| Sea lampreys (Petromyzon marinus), members of the ancient class Agnatha, have as part of their life cycle a metamorphosis when sedentary, filter-feeding larvae transform into free-swimming, parasitic juveniles. As in other vertebrates with a metamorphosis, a change in serum thyroid hormone (TH) levels is found at the time of initiation of lamprey metamorphosis, although the mechanisms involved have yet to be determined. The present study was designed to first provide some information on how THs exert their effects by cloning nuclear thyroid hormone receptors (TRs) and then retinoid X receptors (RXRs). Since the expression of these receptors is correlated with changes in serum TH and morphogenesis during the metamorphoses of anuran amphibians and some fish species, the second phase of the study was to examine their expression patterns in specific tissues and organs in larvae, during lamprey metamorphosis, and in adults using Northern blotting and relative quantitative RT-PCR. Two TR (PmTR1 and PmTR2) and three RXR cDNAs were cloned from sea lampreys whose deduced amino acid sequences were well conserved with those of other vertebrates, particularly in important functional regions. Phylogenetic analysis of amino acid sequences indicated that these lamprey receptors diverged from the vertebrate lineage prior to the gene duplication event that is thought to have occurred in the gnathostomes. Expression data indicated that PmTR1 levels fluctuated over the life cycle in an organ-specific manner, while the expression of PmTR2 and the RXRs was generally constant. The differential expression patterns of these lamprey receptors suggest that their availability might be a component of the peripheral regulation system that mediates TH action during sea lamprey metamorphosis. |
