Characterization of thyroid hormones and thyroid hormone receptors during the early development of Pacific bluefin tuna (Thunnus orientalis)
Introduction
Metamorphosis takes place in certain species of fish in association with a transition to a new habitat. This type of metamorphosis involves an ontogenetic transformation from a larval form to a juvenile one (Wilbur, 1980, Youson, 1988), and is termed first (true) metamorphosis in contrast with second metamorphosis, which is restricted to events related to final sexual maturation and involves the full expression of adult genes involved in reproduction (Youson, 1997). In particular, the morphological changes that occur in eels (Anguilliformes: development from leptocephalus to elver; Smith, 1979) and flounder (Pleuronectiformes: eye migration; Keefe and Able, 1993, Minami, 1982) are representative examples of metamorphosis (Wilbur and Collins, 1973).
Metamorphosis in these species is under the influence of thyroid hormones (THs) (Anguilliformes: Kitajima et al., 1967; Pleuronectiformes: Inui et al., 1994, Schreiber and Specker, 1998, Solbakken et al., 1999). Moreover, the expression patterns of the TH receptor (TR) and the relationships between THs and TR mRNA levels have been studied in Pleureonectiformes (Yamano and Miwa, 1998) and Anguilliformes metamorphosis (Kawakami et al., 2003a, Kawakami et al., 2003b).
Youson (1997) suggested that metamorphosis is a term that has been used rather loosely by fish biologists to describe both major and minor transformations of larval phenotypic characters into those of adults. Like the major transformations influenced by THs seen in Anguilliformes and Pleureonectiformes, the minor transformations seen in species such as black seabream (Perciformes: Acanthopagrus schlegelii; Tanaka et al., 1991), red seabream (Perciformes: Pagrus major; Hirata et al., 1989), and coral trout grouper (Perciformes: Plectropomus leopardus; Trijuno et al., 2002) are also indicated to be under the influence of THs. It is possible that THs play roles in the early development of many fish.
Morphological characteristics common to the early development of scombrid fish include a large head, gape and eyes, development of head spination, and posterior migration of the anus (Collette et al., 1984). However, the role of THs in the early development of scombrid fish is not understood at all. Characterizing the role of TRs in the early development of scombrid fish will increase our understanding of the roles of THs in types of early developments other than those utilized by Pleuroeonectiformes and Anguilliformes. In this study, we use the early development of fish and juveniles of Pacific bluefin tuna (Perciformes, Thunnus orientalis) as a model for the early development of scombrid fishes, and we investigate the whole body TH and TR mRNA transcript levels from embryos to juveniles. In addition, we compare the expression of TRs during early development in Pacific bluefin tuna with previously published data on the Japanese flounder (Pleureonectiformes: Paralichthys olivaceus) and the Japanese conger eel (Anguilliformes: Conger myriaster).
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Animals
Immature 1-year-old Pacific bluefin tuna (10–15 kg body weight), cultivated in a net cage at the Kinki University Fish Nursery Center at Amami in Kagoshima Prefecture, were collected in July 2004. Liver samples from Pacific bluefin tuna were frozen in liquid nitrogen and stored at −80 °C until required.
Fertilized Pacific bluefin tuna eggs, which had been naturally spawned in the Kinki University Fish Nursery Center at Amami, were obtained in July 2006 and transferred to 500-L round plastic and
Nucleotide and predicted amino acid sequences of bTR cDNAs
The nucleotide sequence of bTRβ is shown in Fig. 2. The cDNA encoding bTRβ contains an open reading frame of 1185 bp, encoding a protein of 395 amino acids. This protein contains two putative cysteine-rich zinc fingers (underlined in Fig. 2), which are characteristic of all nuclear hormone receptors, including TRs. In order to isolate a region of sequence for bTRαs, we designed a series of degenerate PCR primers. For each Pacific bluefin tuna RT-PCR, fragments for TRα were obtained at the
Discussion
Hormones, including thyroid hormones, have been shown to pass into fish oocytes from the maternal circulation (Greenblatt et al., 1989). When TH concentrations were analyzed in unfertilized, fertilized and/or ovarian eggs just before spawning, from 26 teleostean species, THs were found in the eggs of all species (Tagawa et al., 1990). In zebrafish (Danio rerio), cotreatment of embryos and fish with an antagonist of TR and an inhibitor of thyroid hormogenesis led to severe retardation of
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