Biology Reference
In-Depth Information
thousands of years (
Smith, 1989
), the current
A. mexicanum
population at
Xochimilco is on the verge of extinction (
Graue, 1998; Zambrano, Vega,
Herrera, Prado, &Reynoso, 2007
).
Smith (1989)
provides an enjoyable histor-
ical account of the original collection of
A. mexicanum
from Xochimilco in
1863. Six
A. mexicanum
were transferred to Paris where
Dumeril (1870)
subse-
quently reported that individuals reproduced in an aquatic, larval state and that
some of the resulting offspring underwent a metamorphosis.
Smith (1989)
sug-
gests that some of the original stock that arrived in Paris included closely related
members of the Tiger salamander species complex, that are capable of
expressing a paedomorphic or metamorphic life history. However, it seems
likely that domestication altered the penetrance for expressing paedomorphosis
and the original axolotl stock was pure
A. mexicanum
that maintained a higher
propensity to express metamorphosis in nature (
Voss & Shaffer, 2000
). Even
though metamorphic forms have been culled from the Ambystoma Genetic
Stock Center axolotl collection for decades, the frequency of spontaneous
metamorphosis is
1-2%, with a 10% frequency observed if
A. mexicanum
experience stressful conditions (Randal Voss, unpublished data).
The simplest explanation for paedomorphosis in
A. mexicanum
is a faulty
thyroid gland. However, the thyroid gland is functional; it can synthesize T4
and release T4 after TSH stimulation (
Prahlad, 1968; Taurog, Oliver, Eskay,
Porter, & Mckenzie, 1974
). Numerous studies, including our own (
Page
et al., 2007, 2008
), have demonstrated that individuals can be stimulated
to undergo morphological metamorphosis by administering T4. This sug-
gests that intracellular receptors and deiodinase enzymes are functional.
Putative T3 receptors have been identified in red blood cells (
Galton,
1991
) and TR function has been shown
in vitro
using mammalian cells
(
Safi et al., 2004
). Also, the activity of deiodinases has been demonstrated
(
Darras et al., 2002; Galton, 1991
).
Blount (1950)
showed that metamor-
phosis is inhibited if the pituitary of
A. mexicanum
is transplanted into the
metamorph
A. tigrinum
, while the reciprocal transplantation experiment
resulted in metamorphosis.
Tassava (1969)
pointed out that these grafts were
probably not precise enough to rule out a hypothalamic contribution. TSH
is present in the
A. mexicanum
pituitary (
Taurog et al., 1974
) at a quantity
sufficient for inducing metamorphosis (
Darras & Kuhn, 1983
). Thus, the pi-
tuitary is capable of producing sufficient functional TSH for metamorphosis,
but for some reason, it is not released. There are at least four explanations for
this observation in
A. mexicanum
: (1) The pituitary does not receive appro-
priate stimulation from the hypothalamus, (2) the pituitary is insensitive to
hypothalamic stimulation, (3) the pituitary is defective in releasing TSH, and
(4) the pituitary is functional but stimulated to release TSH at the wrong
