Background: The functional integration of the cerebellum into a number of different neural systems is governed by the connection of its output axons. In amniotes, the majority of this output is mediated by an evolutionarily diverse array of cerebellar nuclei that, in mice, are derived from the embryonic rhombic lip. To understand the origins of cerebellar nucleus diversity, we have explored how nucleus development is patterned in birds, which notably lack a dentate-like nucleus output to the dorsal thalamus.
Results: Using targeted in ovo electoroporation of green fluorescent protein (GFP) and red fluorescent protein (RFP) in a variety of combinations and with different conditional enhancers, we show that cerebellar nuclei in chicks are produced, as in the mouse, at the rhombic lip. Furthermore, the comparison of fate-mapped neurons with molecular markers reveals a strict temporal sequence of cell fate allocation in establishing the avian lateral and medial cerebellar nuclei. In contrast to the mouse cerebellum, Lhx9 expression is confined to extracerebellar thalamic afferent nuclei corresponding to the absence, in chicks, of a dentate nucleus. Spatiotemporally targeted over-expression of Lhx9 in chick cerebellar nuclei (recapitulating in part the mammalian expression pattern) results in a loss of distinct nuclear boundaries and a change in axon initial trajectories consistent with a role for Lhx9 specifying targeting.
Conclusions: Our results confirm the relationship between cell fate and a fine grain temporal patterning at the rhombic lip. This suggests that the lack of a cerebellar output to the dorsal thalamus of birds corresponds with a restricted expression of the LIM-homeodomain gene Lhx9 to earlier born rhombic lip cohorts when compared to mice. The evolution of cerebellar nucleus diversity in amniotes may hence reflect a heterochronic adaptation of gene expression with respect to the sequential production of rhombic lip derivatives resulting in altered axonal targeting.