The complete mitochondrial genome of the largest amphipod, Alicella gigantea: Insight into its phylogenetic relationships and deep sea adaptive characters.

Alicella gigantea (Alicelloidae) is a scavenger with the largest body size among amphipods. It is a participant in the foodweb of deepsea ecosystem and distributed with vast bathymetric and geographic ranges. In this study, the mitochondrial genome of A. gigantea was completely assembled and characterized. The complete sequence has a total length of 16,851 bp, comprising the usual eukaryotic components, with 13 protein-coding genes (PCGs), 2 ribosomal RNA genes (rRNAs), 22 transfer RNA genes (tRNAs), and 2 noncoding control regions (CRs). The gene rearrangement and reverse nucleotide strand bias of its mitochondrial genome are similar to those observed in the deepsea amphipod Eurythenes maldoror (Eurytheneidae), but different from the characters of Halice sp. MT-2017 (Dexaminoidea), an inhabitant of a deeper environment. Phylogenetic analysis indicates that A. gigantea occupies the basal branch of deepsea species—E. maldoror and Hirondellea gigas. This phylogeny supports the hypothesis that the evolution of hadal amphipods has undergone a transition from the abyssal depth. Compared to 41 available shallow water equivalents, the four accessible mitochondrial genomes from the deep sea, including the one produced in this study, show significantly fewer charged amino acids in the 13 PCGs, which suggests an adaption to the deepsea environment.