Abstract

BackgroundArgeia pugettensis is an isopod species that parasitizes other crustaceans. Its huge native geographic range spans the Pacific from China to California, but molecular data are available only for a handful of specimens from North-American populations. We sequenced and characterised the complete mitogenome of a specimen collected in the Yellow Sea.ResultsIt exhibited a barcode (cox1) similarity level of only 87–89% with North-American populations, which is unusually low for conspecifics. Its mitogenome is among the largest in isopods (≈16.5 Kbp), mostly due to a large duplicated palindromic genomic segment (2 Kbp) comprising three genes. However, it lost a segment comprising three genes, nad4L-trnP-nad6, and many genes exhibited highly divergent sequences in comparison to isopod orthologues, including numerous mutations, deletions and insertions. Phylogenetic and selection analyses corroborated that this is one of the handful of most rapidly evolving available isopod mitogenomes, and that it evolves under highly relaxed selection constraints (as opposed to positive selection). However, its nuclear 18S gene is highly conserved, which suggests that rapid evolution is limited to its mitochondrial genome. The cox1 sequence analysis indicates that elevated mitogenomic evolutionary rates are not shared by North-American conspecifics, which suggests a breakdown of cox1 barcoding in this species.ConclusionsA highly architecturally disrupted mitogenome and decoupling of mitochondrial and nuclear rates would normally be expected to have strong negative impacts on the fitness of the organism, so the existence of this lineage is a puzzling evolutionary question. Additional studies are needed to assess the phylogenetic breadth of this disrupted mitochondrial architecture and its impact on fitness.

Highlights

  • Argeia pugettensis is an isopod species that parasitizes other crustaceans

  • We set out to assess whether the cox1 gene of this species may be evolving at exceptionally elevated evolutionary rates, invalidating the applicability of standard barcode thresholds to this isopod lineage

  • Dataset comprised of entire genes indicates that cox1 of A. pugettensis is the fastest-evolving gene in the entire isopod dataset, closely followed by the only representative of the same family, G. ovalis (Additional file 1)

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Summary

Introduction

Argeia pugettensis is an isopod species that parasitizes other crustaceans. Its huge native geographic range spans the Pacific from China to California, but molecular data are available only for a handful of specimens from NorthAmerican populations. Apart from affecting the mitogenomic replication and (putatively) transcription regulation, ORI events can produce mutational bursts, which can hamper phylogenetic and other evolutionary analyses [22, 26]. Recently it was shown that these multiple ORI events produced strong homoplastic compositional biases in isopods, which in turn cause artefactual clustering in phylogenetic analyses [17, 21]. All these features make isopods an interesting model for studying the evolution of mitochondrial architecture [17]

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