Biology Reference
In-Depth Information
habitats, paedomorphosis is a fixed trait with little or no incidence of
metamorphosis. Within species that use less permanent bodies of water for
breeding, individuals may reproduce initially as paedomorphs but undergo
metamorphosis in subsequent years (
Deno¨l et al., 2007
). The different
patterns of growth and development observed among paedomorphs yield
different adult morphologies, and this suggests different mechanistic bases
for the evolution of paedomorphosis within and among salamander families.
However, exactly how patterns of development and morphology map onto
mechanisms that block TH signaling and metamorphosis, or affect the rate of
gonadal development, is essentially unknown. Without such mechanistic
information, it is not possible to distinguish among examples of paedomor-
phosis that evolve convergently among lineages, which clearly has occurred
during salamander phylogenesis (
Shaffer & Voss, 1996; Weins &Hoverman,
2008
). Thus, all terms that
describe
the evolutionary origin or developmental
expression of paedomorphic salamanders are potentially useful metaphors,
but if they are not based upon mechanistic insight and genetically based
models, they are not likely to resolve outstanding questions concerning
salamander life history variation and life cycle evolution.
It is interesting to consider why paedomorphosis is unique to salamanders
and not other amphibians. In as much as paedomorphosis evolves as a pro-
longation of the larval period, it is notable that salamanders with biphasic life
cycles typically have longer larval periods than caecilians or anurans
(
Duellman & Trueb, 1986
). In biphasic salamander species, metamorphosis
may occur within the same season or year that eggs are laid or larvae may
overwinter in permanent habitats and metamorphose after one or more ad-
ditional years of development (e.g.,
Beachy, 1995; Castanet, Francillon-
Vieillot, & Bruce, 1996; Voss, Prudic, Oliver, & Shaffer, 2003
). Although
there are examples of anurans with multiyear larval periods, salamander lar-
val periods are typically longer. So, there are fundamental differences be-
tween salamanders and other amphibians; salamanders have greater
evolutionary potential to protract (and contract in cases of direct develop-
ment) the length of the larval period. This greater evolutionary potential
is associated with at least five aspects of salamander development and life his-
tory. First, salamanders exhibit slow rates of growth and development. Some
anurans can complete metamorphosis in just a couple of weeks, while the
fastest developing salamander larvae require more than a month, and typi-
cally several months (
Duellman & Trueb, 1986
). In general, salamander
metamorphosis is developmentally less radical and more protracted than that
of anurans. For example, salamander forelimb development does not
