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of paedomorphosis may be pathological, reflecting extreme physiological
conditions that may be exacerbated by inbreeding depression within small,
isolated populations. However,
Matsuda (1982; 1987)
suggested that low
temperatureispivotalintheadaptiveevolution of paedomorphic salamanders;
low temperature initially causes the expression of paedomorphosis in a popula-
tion, and then through genetic assimilation (
Waddington, 1953
)thepae-
domorphic phenotype becomes fixed. The model that we proposed above is
consistent with genetic assimilation because we also assume directional selection
of TH-response alleles that increase an individual's likelihood of expressing a
paedomorphic phenotype. The selection process and allelic effects depend upon
environmental context.
The experimental approaches and evolutionary model outlined above
provide a possible framework from which to initiate studies of the genetic
basis of metamorphic timing in natural amphibian populations. At this point,
it is not clear how many TH-response genes contribute to adaptive differ-
ences in metamorphic timing within natural populations and among species,
or whether the same loci and physiological mechanisms are implicated in
independent examples of paedomorph evolution. Also, much remains to
be learned about facultative paedomorphosis at a mechanistic level. For
example, does facultative expression of paedomorphosis mechanistically
present an intermediated condition between metamorphosis and paedomor-
phosis, and how is it regulated? It is exciting to think that these and other
questions can now be resolved using genetic and genomic methods that
are applicable to nongenetic model organisms. The future looks bright
for investigating evolutionary, developmental, and physiological aspects of
salamander life history and life cycle evolution.
REFERENCES
Armstrong, J. B., Duhon, S. T., &Malacinski, G. M. (1989). Raising the axolotl in captivity.
In J. B. Armstrong & G. M. Malacinski (Eds.),
Developmental biology of the axolotl
(pp. 220-227). New York: Oxford University Press.
Beachy, C. K. (1995). Age at maturation, body size, and life-history evolution in the sala-
mander family Plethodontidae.
Herpetological Review
,
26
, 179-181.
Bizer, J. R. (1978). Growth rates and size at metamorphosis of high elevation populations of
Ambystoma tigrinum
.
Oecologia
,
34
, 175-184.
Blount, R. F. (1950). The effects of heteroplastic hypophyseal grafts upon the axolotl,
Ambystoma mexicanum
.
The Journal of Experimental Zoology
,
113
, 717-739.
Brown, D. D., & Cai, L. (2007). Amphibian metamorphosis.
Developmental Biology
,
306
,
20-33.
Bruce, R. C. (2003). Life histories. In D. M. Sever (Ed.),
Reproductive biology and phylogeny of
Urodela
(pp. 477-525). Enfield, NH: Science Publishers, Inc.
