Comparative analysis of the circadian clock in Metaseiulus occidentalis, Ixodes scapularis, and Tetranychus urticae indicates a variety of mechanisms among the acarines

Acarines represent a large proportion of chelicerate arthropods and are of great importance to agriculture and public health. Although the circadian system has been thoroughly investigated in several insect species and the Drosophila melanogaster clock has become the standard, little study has been done on the clockworks of other arthropod classes. Two recent bioinformatic analyses of crustaceans (the water flea Daphnia pulex and the copepod Calanus finmarchicus) represent most of the work that has been published outside the Insecta. Acarines, as representatives of the chelicerate clade, have been shown to exhibit phototaxis and cyclical behaviors, but to our knowledge, no specific study of circadian genes has been published. In this study, we broaden the knowledge base concerning putative circadian proteins in acarines for comparison to other arthropods. By comparison to D. melanogaster, we identify nearly all the putative circadian components in the acarines Tetranychus urticae (Arthropoda: Chelicerata: Arachnida: Acarina: Tetranychidae) and Ixodes scapularis (Chelicerata: Arachnida: Acarina: Ixodidae). A majority of these same components are identified in the blind predatory mite Metaseiulus occidentalis (Arachnida: Acarina: Phytoseiidae) but with notable exceptions being the cryptochromes, cycle, TIMELESS, and periodproteins, which appear to be missing or highly divergent.  To assess whether this variant system in M. occidentalis is functional, we confirmed effective clock output by investigating periodicity in oviposition. To our knowledge, this is the first comparative analysis of circadian genes in acarines and is a basis for future studies of variant clock mechanisms.