The expression of FoxG1 in the early development of the European river lamprey Lampetra fluviatilis demonstrates significant heterochrony with that in other vertebrates.

FoxG1, a member of the Fox/Forkhead family of winged helix transcription factors, plays key roles in the induction and spatial compartmentalization of the telencephalon in vertebrates. Loss- and gain-of-function experiments have established FoxG1 as a maintenance factor for neural progenitors and a crucial player in the specification of the ventral telencephalon (subpallium). For the first time in evolution, the telencephalon appeared in the ancestors of vertebrates, including cyclostomes. However, although FoxG1 homologues are present in cyclostomes (i.e., in lampreys and hagfishes), no systematic study of the spatial-temporal expression of FoxG1 during the embryonic development of these animals has been carried out. Given these findings, we have now studied FoxG1 spatial-temporal expression patterns in the early development of the European river lamprey Lampetra fluviatilis. We show that in contrast to other vertebrates, in which the expression of FoxG1 begins during neurulation, the expression of this gene in L. fluviatilis starts after neurulation, first at stage 21 (early head protrusion) in the area of the otic placodes and then, beginning from stage 22, in the telencephalon. Such heterochrony of FoxG1 expression in the lamprey may reflect the fact that in this basally divergent representative of vertebrates, telencephalon specification occurs relatively late. This heterochrony could be related to the evolutionary history of the telencephalon, with a recent appearance in vertebrates as an extension to more ancient anterior brain regions. Another peculiarity of FoxG1 expression in lamprey, compared to other vertebrates, is that it is not expressed in the lamprey optic structures.

The presence of Anf/Hesx1 homeobox gene in lampreys suggests that it could play an important role in emergence of telencephalon.

Accumulated evidence indicates that the core genetic mechanisms regulating early patterning of the brain rudiment in vertebrates are very similar to those operating during development of the anterior region of invertebrate embryos. However, the mechanisms underlying the morphological differences between the elaborate vertebrate brain and its simpler invertebrate counterpart remain poorly understood. Recently, we hypothesized that the emergence of the most anterior unit of the vertebrate brain, the telencephalon, could be related to the appearance in vertebrates' ancestors of a unique homeobox gene, Anf/Hesx1(further Anf), which is absent from all invertebrates and regulates the earliest steps of telencephalon development in vertebrates. However, the failure of Anf to be detected in one of the most basal extant vertebrate species, the lamprey, seriously compromises this hypothesis. Here, we report the cloning of Anf in three lamprey species and demonstrate that this gene is indeed expressed in embryos in the same pattern as in other vertebrates and executes the same functions by inhibiting the expression of the anterior general regulator Otx2 in favour of the telencephalic regulator FoxG1. These results are consistent with the hypothesis that the Anf homeobox gene may have been important in the evolution of the telencephalon.