Vol.:(0123456789) 1 3 Applied Entomology and Zoology https://doi.org/10.1007/s13355-019-00631-3 REVIEW Steroid hormones in Acari, their functions and synthesis Mari H. Ogihara 1  · DeMar Taylor 2  · Hiroshi Kataoka 3 Received: 16 November 2018 / Accepted: 26 June 2019 © The Japanese Society of Applied Entomology and Zoology 2019 Abstract Acari are arthropods with the conserved features of an exoskeleton that require the shedding of cuticle for growth. Steroid hormones, ecdysteroids, regulate the mechanisms necessary for growth and development. Acari lack juvenile hormones, so they utilize ecdysteroids as the solo lipophilic hormone throughout their life for molting as well as for reproduction. Although some research on ecdysteroidogenesis has been conducted in the Acari, two important questions remain unclear: the active form of ecdysteroid and sites for ecdysteroidogenesis. The active form of ecdysteroids in Acari is controversial. Analysis by mass spectrometry confrms that most Acari use 20-hydroxyecdysone (20E), but the spider mite Tetranychus urticae (Koch) (Acari: Tetranychidae) uses Ponasterone A (25-deoxy-20-hydroxyecdysone). Analyses of ecdysteoridogenic genes provide clear evidence that the ovary is the primary site of ecdysteroidogenesis in both immature and mature ticks. In this review, the known functions of ecdysteroids are summarized and recent progress on ecdysteroidogenesis is introduced. Keywords Acari · Arthropod · Ecdysteroid · Ecdysteroidogenesis · Hormone · Development · Reproduction Introduction Acari, ticks and mites, are chelicerate arachnids in the same group as spiders and scorpions. The Acari are divided into two superorders, acariformes and parasitiformes, with a part of the parasitiformes (Ixodida) commonly called ticks, and all other groups commonly referred to as mites (Fig. 1). Acari include large numbers of species showing highly divergent life cycles and morphology. Some are tiny and live on plants such as spider mites (Acariformes: Prostigmata), and some are large and feed on the blood of animals such as cattle ticks (Parasitiformes: Ixodida) (Fig. 1). Although Acari show extensive diversity, they conserve the common features of the arthropod exoskeleton. Therefore, molting is necessary for development. Similar to other arthropods, molting in Acari is regulated by the arthropod steroid hor- mones, ecdysteroids. On the other hand, Acari lack juvenile hormones or their precursor methyl farnesoate (Neese et al. 2000). Genomic and transcriptome analyses also support the theory that Acari lack enzymes to synthesize juvenile hormones (Zhu et al. 2016). Hence, ecdysteroids appear to be the only lipophilic hormone regulating development in the Acari. In addition, ecdysteroids regulate reproduction of Acari, so this hormone is essential for the complete life cycle. Since Wright (1969) demonstrated that ecdysteroids ter- minate diapause in the hard tick Dermacentor albopictus (Packard) (Acari: Parasitiformes: Ixodidae), numerous stud- ies have investigated the functions of ecdysteroids in the Acari (Rees 2004). On the other hand, the understanding of ecdysteroid synthesis, ecdysteroidogenesis, of Acari has shown little progress. New approaches such as transcrip- tome and metabolome have opened a new era for studies on Acari endocrinology. In this review, we summarize recent studies on ecdysteroid functions and ecdysteroidogenesis in Acari to clarify misconceptions that may have arisen. First, we discuss the functions of ecdysteroids in Acari, second the pathway of ecdysteroid synthesis in insects and crus- taceans, third the conservation of ecdysteroidogenesis in Acari species, fourth the tissues for ecdysteroid synthesis in Acari and fnally the regulation of ecdysteroidogenesis in Acari before presenting our conclusions on the impor- tance of steroid hormones in the Acari. According to precise * Mari H. Ogihara ogiharam590@afrc.go.jp 1 Institute of Livestock and Grassland Science, NARO, 2-1 Ikenodai, Tsukuba, Ibaraki, Japan 2 Faculty of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki, Japan 3 Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwano-ha, Kashiwa, Chiba, Japan