Biology Reference
In-Depth Information
Aquatic larva
(+)
(
-
)
Thyroid
hormone
Terrestrial adult
metamorph
Genes
environment
Aquatic adult
paedomorph
Figure 8.1 A representative salamander larva, adult metamorph (Ambystoma tigrinum),
and adult paedomorph (A. mexicanum). Critical levels of thyroid hormone induce meta-
morphosis in salamanders. Environmental and genetic factors may cause thyroid hor-
mone levels to be low, resulting in paedomorphic salamanders. The pictures were taken
by Jeramiah Smith.
Boley, Smith, Putta, & Voss, 2010
). However, TH secretion is regulated by
complex, neuroenodrine pathways that must first develop during the larval
period before these systems become competent to signal TH release in re-
sponse to intrinsic and extrinsic cues (
Denver, Glennemeier, & Boorse,
2002
). Thus, paedomorphosis could conceivably evolve by altering the de-
velopment or function of a number of different cells, tissues, and organs that
regulate the release or reception of TH. Given this complexity and the broad
pleiotropic role that TH assumes in amphibian development and physiol-
ogy, it seems likely that paedomorph evolution requires multiple changes
across many genes.
The specific heterochronic process most commonly associated with sal-
amander paedomorphosis is neoteny (
Gould, 1977
)—relative to the ances-
tor or metamorphic condition, somatic development is delayed but rate or
timing of reproductive maturation is the same. However, paedomorphosis
in some species is associated with an earlier time to first reproduction,
achieved by accelerating the rate of gonadal development (progenesis in
Trit-
urus alpestris
;
Denoel & Joly, 2000
) or by initiating reproductive maturation
earlier in the life cycle (peramorphosis in
Ambystoma talpoideum
;
Ryan &
Semlitsch, 1998
). Thus, very different patterns of growth and development
may be associated with the expression of paedomorphosis within and be-
tween species (
Whiteman, 1994
). Among species that occur in stable aquatic
