Biology Reference
In-Depth Information
stay on the bottom on their future blind side. The period of staying on the
bottom becomes longer as metamorphosis progresses. By doing so, they
might suppress the development of dark pigmentation on the blind side.
4.5 Molecular System to Control the Asymmetry Formation
In most vertebrates, the shape or position of their internal organs such
as the heart, digestive organs and some part of the brain are known to be
asymmetrical. Asymmetric orientation of these internal organs is controlled
by the activation of a molecular pathway (Nodal-pathway), which precedes
the organogenesis (Tamura et al., 1999; Capdevila et al., 2000; Hamada et al.
2002;) including zebrafi sh and fl ounders (Concha et al., 2000; Hashimoto
et al., 2004, 2007). The principal pathway of left and right (L/R)-biased
signal in zebrafi sh and fl ounder is considered as follows. An embryonic
organ called Kupffer's vesicle, which is teleost-specifi c and homologous
to the “node” in higher vertebrates, is transiently formed at the tail bud in
the segmental period (Cooper and D'Amico, 1996; Essner et al., 2005). The
organ is considered to initiate sequential molecular signaling for L/R axis
determination (Hashimoto et al., 2004, 2007; Essner et al., 2005; Kramer-
Zucker et al., 2005). In the Kupffer's vesicle, two nodal pathway genes,
spaw
and
charon
, both of which are secretory signals of the transforming
growth factor-B superfamily, are expressed. Leftward water fl ow driven by
cilia in the Kupffer's vesicle transfers spaw from the Kupffer's vesicle to
the posterior left lateral plate mesoderm (LPM). On the other hand, charon
expressed in the Kupffer's vesicle blocks induction of spaw in the right
LPM. The transferred spaw in turn leads expressions of spaw itself, lefty
and pitx2, in the left LPM (Long et al., 2003; Hashimoto et al., 2004, 2007),
where spaw and lefty are considered to cooperatively induce pitx2 and
restrict pitx2 expression in the left side of the body (Hashimoto et al., 2004,
2007). Pitx2 is expressed fi rst in the gut fi eld and then at the heart fi eld in
left side LPM, and thereafter, expressed in the left side of diencephalons.
Thus expressed pitx2 induces asymmetrical development of the heart, gut
and diencephalons: heart jogs to the left, gut coils to rightward and dorsal
diencephalons are asymmetrically formed. To determine whether the
laterality of Nodal-pathway expression differs between dextral and sinistral
fl atfi sh, Hashimoto et al. (2007) examined the expression domains of charon,
lefty and pitx2 in embryos of the spotted halibut,
Varasper variegates
, whose
members are normally dextral. Also in this species, charon was expressed
at Kupffer's vesicle, and lefty and pitx2 were expressed at left LPM and
left side of the dorsal diencephalon as is the case in the Japanese fl ounder,
which is all sinistral. Moreover, the laterality of gut looping in the spotted
halibut was the same as that in the Japanese fl ounder. They also examined
expression domains of lefty and pitx2 for another sinistral fl ounder, summer
