Ambient Developmental Temperatures and Maternal Thyroid Supply Promote Early Life Diversification in Salmonids

The interplay between ambient temperature and thyroid signaling controls the morphogenesis of poikilotherms in the early stages of life. These two metabolic conditioning factors are crucial for the divergence of fish morphs undergoing adaptive diversification. Using Salvelinus malma, a model salmonid species with high phenotypic plasticity and a capacity for adaptive radiation, we conducted a series of experiments by rearing sympatric large predatory, mid-sized generalized and dwarf insectivorous morphs from both lacustrine and riverine environments. First, we compared the developmental rates and thyroid hormone profiles of these morphs reared under morph-specific and standard temperature regimes. Next, we reared the offspring of the generalized morph at temperatures imitating those at the spawning site of the morphs comprising the sympatric triplets. Finally, we reared the generalized morph under altered thyroid status at a standard temperature. Our findings revealed that morphs adapted to different developmental temperatures exhibit specific profiles of maternal thyroid hormone consumption, as well as discrepancies in the rate of life stage transition and cranial ossification. Changes in developmental temperature induce compensatory changes in the rate of thyroid hormone supply and heterochronic shifts in life stage transition and skull ossification. Lacustrine and riverine morphs demonstrate a similar developmental bias: fish with an elevated rate of thyroid consumption, developing in cold conditions follow a fast-developing predatory morphogenesis pathway, whereas those with a reduced maternal thyroid endowment and forced to develop in warm water become dwarfs.