This is the first report of the mitochondrial gene order and almost-complete DNA sequence of a representative of the Neritimorpha, the highest-ranking gastropod clade lacking such data. Mitochondrial gene order in Nerita is largely plesiomorphic. Its only difference from the cephalopod Octopus vulgaris is a tRNA transposition shared by Vetigastropoda and Caenogastropoda. Genome arrangements were not informative enough to resolve the evolutionary relationships of Neritimorpha, Vetigastropoda and Caenogastropoda. The sister-group taxon of Neritimorpha varied in sequence-based analyses. Some suggested that Neritimorpha is the sister group of Caenogastropoda plus Heterobranchia and some that Neritimorpha and Caenogastropoda are sister groups. No analysis significantly supported the hypothesis that Vetigastroda is more closely related to Caenogastropoda than is Neritimorpha.
The mitochondrial gene order and almost-complete DNA sequence of a representative of the Neritimorpha is reported. -The gene order in Nerita melanotragus differs from that in the vetigastropoda Haliotis rubra only by two tRNA transpositions, from that in the caenogastropod Ilyanassa obsoleta by one tRNA transposition and a large inversion and from that in the cephalopod Octpopus vulgaris by only one tRNA transposition. -Genome arrangement analyses group Neritomorpha and Vetigastropoda as sister taxa, but with a low support. -The sister-group taxon of Neritimorpha varied in sequence-based analyses. However, all analyses either supported or did not contradict the hypothesis that Neritimorpha is closer than Vetigastropoda are to either Caenogastropoda plus Heterobranchia.
Keywords: Amino acid alignment; Gastropod phylogeny; Gene order; Mitochondrion; Nerita