Mollusca have always been and are still considered one of the biggest questions marks across the evolutionary tree of Metazoa. In recent times, a close relationship with Annelida has been strongly advocated, if not accepted (e.g., Struck and Fisse, Mol. Biol. Evol. 2008 25: 728-736; Andrade et al., Mol. Biol. Evol. 2014 31:3206-3215). Conversely, the internal relationships of molluscs are widely debated: almost all conceivable combinations of molluscan classes have been proposed as monophyletic clades (Sigwart and Lindberg, Syst. Biol. 2015 64:384-395).

Unraveling the phylogeny of single molluscan classes is therefore mandatory to gain a clearer understanding of the evolution of the phylum as a whole. A research line of our lab focuses on the phylogeny of the class Bivalvia, with special emphasis on deep nodes.

To this purpose, mitochondrial markers were used, as well as complete mitochondrial genomes. In this regard, the sequencing of the complete mitochondrial genome of Solemya velum in 2013 (Plazzi et al., BMC Genomics 2013 14:409) was a key achievement of our group, in that this species retains many ancestral mitogenomic (as well as morphological) features.

A long-debated issue on bivalve phylogeny is the sister group of the subclass Heterodonta (Venus clams, cockles, quahogs, razor clams, and their kin): the use of mitochondrial markers leads us to favor the Heterodonta+Pteriomorphia hypothesis (where Pteriomorphia includes mussels, oysters, scallops, and their kin), a clade we called Amarsipobranchia (Plazzi et al., PLoS One 2011 6:e27147). Recent findings based on high-throughput data are currently challenging this view, while the fossil record would strengthen our hypothesis (Plazzi et al., Genome Biol. Evol. 2016 8:2544-2564).

Morevoer, mitochondrial genomics of bivalves is interesting per se, in that bivalve mitochondrial genomes show many uncommon aspects. Outstanding is the presence in some species of the DUI mechanism for mitochondrial heredity (which is the focus of another research line of our lab), but there are many other peculiarities of bivalve mitochondrial genomes. A sound phylogenetic framework is mandatory to fully evaluate the evolution of these unique organellar genomes.