Evolution of the vertebrate neurotrophin and Trk receptor gene families.

Studies of neurotrophins and Trk receptors in jawless fish have shed light on the course of events underlying the formation of these gene families. They evolved early in vertebrate history during major gene duplication events, before the appearance of cartilaginous fish. The existence of multiple genes has permitted the diversification of neurotrophin and Trk receptor expression, and thereby enabling the acquisition of specific functions in selective neuronal populations.