Contrasting Modes of Mitochondrial Genome Evolution in Sister Taxa of Wood-Eating Marine Bivalves (Teredinidae and Xylophagaidae)

Abstract

The bivalve families Teredinidae and Xylophagaidae include voracious consumers of wood in shallow and deep-water marine environments, respectively. The taxa are sister clades whose members consume wood as food with the aid of intracellular cellulolytic endosymbionts housed in their gills. This combination of adaptations is found in no other group of animals and was likely present in the common ancestor of both families. Despite these commonalities, the two families have followed dramatically different evolutionary paths with respect to anatomy, life history and distribution. Here we present 42 new mitochondrial genome sequences from Teredinidae and Xylophagaidae and show that distinct trajectories have also occurred in the evolution and organization of their mitochondrial genomes. Teredinidae display significantly greater rates of amino acid substitution but absolute conservation of protein-coding gene order, whereas Xylophagaidae display significantly less amino acid change but have undergone numerous and diverse changes in genome organization since their divergence from a common ancestor. As with many bivalves, these mitochondrial genomes encode two ribosomal RNAs, 12 protein coding genes, and 22 tRNAs; atp8 was not detected. We further show that their phylogeny, as inferred from amino acid sequences of 12 concatenated mitochondrial protein-coding genes, is largely congruent with those inferred from their nuclear genomes based on 18S and 28S ribosomal RNA sequences. Our results provide a robust phylogenetic framework to explore the tempo and mode of mitochondrial genome evolution and offer directions for future phylogenetic and taxonomic studies of wood-boring bivalves.

The complete mitochondrial genome data of Pholas orientalis (Gmelin, 1791) from Malaysia

“…83 %). However, the ML trees based on the 12 taxa reaffirms the monophyly of Pholadoidea, which is similar to the previous study that is based on the nuclear 18S and 28S rDNA sequences [ 7 ]. However, it is noteworthy that the amino acid-based tree that is derived from the second dataset ( Fig.…”

“…2 and 3 ). Eventually, the findings from the nucleotide-based trees is congruent with the morphological classification and genetic analyses that support the sister relationship between Teredinidae and Xylophagaidae [ 6 , 7 , 9 ].

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“…The mitogenome has an overall base composition of 25.4 % A, 41.5 % T, 22.1% G, and 11 % C, demonstrating a bias towards AT bases (66.9 %). All published mitogenomes of Pholadoidea so far exhibited the same AT-skew [ 7 ]. The overall GC-content was 33.1 %, which is lower than most published mitogenome sequences of Pholadoidea, but higher when compared to species such as Xyloredo spp.…”

“…The overall GC-content was 33.1 %, which is lower than most published mitogenome sequences of Pholadoidea, but higher when compared to species such as Xyloredo spp. [ 7 ].…”

“…To date, the phylogenetic relationships within Pholadoidea have been investigated using the 18S and 28S rRNA gene sequences, but the limited molecular resolution has led to an unresolved relationship among Pholas species and their relatives [ 6 ]. A subsequent attempt to reconstruct the phylogenetic tree using complete mitochondrial genome (mitogenome) data was carried out by involving most of the species of Pholadoidea, but members of Pholadidae were not included in the study [ 7 ]. Thus, the relationship between Pholadidae and its sister family is still unknown at the mitogenome level.…”