Partial mitochondrial gene arrangements support a close relationship between tardigrada and arthropoda

Regions (about 3.7-3.8 kb) of the mitochondrial genomes (rrnL-cox1) of two tardigrades, a heterotardigrade Batillipes pennaki, and a eutardigrade, Pseudobiotus spinifer, were sequenced and characterized. The gene order in Batillipes was rrnl-V-rrnS-Q-I-M-nad2-W-C-Y-cox1, and in Pseudobiotus it was rrnlL-V-rrnS-O-M-nad2-W-C-Y-cox1. With the exception of the trnI gene, the two tardigrade regions have the same gene content and order. Their gene orders an strikingly similar to that of the chelicerate Limulus polyphemus (rrnL-V-rrnS-CR-I-Q-M-nad2-W-C-Y-cox1), which is considered to be ancestral for arthropods. Although the tardi-grades do not have a distinct control region (CR) within this segment, the trnI gene in Pseudobiotus is located between rrnL-trnL1 and trnL2-nad1, and the trnI gene in Batillipes is located between trnQ and trnM. In addition, the 106-bp region between trnQ and trnM in Batillipes not only contains two plausible trn1 genes with opposite orientations, but also exhibits some CR-like characteristics. The mitochondrial gene arrangements of 183 other protostomes were compared. 60 (52.2%) of the 115 arthropods examined have the M-nad2-W-C-Y-cox1 arrangement and 88 (76.5%) the M-nad2-W arrangement, as found in the tardigrades. In contrast, no such arrangement was seen in the 70 non-arthropod Protostomes studied. These are the first non-sequence molecular data that support the close relationship of tardigrades and arthropods.