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approach would identify differentially expressed genes that map to the
position of
met1
, potentially identifying candidate genes. As predicted, the
majority of differentially expressed genes from LG2 mapped within
10 cm of
met1
, and six of the 14 genes that located within 2 cm of
met1
were
differentially expressed as a function of genotype during larval development
and/or metamorphosis. At this point, it is not clear if the
met1
genomic region
is more enriched for differentially expressed genes than other regions of the
Ambystoma
genome, and perhaps genes that are TH responsive and associated
with brain development. Regardless, it is clear from microarray studies that
many genes are differentially expressed between paedomorphic
A. mexicanum
and metamorphic
A. t. tigrinum
,and
met1
locates to a uniquely structured
genomic region with several candidate genes for paedomorphosis.
10. SYNTHESIS: EVOLUTION OF SALAMANDER
PAEDOMORPHOSIS
Many times during salamander evolution, mechanisms that regulate
TH induction of metamorphosis were altered, resulting in paedomorphic
salamanders that retain larval traits into the adult, breeding phase. Over
the past 100 years, researchers have treated paedomorphic salamanders with
TH to investigate the physiological and evolutionary basis of this unique
salamander trait. Studies show that tissues within some species are entirely
resistant to TH, while others show partial or complete metamorphosis.
These varied responses are not explained by phylogeny or morphology.
Salamanders are a morphologically conservative group and paedomorphosis
has evolved independently during salamander evolution. It is not likely that
investigations of convergent morphological differences among deeply diver-
gent paedomorphic species will yield a synthetic model for the origin of
salamander paedomorphosis. Instead, an understanding of the origins
of paedomorphosis requires knowledge of physiological and genetic mech-
anism, environmental context, and life history traits associated with fitness
(
Fig. 8.5
). In the case of ambystomatid salamanders, paedomorph expression
is associated with TH-responsive loci that pleiotropically affect metamor-
phic timing and adult fitness traits. According to our model, paedomorphosis
and metamorphosis are mechanistically connected by loci that segregate al-
lelic variation for responsiveness to T4. Phenotypes arising from hybrid
ambystomatid crosses clearly show that it is possible to artificially phenocopy
the complete spectrum of metamorphic timing variation observed among
species in nature. Given variation in habitat stability, selection should favor
