Is Lamprey Metamorphosis Regulated by Thyroid Hormones?

Lampreys are one of just a few fishes which have a true (first or first type) of metamorphosis in their life cycle. In the sea lamprey ( Petromyzon marinus ), spontaneous metamorphosis is initiated when the size (length and weight), condition factor, and lipid stores reach appropriate levels and coincide with the postwinter rise in water temperature. The serum levels of the thyroid hormones, thyroxine (T4) and triiodothyronine (T3), drop dramatically at the onset of metamorphosis and metamorphosis can be induced with treatment of animals with the goitrogen, KCIO4, which also results in a decline in serum levels of thyroid hormones. The fact that thyroid hormone treatment can block spontaneous and induced metamorphosis is support for the view that thyroid hormones, particularly T3, operates like a juvenile hormone in lamprey metamorphosis; this view is counter to the role of thyroid hormones in metamorphosis of other vertebrates. The monodeiodinase pathways, whereby T4 is converted to T3 or to the biologically inactive reverse T3, and even further degradation of T3, may be a significant mechanism directing metamorphic change. Lamprey metamorphosis is facultative in that it is initiated or inhibited depending upon the coordination of a complex integration of environmental, metabolic and hormonal cues. Thyroid hormones do not regulate lamprey metamorphosis in the sense observed in other vertebrate metamorphoses but they are important to the developmental process. Some of the features of the involvement of thyroid hormones in lamprey metamorphosis may be related to the presence of the endostyle in larvae which in turn reflects the ancient origins of this vertebrate and perhaps the conservation of an ancient method of induction of metamorphosis. Some clue for other factors which initiate lamprey metamorphosis may come through the examination of inducers of metamorphosis in lower chordates