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fl ounder,
Paralichthys stellatus
and dextral fl athead fl ounder,
Hippoglossoides
dubius
, with the result that left-sided expression of the Nodal-pathway
genes was found in both species. Thus, the embryonic laterality determined
by Nodal-pathway genes is identical in dextral and sinistral fl atfi sh, even
though external asymmetry is exactly opposite.
In contrast to asymmetrical development of these internal organs,
eye migration and/or asymmetrical cranium development, as well as
asymmetrical pigmentation of fl atfi sh occur during metamorphosis: more
than 2 weeks after the termination of pix2 expression in the left LPM and
diencephalon at 20
O
C in the Japanese fl ounder. However, Nodal pathway
also seems to be involved in triggering the development of external
asymmetry. Left-sidedness in the Japanese fl ounder is strongly determined
genetically and occurrence of reverse (dextral) is very rare (less than 1%)
in the hatchery-reared fi sh. Hashimoto et al. (2002) isolated a clonal line
“
reversed
(
rev
)” of the Japanese fl ounder by gynogenesis. The homozygous
offsprings produced from
revs
exhibit reversed eye position at a high
frequency of 20-30%. Hashimoto et al. (2002) examined pitx2 expression in
the LPM in
rev
embryo and found that 43% of the embryo exhibited pitx2
only in the left LPM, 53% in both left and right, and 4% only in the right
side LPM. After metamorphosis, 20% of the juveniles of the same siblings
displayed dextral-type eye position, while the remaining fi sh were synistral
as the normal Japanese fl ounder. These results strongly suggest that the
molecular pathway controlling the left-sided expression of pitx2 in the
LPM is genetically disturbed in
rev
, and that left-biased pitx2 expression
in normal fl ounder is involved in the right eye migration to the left side
during metamorphosis in this species.
On the other hand, 15% of the juveniles of above mentioned siblings
showed reversed gut coiling (leftward coiling). However, reversed gut
coiling was not always accompanied by reversal of the eye migration,
suggesting that a process of Nodal-pathway that is different from that for
embryonic asymmetric development of internal organs is functioning for
the asymmetry formation during metamorphosis.
Finally, Suzuki et al. (2009) found that pitx2 is re-expressed at the left of
the diencephalon (habenular nuclei) at premetamorphosis, earlier than the
initiation of eye migration in both sinistral Japanese fl ounder and dextral
spotted halibut,
Varasper variegatus
.
Although the eyed-side in the fl atfi sh is usually the same within
the same species, the steadiness of eye-position within a species differs
by species. While left-sidedness is fi rmly determined in the Japanese
flounder
Paralichtys olivaceus
, the spotted halibut
Varasper variegates
,
which is normally dextal, often exhibits reversed eyed-side (synistral) and
bilateral eye-position (each eye on both side), especially in laboratory-
reared animals (Aritaki et al., 2004; Tagawa and Aritaki, 2005). Suzuki et
