Gene Expression Analysis of Developing Cell Groups in the Pretectal Region of Xenopus laevis

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Our previous analysis of progenitor domains in the pretectum of Xenopus revealed three molecularly distinct anteroposterior subdivisions, identified as precommissural (PcP), juxtacommissural (JcP), and commissural (CoP) histogenetic domains (Morona et al. [2011] J Comp Neurol 519:1024–1050). Here we analyzed at later developmental stages the nuclei derived from these areas, attending to their gene expression patterns and histogenesis. Transcription-factor gene markers were used to selectively map derivatives of each domain: Pax7 and Pax6 (CoP); Foxp1 and Six3 (JcP); and Xiro1, VGlut2, Ebf1, and Ebf3 (PcP). Additional genoarchitectural information was provided by the expression of Gbx2, NPY, Lhx1, and Lhx9. This allowed both unambiguous characterization of the anuran pretectal nuclei with regard to their origin in the three early anteroposterior progenitor domains, and their comparison with counterparts in the chick and mouse pretectum. Our observations demonstrated a molecular conservation, during practically all the stages analyzed, for most of the main markers used to define genoarchitecturally the main derivatives of each pretectal domain. We found molecular evidence to propose homologous derivatives from the CoP (olivary pretectal, parvocellular, and magnocellular posterior commissure and lateral terminal nuclei), JcP (spiriformis lateral and lateral terminal nuclei), and PcP (anterior pretectal nucleus) to those described in avian studies. These results represent significant progress in the comprehension of the diencephalic region of Xenopus and show that the organization of the pretectum possesses many features shared with birds.