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Increased understanding of thyroxine in urodele metamorphosis has been complemented by studies in genetics. Endocrinological research has now determined that thyroid hormone binds to nuclear receptors that are in immediate contact with DNA. Geneticists have determined that the hormone then works to change transcription of genes from those that develop larval characteristics to ones that generate juvenile and adult characteristics (Rosenkilde and Ussing 1996). |
 Image courtesy of Diane Malacinski) |
The working genetics of paedomorphosis have not been made as abundantly clear as those of metamorphosis. However, a "single-gene hypothesis" thought to control the axolotl's fully aquatic life cycle, has emerged (Voss 1995). Researchers base this on 1:1 Mendelian segregation outcomes of paedomorph/ metamorph backcrosses between A. mexicanum and A. tigrinum (Voss and Shaffer 1996). If this theory is correct, then their are two alternate alleles at a major locus primarily responsible for determining the expression of metamorphosis or paedomorphosis. |
| Further investigation, via genetic
linkage analysis, has identified a major quantitative trait locus for expression
of metamorphosis. This finding supports and is consistent with the
single-gene hypothesis (Voss and Shaffer 1997).
There are few examples of adaptive single genes in nature. Although viability of the major gene hypothesis is pending, this does not detract from speculation of its purpose. The evolution of paedomorphosis from an ancestral obligate metamorph may have been selected for as the result of ecological opportunity. Phylogenetic studies indicate that paedomorphosis has evolved independently in several ambystomid salamanders that inhabit waters devoid of aquatic predators (Voss and Shaffer 1997). Whether or not this speculation is upheld, a complete understanding of paedomorphosis remains illusive. However, integration of endocrinology and genetics has become a productive and promising future for developmental research on the axolotl. |
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This page was last updated on October 19, 2000
