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changes (gill and tail reabsorption) in response to T4, but other larval features
were resistant (histological sections of skin revealed only larval characteristics).
However, related paedomorphic species (
E. tynerensis
and
E. neotenes
) complete
metamorphosis after T4 treatment in less than 2 weeks (
Kezer, 1952
). Pae-
domorphic species of
Ambystoma
also complete metamorphosis when treated
with T4 but show species-specific variation in metamorphic timing
(
Voss et al., 2012
). Collectively, these studies establish that paedomorphosis
is associated with TH regulation and tissue responsiveness, and these char-
acteristics vary among closely and distantly related species.
Based on results of early TH induction experiments, it was proposed that
paedomorphic species evolve via several mechanisms, including disruption
of peripheral and central mechanisms of TH regulation (
Dent, 1968
;
Kezer,
1952
). Species that were shown to be refractory to TH but had active
thyroid glands were presumed to be deficient in peripheral mechanisms that
receive and transduce the T4 signal, such as TRs. Species that responded to
high levels of TH but did not apparently metamorphose in nature were
assumed to be deficient in some aspect of central regulation within the hypo-
thalamus or pituitary. Such a deficiency would normally prevent T4 release
but could artificially be over-ridden in the lab by T4 treatment. It also seems
likely that central regulatory mechanisms have been altered in the evolution of
facultative species from biphasic ancestors. This is because the brain and pitu-
itary regulate appropriate physiological and developmental responses in re-
sponse to environmental cues. In the case of facultative paedomorphosis,
the HPT axis is not sufficiently activated if cues favor a paedomorphic life
history, or environmental conditions suppress hormone activity, or if larval
development and maturation is delayed (
Whiteman, 1994
). Several environ-
mental conditions that are associated with habitat quality affect the probability
that an individual will express paedomorphosis, including low density of
conspecifics (
Harris, 1987; Semlitsch, 1987
), food availability (
Denoel &
Poncin, 2001; Ryan & Semlitsch, 2003; Semlitsch, 1987
), pond permanence
(
Semlitsch, 1987; Semlitsch et al., 1990
), and low temperature (
Eagleson &
McKeown, 1980; Snyder, 1956; Sprules, 1974
).
5. PAEDOMORPHOSIS IN THE MEXICAN AXOLOTL
The exemplar of all paedomorphic salamanders is the Mexican axolotl
(
A. mexicanum
). The Mexican axolotl is native to the Xochimilco lake sys-
tem, near present-day Mexico City. Whereas historical populations were
sufficiently large to provide an important food source to Indians for
