-rich benthic fauna
Using a BLAST-based approach[33], we identified mitochondrial DNA sequences (mtDNA) contigs that were roughly 15,000–16,000 bp in size from the genome assembly
Contigs of interest where identified by using blast with previously published terebellomorph mtDNA genomes[18] against the assembled genomic data
Summary
The insertion position into the cox[1] gene and size of the introns were identical within the three Decemunciger mitochondrial genomes sequenced. Multiple introns were first identified on mitochondrial genes (cox[1] and nad5) of sea anemones (Group I intron[37], and recently Group II introns have been reported on a cox[1] gene of a Nephtydae (Nephtys sp.) and glycerid polychaetes[23, 30] Intron sizes, their position within the cox[1] gene and their coding protein sequences, differ between Nephtys sp., Decemunciger sp. Decemunciger has marked morphological similarities (e.g. branchiae position and number) with the vent ampharetid genus Paramytha gen nov., which is a sister group to other vent/seep Ampharetinae clades based on cox1, 16S and 18S genes[25, 32]. Our phylogenetic analysis support Decemunciger as within the Ampharetinae, within a clade comprised of several described species from chemosynthetic ecosystems in the North Atlantic and Arctic basins
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