Does atp8 exist in the mitochondrial genome of Proseriata (Metazoa: Platyhelminthes)?

The adenosine triphosphate (ATP) synthase F₀ subunit 8 gene (atp8) had been believed to be absent in mitochondrial genomes of platyhelminths until the late 2010s, since when multiple lines of emergent evidence have suggested that this gene is actually present, albeit in highly derived forms, throughout the entire phylum except for the parasitic Neodermata (tapeworms, flukes, and their kin). Of about 11 non-parasitic (turbellarian) major platyhelminth subtaxa, the existence of atp8 has hitherto been documented in five (Catenulida, Macrostomorpha, Polycladida, Rhabdocoela, and Tricladida), while it remains open in the remaining six (Prorhynchida, Gnosonesmida, Proseriata, Fecampiida, Prolecithophora, and Bothrioplanida). Here we report the mitochondrial genome sequence of an undetermined marine interstitial turbellarian species in the genus Nematoplana Meixner, 1938 as the first representative of Proseriata. This circular genome comprises 16,106 bp (but potentially 18,812–19,277 bp when unresolved, non-coding tandem repeats are considered) and includes 38 genes, viz. 23 transfer RNA genes, 13 protein-coding genes (including the putative atp8), and two ribosomal RNA genes. The putative atp8 in Nematoplana sp. was not annotated by a standard automated procedure but was detected by manual inspection. If it encodes a translated product, it consists of 156 bp, with the potential 52-amino-acid-residue product beginning with MPHV, instead of the metazoan-canonical MPQL, and containing a single putative transmembrane region expanding from the 7th to the 29th amino-acid positions. While our finding seemingly strengthens the hypothesis that atp8 is in the ground pattern of flatworm mitochondrial genomes, whether the putative atp8 in flatworms is actually transcribed and translated to form a functional ATP synthetase F₀ subunit should be tested in future studies.