Biology Reference
In-Depth Information
Hiroi et al. (1997) reported that the tolerance of the Japanese fl ounder
against low salinity was fairly high in larvae immediately after hatching,
decreased toward the beginning of the metamorphosis and rapidly increased
during metamorphosis, reaching the highest level thereafter. Schreiber and
Specker (1999b) also reported that, in the summer fl ounder, tolerances to
both low and high salinity decreased toward the late prometamorphosis to
middle climax and subsequently increased rapidly reaching to the highest
level in juveniles.
The above mentioned turning point of salinity tolerance coincides with
the timing of the shift from cutaneous chloride cells to gill chloride cells. In
addition, the decrease of the salinity tolerance toward the metamorphosis
coincide with the decreases in size and density of cutaneous choloride cells,
and the subsequent increase of the tolerance coincide with the development
of juvenile type gill chloride cells, respectively. Thus, the change in salinity
tolerance of fl ounder larvae seems to refl ect the shift of chloride cells from
the cutaneous to the gill and developmental changes in these chloride
cells.
4.2.6 Thyroid hormone receptors and their dynamics during
metamorphosis
The action of thyroid hormone is mediated by thyroid hormone receptors
(THRs). THRs are members of nuclear receptor family (for the review
see Evans, 1988), and THRs activated by thyroid hormone directly bind
to a specifi c DNA element of a gene (thyroid hormone receptor element)
to regulate expression of the gene positively or negatively (Glass et al.,
1987; Chatterjee et al., 1989). Two major types of THRs (α and β), which
are produced from distinct genes, have been identifi ed in various classes
of vertebrates such as mammals (Weinberger et al., 1986; Thompson et al.,
1987), chickens (Sap et al., 1986) and amphibians ( Brooks et al., 1989; Yaoita
et al., 1990). In fi sh, THR genes were fi rst cloned from the Japanese fl ounder
(Yamano et al., 1994a), and so far, four cDNAs for the fl ounder THRs have
been cloned: two αtypes (αA and αB) and two βtypes (β1 and β2) (Yamano
et al., 1994a; Yamano and Inui, 1995). In mammals, two αtype THR variants
(α1 and α2) are generated from a single gene by alternative splicing (Izumo
and Mahdavi, 1988), whereas
Xenopus
possesses two distinctive genes for
αtype THRs (αA and αB )(Yaoita et al., 1990). As is the case in
Xenopus
, the
Japanese fl ounder appears to have distinct genes for two THRαs. On the
other hand, two βtype THRs (β1 and β2) of the Japanese fl ounder share a
constant sequence except for a 60 base additional sequence found only in
THRβ2, suggesting that two fl ounder THRβs are produced from the same
gene through an alternative splicing system (Yamano et al., 1994a; Yamano
and Inui, 1995).
