Comparative mitogenomic analyses and gene rearrangements reject the alleged polyphyly of a bivalve genus.

BACKGROUND

The order and orientation of genes encoded by animal mitogenomes are typically conserved, although there is increasing evidence of multiple rearrangements among mollusks. The mitogenome from a Brazilian brown mussel (hereafter named B1) classified as Linnaeus, 1758 and assembled from Illumina short-length reads revealed an unusual gene order very different from other congeneric species. Previous mitogenomic analyses based on the Brazilian specimen and other Mytilidae suggested the polyphyly of the genus .

METHODS

To confirm the proposed gene rearrangements, we sequenced a second Brazilian specimen using the "primer-walking" method and performed the assembly using as reference . This time-consuming sequencing method is highly effective when assessing gene order because it relies on sequentially-determined, overlapping fragments. We also sequenced the mitogenomes of eastern and southwestern South African lineages to analyze the existence of putative intraspecific gene order changes as the two lineages show overlapping distributions but do not exhibit a sister relationship.

RESULTS

The three mitogenomes sequenced in this study exhibit the same gene order as the reference. CREx, a software that heuristically determines rearrangement scenarios, identified numerous gene order changes between B1 and our mitogenomes, rejecting the previously proposed gene order for the species. Our results validate the monophyly of the genus and indicate a misidentification of B1.