Flatfish: An Asymmetric Perspective on Metamorphosis - Current Topics in Developmental Biology - page 172

innervation of dorsal and ventral midbrain targets by left and right habenulae
that is established during embryogenesis. Nonetheless, the observations of
Suzuki et al. (2009) clearly suggest that in flatfish the expression of pitx2
in the left habenula at metamorphosis corresponds with the development
of eye-sidedness that is characteristic for the species in question (i.e., a dextral
or sinistral species), and that the absence of this expression is accompanied by
randomized sidedness that yields an abnormally high percentage of “re-
versed” individuals. The mechanism that specifies whether a monomorphic
species metamorphoses with specifically sinistral or specifically dextral mor-
phology, however, remains to be discovered.
In addition to eye-sidedness, another example of postembryonic asym-
metry that is particularly evident in the largest of the flatfishes, the Atlantic
halibut ( Hippoglossus hippoglossus ), is the presence of a much larger fast skel-
etal muscle mass on the eyed side of adults compared with the blind side
( Hagen, Vieira, Solberg, & Johnston, 2008; Johnston, 2004 ). Interestingly,
Galloway et al. (2006) reported that mRNA for the myogenic regulatory
factor myod2 is expressed with a left-right asymmetric pattern in fast muscle
precursor cells during halibut somitogenesis, leading to an initial speculation
that myod2 may play a role in the development of the thicker muscle on the
eyed side. Upon further analysis, the researchers discovered that embryonic
myod2 is expressed in a randomized left-right pattern (displaying left-sided,
right-sided, or bilateral expression) (see Fig. 6.6 ), which did not correspond
with the development of asymmetric muscle thickness in the majority of the
halibut in the study that metamorphosed with dextral morphology, with
very low incidence of reversal ( Andersen et al., 2009 ). However, there still
does remain the intriguing possibility that halibut embryos expressing myod2
on their left sides are fated to metamorphose with reversed (sinistral) muscle
morphology, but that for unknown reasons experience higher mortality
such that only dextral morphs typically survive. In contrast to the asymmet-
ric expression patterns of halibut myod2 , its gene duplicate, myod1 ,is
expressed in a bilaterally symmetrical fashion during embryogenesis
( Andersen et al., 2009 ). Also, in spite of asymmetric myod2 expression during
embryogenesis, the somite pairs have been shown to form symmetrically
along the halibut embryo midline, and that morphological muscle symmetry
is maintained throughout metamorphosis and into the juvenile stage
( Galloway, Kjørsvik, & Kryvi, 1999 ), suggesting that the development of
muscle thickness asymmetry is a postmetamorphic event. The functional in-
fluence of asymmetric myod2 expression during halibut embryogenesis on
metamorphic asymmetry, if any, remains unresolved.