Biology Reference
In-Depth Information
abundance. The most dramatic changes are seen for basal keratinocytes that
proliferate and differentiate duringmetamorphosis and transcribe adult specific
keratins that generate a more cornified epithelial layer to limit desiccation
during the adult phase. Genes that transiently increase in abundance code
for matrix-remodeling proteins (e.g., MMPs) and cellular processes like meta-
bolism, transcription, and translation, while genes that decrease in abundance
include negative regulators of cellular processes (
Page et al., 2008
). These
gene expression patterns illuminate tissue remodeling and macromolecular
biosynthetic processes that occur during metamorphic climax.
Although some genes are expressed similarly between metamorphic and paedomor-
phic individuals during larval development
,
the majority of the differentially expressed
genes are unique.
For example, brain transcription has been compared be-
tween
A. mexicanum
and
A. t. tigrinum
individuals during larval development
and the period spanning the metamorphic period of the later species (
Page
et al., 2010
). As
A. t. tigrinum
individuals near the onset of anatomical meta-
morphosis, more gene expression differences are observed between larvae of
these species. Presumably, many of these expression differences trace to in-
creasing levels of glucocorticoids and THs in
A. t. tigrinum
individuals as they
initiate early metamorphic changes. In support of this idea, a glucocorticoid
receptor (
nr3c2
) showed significantly higher expression in
A. t. tigrinum
.
Also, some genes that were more highly expressed during natural metamor-
phosis in
A. t. tigrinum
were also similarly upregulated in
A. mexicanum
brains
when individuals were induced with T4 (
Huggins et al., 2012
). However,
hundreds of genes were expressed more highly in
A. t. tigrinum
than
A. mexicanum
throughout the larval period. At this point, it is not clear if
all of these differences are associated with metamorphic and paedomorphic
life histories; however, the observed systematic bias in transcription suggests
more than species-specific divergence of gene expression.
7. GENETIC ANALYSIS OF PAEDOMORPHOSIS
The idea that paedomorphic salamanders simply need a dose of TH
to induce the second half of the ancestral ontogeny influenced the way
paedomorphic life histories were thought to originate and evolve. For ex-
ample,
Goldschmidt (1940)
cited
A. mexicanum
as the “hopeful monster”
(
Page et al., 2010
), an example of evolution waiting around for the right
macromutation—simply block a single physiological step in TH regulation
and instantly a novel form is originated. The idea that paedomorphosis re-
sults from a simple mechanistic change gained further support when genetic
