Biology Reference
In-Depth Information
Figure 12.
Phylogenetic tree of the thyroid hormone receptor (TR). Deduced amino acid
sequences of B to D domains, which include DNA- and ligand-binding domains, were used
for the comparison. The alignments were performed on the ClustalW program with a pairwise
alignment algorithm. The lengths of lines indicate the genetic distances.
is specifi c to
A. anguilla
has the highest similarity with conger eel TRβB.
Whether
A. anguilla
has a single TRβ or also possesses a second TRβ similar
to TRβA of the conger eel, is open to further study.
The existence of multiple TRs within a species could imply a distinct and
indispensable function of each TR type. Indeed, different expression profi les
of TRs were revealed in conger eel metamorphosis (Fig. 13) (Kawakami
et al., 2003a; Kawakami et al., 2003b). TRαA and TRαB transcript levels
peaked at metamorphic climax and then declined toward the completion
of metamorphosis. TRβA levels also increased and reached a peak during
the metamorphic period but, unlike TRαs, the peak was maintained during
the glass eel, elver and young eel stages. TRαA, TRαB and TRβA transcripts
were ubiquitously detected in various tissues in the adult conger eel whereas
TRβB transcripts were predominantly expressed in the brain and pituitary.
The expression of TRβB increased in the whole head during metamorphosis
and reached a peak in elvers. The specifi c expression of TRβB in the brain
and pituitary was also seen in the Japanese eel (Kawakami et al., 2007).
These differential patterns in the levels and distribution of each TR
strongly suggest diverse function for each of the TRs. Metamorphosis is, as
described earlier, accompanied by various morphological and biochemical
changes in the body, which can be driven by thyroid hormone. Thyroid
hormone is capable of modulating its function at the receptor level by using
