Biology Reference
In-Depth Information
Figure 8.
125
I
-
uptake, expressed as a percentage of control, by larval lamprey endostyles
following a 4 hour
in vitro
incubation in the presence of 0.72 nM (L-KClO
4
) or 3.6 mM (H-KClO
4
)
potassium perchlorate. Endostyles were incubated with 3 or 30 µCi of Na
125
I in the presence
or absence (controls) of KClO
4
. In all instances,
125
I
-
uptake by KClO
4
-treated endostyles was
signifi cantly lower than controls (ANOVA, P <0.05). (From R.G. Manzon and Youson, 2002).
Collectively, the work discussed above convincingly shows that
a decline in serum TH levels is required for induced metamorphosis.
When coupled with observations that serum TH levels gradually increase
throughout the larval period in preparation for metamorphosis and peak
prior to metamorphosis in
P. marinus
(Youson et al
.
, 1994), decrease early
in spontaneous metamorphosis in
P. marinus
(Wright and Youson, 1977;
Lintlop and Youson, 1983b; Youson et al
.
, 1994),
G. australis
(Leatherland
et al
.
, 1990), and
L. appendix
(Holmes et al
.
, 1999), and that exogenous T
3
can disrupt spontaneous metamorphosis in
P. marinus
(Youson
et al
.
, 1997)
it is diffi cult to discount the importance of a decline in TH levels during
lamprey metamorphosis. Although a decline appears to be essential for
metamorphosis, it is likely not the only factor required. For instance, PTU
suppresses TH levels in three species of lamprey (Leatherland et al
.
, 1990;
Holmes et al
.
, 1999; R.G. Manzon et al
.
, 2001), but unlike other goitrogens does
not induce metamorphosis. KClO
4
cannot supersede the water temperature
requirement for metamorphosis and is not able to induce metamorphosis
when lampreys are maintained at winter water temperatures (2-3°C) (R.G.
Manzon and Youson, 1999). Finally goitrogen-induced metamorphosis is
not as synchronized as spontaneous metamorphosis and, to date, there is
no evidence that animals that are the product of inducement are capable of
