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(
Logan, Pagan-Westphal, Smith, Paganessi, & Tabin, 1998; Piedra, Icardo,
Albajar, Rodriguez-Rey, & Ros, 1998; Ryan et al., 1998
), the findings sug-
gest that the
rev
locus plays a role in patterning the embryonic left-right
axis. Furthermore,
rev
offspring exhibit a high proportion (20-30%) of “re-
versed” (dextral) flounder, suggesting that
rev
also has a role in controlling
the directionality of metamorphic eye migration (
Hashimoto et al.,
2002
). Considering the previously discussed observation by
Suzuki et al.
(2009)
that
pitx2
expression occurs twice in developing flatfish (once during
embryogenesis and once prior to metamorphosis), it is interesting to note
their additional observation that the establishment of eye-sidedness at meta-
morphosis does not correlate with the laterality of either
pitx2
expression or
visceral organ placement during embryogenesis. However, the expression of
pitx2
in the left habenula at metamorphosis does indeed correspond with the
establishment of eye-sidedness, as loss of
pitx2
expression in the habenula at
this time (induced in wild-type flounder via treatment with ouabain to
induce laterality defects in the NLP pathway) results in the randomization
of eye-sidedness and the production of a population of juveniles containing
both normal morphology (i.e., sinistral for
P. olivaceus
and dextral for
V. vari-
egatus
) and “reversed” morphology (
Suzuki et al., 2009
). Interestingly,
whereas in both dextral and sinistral flatfish species the right habenula always
grows larger than the left, in flatfish with reversed metamorphic morphology
the volumetric asymmetry becomes inverted and the left habenula grows
larger than the right (
Fig. 6.4
E). In all vertebrates, axons projecting from
the habenula are known to relay thalamic, telencephalic, and other inputs
to the ventral midbrain (
Butler & Hodos, 1996
), and in the zebrafish, it
has been shown that axons emanating from the left and right habenula nuclei
innervate different dorsal and ventral regions of the midbrain in a pattern
established by the NLP pathway during embryogenesis (see review by
Snelson & Gamse, 2009
). Although southern flounder (
P. lethostigma)
juve-
niles have also been shown to display a pattern of habenular-midbrain in-
nervation similar to zebrafish, the innervation pattern is not different
between siblings with sinistral and reversed (dextral) eye-sidedness (
Kuan,
Gamse, Schreiber, & Halpern, 2007
), suggesting that the development of
eye-sidedness during metamorphosis is independent of the differential
regions of the embryos localize to the cephalic mesoderm. (G, H) Posterior view, dorsal
to the top. nt, neural tube; nc, notochord; sm, somite. Reproduced with permission from
Hashimoto et al. (2002)
.
