Lamprey Metamorphosis - Metamorphosis in Fish - page 43

of the thyroid axis during KClO 4 -induced metamorphosis in P. marinus
(Youson and Sower, 2001). As with spontaneous metamorphosis, a rise in
brain GnRH-I and III levels occurs during KClO 4 -induced metamorphosis,
however, as with other aspects of induced metamorphosis, the changes
in GnRH levels do not closely parallel those that occur in spontaneous
metamorphosis (Youson and Sower, 2001). A signifi cant increase in GnRH-
III is observed between stages 3 and 4 of KClO 4 -induced metamorphosis,
however during spontaneous metamorphosis this increase does not occur
until stage 6. Likewise, GnRH-I in the brain increases markedly at stage
7 during spontaneous metamorphosis; not only is this increase absent in
KClO 4 -induced metamorphosis, but a signifi cant decline occurs at stage 5
of induced metamorphosis (Youson and Sower, 2001). These differences
in GnRH levels between spontaneous and induced metamorphosis may
be related to the asynchronous development associated with induced
metamorphosis. It is difficult to make direct comparisons between
spontaneous and induced metamorphosis as staging in the latter case is at
best an approximation (Holmes and Youson, 1993; R.G. Manzon and Youson,
1997; R.G. Manzon et al . , 1998) (Fig. 1). Stage 5 is the most advanced stage
of induced metamorphosis observed by Youson and Sower (1991), despite
a 4 month KClO 4 exposure which is equivalent to the time required to
complete spontaneous metamorphosis. A better understanding of cause and
effect relationships associated with asynchrony in induced metamorphosis
could prove invaluable to our understanding of the regulation of lamprey
metamorphosis. For instance, the failure to complete metamorphosis in
some individuals might in part be related to the need for a peak in GnRH-I
and GnRH-III signaling from the hypothalamus (Youson and Sower, 2001).
An alternative explanation is that peak concentrations in GnRH are not
realized during induced metamorphosis because the appropriate positive or
negative feedback signals are absent. Is the lack of complete metamorphosis
due to the absence of a peak in GnRH or visa versa? In either case, these data
support a role for GnRH in metamorphosis, they show that manipulation
of the thyroid axis results in some changes in the reproductive axis, and
they indicate that there is some overlap between these two axes.
That there is the potential for cross regulation between the lamprey
gonadal and thyroid axes was fi rst supported by observations that a single
injection of either salmon GTH or a GnRH analog produces signifi cant
elevations in serum T 4 in adult P. marinus (Sower et al . , 1985). To date, one
lamprey GTH-β (Sower et al . , 2006) and one GpH-α subunit, with homology
to GpH-A2 (TSM-α), have been identifi ed (Sower et al . , 2009). Preliminary
data also exist for a second lamprey GpH-β (lGpHB5) with homology to
GpHB5 (TSM-β) (Sower et al . , 2009). Sower and co-workers (Sower et al . ,
2009) propose that an ancestral GTH-β gave rise to one lamprey GTH-β
and it was gnathostome-specifi c duplications that gave rise to FSH-β,