The term "larva" is usually defined as the stage from hatching to metamorphosis, but as mentioned for example by Hadfield (in [ 36]), this definition is more related to methods of dispersal than to changes in morphology; in most "direct-developing" gastropods, the morphological metamorphosis, with the loss of a velum or only of a prototroch, occurs before hatching from the egg capsule.
Using 5 and 50 nM T4 microarray datasets, we describe the temporal transcriptional response of T4 induced metamorphosis and specifically address the following question: Does T4 concentration affect morphological metamorphosis and gene expression in the axolotl?
Moreover, some of the genes that were differentially expressed were identified early in the larval period, well before the onset of morphological metamorphosis.
This analysis shows that global gene expression and morphological metamorphosis are strongly correlated, but there is little or no correlation between gene expression and T4 treatment.
At Day 28, axolotls reared in 50 nM T4 had fully resorbed tailfins and gills, and thus had completed morphological metamorphosis (Stage IV).
These patterns indicate that met1 genotype is associated with cis and trans transcriptional changes that commence late in the larval period, before morphological metamorphosis.
Thus, a low concentration of T4 delays the initiation timing of morphological metamorphosis but not the length of the metamorphic period.
In that study, A. t. tigrinum larvae grew at a faster rate, and all A. t. tigrinum showed definitive signs of morphological metamorphosis by 84 DPH.
These time points were sampled to test for early gene expression changes that might precede morphological metamorphosis, and because 28 days is a sufficient period for complete metamorphosis of 50 nM T4 induced A. mexicanum [ 11].
Transcripts were sampled from both species at four chronologically matched times post hatching to obtain temporal profiles of gene expression during the early larval period and during early stages of morphological metamorphosis in A. t. tigrinum.
