Biology Reference
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data). On the fi rst day of the experiment, the value of anal length/total
length was > 85% in all but two of the leptocephali (upper panel), and
the blood color was indiscernible (lower panel). These leptocephali were
separated into three tanks with either normal seawater (n=11), seawater
with T
4
at 100 nM (n=12), or with thiourea at 5 mM (n=11). One week later,
half of the control fi sh had naturally started metamorphosis, as revealed
by the decrease in the value of anal length/total length, and by erythrocyte
production, which was recognizable through the transparent skin. By four
weeks, nine fi sh had completed metamorphosis and 2 fi sh remained as
leptocephali. The treatment with T
4
induced metamorphosis in all fi sh.
The induced metamorphosis progressed faster than metamorphosis of
controls. Thiourea barely inhibited the change in body shape, but delayed
the production of erythrocytes.
3.5.2 Thyroid hormone receptor (TR)
It has been well demonstrated, in higher vertebrates, that the ligand-TR
complex forms a homodimer or heterodimer with the retinoid receptor and
then modulates gene expression through binding to a specifi c regulatory
nucleotide sequence of a target gene. Accordingly, eel metamorphosis
must be triggered by the modifi cation of gene expression by TR. Fish TR
genes have already been cloned from more than ten fi sh species, including
anguilliform eels (Marchand et al., 2001; Kawakami et al., 2003a; Kawakami
et al., 2003b; Kawakami et al., 2007).
Phylogenetic relationships based on the amino acid sequences of TRs are
illustrated in Fig. 12. Two types of TR, TRα and TRβ were widely distributed
in fi shes, as observed in higher vertebrates. The TRα of a given species
has a higher similarity to the TRα of other species than to its own TRβ,
even between fi sh and humans. A corresponding situation exists for TRβ.
With respect to TRα, two subtypes, TRαA and TRαB, were identifi ed in the
more advanced fi sh groups, providing distinct groups in the phylogenetic
analysis. Although the conger eel also has both TRαA and TRαB, the origin
of these subtypes in the conger eel and in the more advanced fi shes seems
to be different. Conger eel TRαA and TRαB are positioned apart from those
of more advanced fi sh groups and possessed a stronger similarity with each
other. Thus, phylogenetic analysis strongly suggested that a TRα gene in
the conger eel lineage may have been duplicated independently from the
duplication in the more advanced fi sh groups. It remains to be resolved
whether this is a feature common to elopomorph fi shes or is peculiar to
specifi c taxonomic groups of the Elopomorpha.
Unlike TRα, the single TRβ gene has been identifi ed in all fi shes except
for the conger eel. Therefore, a gene duplication event, specifi c for the conger
eel lineage, also seems to have taken place in the TRβ gene. The TRβ that
