Selection effect of the outgroup in the recovering the phylogenetic relationships of Gastropoda (Mollusca) using complete mitochondrial genomes

Abstract

Gasteropod evolutionary relationships are still not resolved and there are different publications with different hypothesis. The main objective of this study was to evaluate the effect of outgroup choice on the topology of the ingroup, taking into account that there is not an agreement in regards to which is the more closely related group to gastropods. Four different treatments were performed using the different classes of Mollusks hypothetically closer to gastropods as outgroups (Cephalopoda, Scaphopoda, Bivalvia and a all combined). These treatments were performed with two different datasets (aminoacids and nucleotide sequences excluding third codon positions) and two analyses were applied to each dataset (Parsimony and Bayesian), for a total of 16 analyses. In all the analyses, the ingroup relationships are not well recovered when using Scaphopoda as outgroup, as it gets recovered within the ingroup. The same happened with Bivalvia when using Parsimony and the nucleotide dataset. The more stable relatioships were observed when Cephalopoda was used as the outgroup, both in Parsimony and Bayesian analyses and with both type of datasets. In general, in most of the analysis we recovered the group ((Caenogastropoda+Neritimorpha)+V etigastropoda), as well as the group Patelogastropoda+Heterobranchia) with good statistical support. The Eutyneura group was recovered as paraphyletic for the inclusion of Pyramidelloidea dolabrata a Heterostropha member. The analyses including all the outgroups did not recovered well the relationship neither for Gastropoda nor for Mollusca. In general, the topologies of the phylogenetic trees using complete mitocondrial genomes on gastropods can be very affected by outgroup choice. We recommend precaution on the evaluation and selection of outgroups when working with this group.