YOUSON, J.H.; University of Toronto at Scarborough: Lamprey metamorphosis and a theory on the evolution of adult life history types
Lampreys, as agnathans, have a direct lineage to the most ancient of vertebrates. Although the other extant agnathan, the hagfish, has direct development, all lamprey species have a free-living larva that undergoes a true metamorphosis into a juvenile. Two adult life histories, nonparasitic and parasitic, are seen among the 38 extant lamprey species with 20 of these being nonparasitic, the more recently evolved adult life history. Paired species, with a nonparasitic and parasitic member, have arisen from a parasitic ancestor. Recent findings of hormone profiles during metamorphosis, the existence of a certain degree of plasticity in the timing of development in wild populations, and the ability to induce precocious metamorphosis have allowed some speculation on both the evolution of metamorphosis as a developmental strategy and the evolution of adult life history types. An attempt will be made to explain the evolutionary origins of the unique thyroid hormone involvement in lamprey metamorphosis. Recent results comparing profiles of the hormone, gonadotropin-releasing hormone, and the prohormone, proopiomelanocortin, during metamorphosis and sexual maturation in a parasitic and nonparasitic species will be integrated into a view of how adult life history is directed during these developmental events. Many of the answers to questions on the control and evolution of metamorphosis in lampreys may be found among organisms that shared their time of origin and their long evolutionary history.