Biology Reference
In-Depth Information
metamorphosis is also infl uenced by the hormone. It is unknown, at present,
whether anomalous pigmentation induced by thyroid hormone is caused
through the same mechanism for the anomalous pigmentation induced
by dietary factors. However, the similarities of the sensitive period for
the two cases suggest that at least a part of the mechanism is shared for
the development of albinism induced either by the manipulation of the
hormone or by diet. In fact, thyroid hormone and vitamin A (retinoic acid)
interact through their receptors in vertebrates. There are two biologically
important isomers of retinoic acid (RA), all-trans retinoic acid (atRA)
and 9-cis retinoic acid (9cRA). Biological effects of these retinoic acids are
mediated by specifi c receptors: retinoic acid receptor (RAR) for atRA and
retinoid X receptor (RXR) for 9cRA, although 9cRA can bind to both RAR
and RXR (Heyman et al., 1992; Allenby et al., 1993). Both thyroid hormone
receptors (TRs), and RAR are known to heterodimerize with RXR to exert
pleiotropic effects of their legands. Therefore, it is quite possible that thyroid
hormone and retinoic acid or its precursor, vitamin A, may affect each
other in the exertion of their biological effects. Schuh et al. (1993) reported
that v-erb A, an oncogene encoding TR, reduced the teratogenic effects
of atRA in
Xenopus
embryogenesis and suggested a possible loss of RXR
that is able to form heterodimers with RAR due to v-ervA/RXR dimmer
formation. On the other hand, Forman et al. (1995) found that transcriptional
activation by thyroid hormone through TR/RXR heterodimer was reduced
by RXR-specifi c synthetic ligand LG69. The interaction of thyroid hormone
and retinoids and its mechanism in fl ounder metamorphosis should be
clarifi ed.
Albinism and above mentioned ambicoloration are irreversible and
the causes for these anomalies reside during metamorphosis. On the other
hand, “staining” seems to develop after metamorphosis and at least partly
reversible (Stickney and White, 1975). Staining is considered to be caused
by environmental factors (for reviews see Norman, 1934; Bolker and Hill,
2000). Light irradiation to the blind side has been reported to cause staining
(Cunningham, 1891, 1893, 1895; Osborn, 1940, 1941). On the other hand,
some authors reported that coarse bottom surface, e.g. sand, of the rearing
aquaria prevented staining (Stickney and White, 1975; Iwata and Kikuchi,
1998). Iwata and Kikuchi (1998) examined the effects of light and the surface
texture of the bottom of aquaria on hypermelanosis (staining) of the blind
side. They found that, when they illuminated transparent aquaria from
under the bottom through glass sand, staining of the blind side did not
develop. Thus, surface texture of the bottom of rearing aquaria seems more
important than lighting condition as a factor affecting hypermelanosis of
the blind side. These observations clearly indicate that the development
of asymmetrical pigmentation of fl atfi sh is controlled by environmental
factors, such as physical stimuli on the skin. Flatfi sh larvae sometimes
