[Thyroid incorporation of radioactive iodine in neotenic tritons (Triturus helveticus Raz.). In vivo studies by high resolution radioautography. Comparison of iodine incorporation in adults and larvae].

In vivo kinetics of radioiodine incorporation have been established for 70 newts, collected from Triturus helveticus Raz. populations where spontaneous neoteny occurs. Fifty of these newts which appeared to present four degrees of neoteny (more or less metamorphosed), have been studied. Main results obtained were: 1) The intensity of the iodine incorporation varies according to the degree of neoteny. The higher is this one, the lower is the intensity. 2) In general, iodine uptake rates are more or less similar to these of adults, but more often the evolution of the 125I uptake approaches a plateau-curve. From thyroid radioautographies on some animals (17 neotenic and 2 adult newts) for which in vivo iodine kinetics have been examined and on 9 normal larvae in premetamorphosis or in metamorphosis, we have noticed in those neotenic thyroids, a very small number of silver grains on apical vesicles and colloidal lumina when the degree of neoteny is the highest. This number of marked apical vesicles and colloid is more important in the neotenics which have lost some larval characters. Silver grains on colloid droplets indicating thyroid hormone excretion are inexistent in the most larval neotenics, more numerous in most metamorphosed neotenics. But they always are less numerous than in metamorphosing larvae. For a few animals, the fixation curve is very high, probably because of a stimulation of thyroid function, induced by the test and not because of a goitrous tendancy, as radioautographic observation of their thyroid shows. Thyroid function and its relation with the origin of this total and accidental neoteny are discussed.