Summary of the hypothesized evolutionary history of the GnRH receptor family in vertebrates. Boxes and their respective colors represent major groups of paralogous gene family members, as illustrated in Figure 5. Filled boxes symbolize gene presence and open boxes symbolize gene absence. “-” in an open box indicates the initial loss of a gene and “+” indicates the initial gain of a gene. “?” indicates that insufficient data are available to determine whether the ancestral condition is presence of one, two, or three copies of a Type IIa-3 receptor, and “*” indicates receptors that arose either through duplication or divergence. “WGD” signifies the third lineage-specific whole genome duplication that occurred in teleosts. Amphioxus possess four copies of a gene that is closely related to the GnRH receptors, as illustrated in Figure 5.