Mitochondrial genome of Parborlasia corrugatus (Nemertea: Lineidae)

Abstract

We present the complete mitochondrial genome of the herteronemertean Parborlasia corrugatus. Within the 15,499 bp genome, we were able to recover 13 protein-coding genes as well as two rRNA and 22 tRNA. The order of tRNA genes was relatively conserved save one inversion of tRNAL1 and tRNAL2 when compared to other heteronemertean taxa. The GC% was 35.9% across the genome.

Keywords:

• Mitochondrial genome
• heteronemertean
• gene-order

Parborlasia corrugatus is a large (up to 2 m in length) dioecious, broadcast-spawning heteronemertean (Pearse et al.1991) found in southern marine systems from intertidal to 3950 m in depth (Gibson 1983). Like other nemerteans, P. corrugatus has limited external morphology but can display a range of colour variation. These scavenging worms have a broad diet and can occur in high densities on larger food sources (e.g. penguin or fish falls). Interestingly, their highly-secretory epidermis can produce acidic mucous that contains a cytolysin A-III protein called parborlysin (aids penetration of water-soluble molecules into lipid membranes) (Berne et al. 2003). Additionally, there are at least two populations of P. corrugatus with one on the Antarctic continental shelf and one on the South American shelf. Despite having pelagic larvae, these populations have been reproductively isolated by temperature differences at the Antarctic Polar Front, an open-ocean barrier to gene flow (Thornhill et al. 2008).

Animals were collected in Antarctica (65° 01.55 S, 63° 18.23 W) at a depth of 400 m in 2006 and frozen at −80 °C until DNA was extracted using a Qiagin DNEasy blood and tissue extraction kit (Qiagen Inc., Valencia, CA). The remaining frozen tissue from the collected specimen is accessioned in the Auburn Museum of Natural History (Accession #44822). Samples were identified by morphology and CO1 barcoding. Illumina’s Nextera DNA sample preparation kit (Illumina, San Diego, CA) was used for sequencing on an Illumina MiSeq sequencer using a 2 × 150 paired-end protocol at Auburn University. Genome assembly used Ray 2.2.0 (Boisvert et al. 2010) using default parameters and a kmer of 31. We used MITOS 2 (Bernt et al. 2013) to annotate the contig containing the putative Parborlasia corrugatus mitochondrial genome that was recovered with a BLASTn (Altschul et al. 1997) search that used a P. corrugatus CO1 fragment as bait.

The full contig (GenBank Accession MH630149) is 15,499 bp in length with a GC content of 35.9%. We recovered 13 protein coding, two rRNA, and 22 tRNA genes. One tRNA, trnP, not found by the initial MITOS 2 annotation, was recovered when analyzed using the ARWEN v1.2 server (Laslett and Canbäck 2008). With the exception of tRNAL1 and tRNAL2, the order of tRNA in the P. corrugatus mtDNA genome was consistent with other heteronemertean mtDNAs (Podsiaslowski et al. 2009; Gonzalez-Cueto et al. 2015; Shen and Shi-Chun 2016). Of the 17 complete mtDNA genomes in GenBank 6 are heteronemerteans and all of these are from Lineidae.

Phylogenetic analyses yielded a topology consist with the current understanding of heteronemertean relationships (Shen and Shi-Chun 2016) (Figure 1). Interestingly, P. corrugatus exhibited a long branch in our analysis. The reason for this is unclear. The nucleotide base composition of the genome is 22.7% G, 13.2% C, 42.2% T, and 21.8% A, consistent with other nemertean mitogenomes (Shen et al. 2015).

Figure 1. Maximum-likelihood tree with bootstrap support (1000 iterations) of concatenated amino acid sequences for 13 mitochondrial protein-coding genes in eight nemertean species (see text). Data were alignment in Mafft (Katoh and Standley 2013) and a PROTGAMMAAUTO model was employed in RAxML (Stamatakis 2014). Bootstrap values above 50% are shown on the tree. The hoplonemertean Nipponnemertes punctatula was used as the outgroup.

Acknowledgements

This research was made possible with assistance from the Captains and crews of the ASRV Laurence M. Gould. This is Molette Biology Laboratory contribution #84 and Auburn University Marine Biology Program contribution #181.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

Funding from the National Science Foundation (NSF OPP-0132032, OPP-0338218, and ANT- 1043745) is gratefully acknowledged.