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al. (2009), utilizing this species difference, examined how pitx2 expression
in embryonic stage and its re-expression in premetamorphic stage are
related to the determination of eyed-side during metamorphosis. They
found that pitx2 was normally expressed at left side of diencephalon and
gut fi eld in 98-99% embryo in both species. Gut looping and asymmetry-
formation of diencephalon were also normal in them. However, 10% of
the metamorphosed
V. varigatus
became synistral (reversed) and 24%
became bilateral, while all the metamorphosed
P. olivaceus
became normally
synistal. In contrast, only 40% of the
V. varigatus
larvae re-expressed pit-x2
in premetmorphosis, while 100% of
P. olivaceus
larvae re-expressed pitx2.
These results strongly suggest that re-expression of pitx2 in premetamorphic
stage is directly related to eye migration during metamorphosis.
Suzuki et al. (2009) concluded that although the fi rst expression of pitx2
expression in embryonic stage forms asymmetrical formation of habenular
nuclei, re-expressed pitx2 at premetamorphosis is directly responsible for
lateralizing habenular nuclei to rightward in sinistral fl ounder and leftward
in dextral fl ounder respectively, thus inducing asymmetric development
of the brain and eyes.
Clarifi cation of details of molecular mechanisms of pitx2 re-expression
and its function in regulating metamorphosis is eagerly needed. In
particular, possible involvement of thyroid hormone in the re-expression
of pitx2 should be a subject of future study.
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