The LIM-Homeodomain Gene Family in the DevelopingXenopusBrain: Conservation and Divergences with the Mouse Related to the Evolution of the Forebrain

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Abstract
A comparative analysis of LIM-homeodomain (LIM-hd) expression patterns in the developing stage 32 Xenopus brain is presented. x-Lhx2, x-Lhx7, andx-Lhx9 were isolated and their expression, together with that of x-Lhx1 and x-Lhx5,was analyzed in terms of prosomeric brain development and LIM-hd combinatorial code and compared with mouse expression data. The results show an almost complete conservation of expression patterns in the diencephalon. The Lhx1/5 and Lhx2/9subgroups label the pretectum/ventral thalamus/zona limitans versus the dorsal thalamus, respectively, in alternating stripes of expression in both species. Conversely, strong divergences in expression patterns are observed between the telencephalon of the two species for Lhx1/5 andLhx2/9. Lhx7 exhibits particularly conservative patterns and is proposed as a medial ganglionic eminence marker. The conservation of diencephalic segments is proposed to mirror the conservative nature of diencephalic structures across vertebrates. In contrast, the telencephalic divergences are proposed to reflect the emergence of significant novelty in the telencephalon (connectivity changes) at the anamniote/amniote transition. Moreover, the data allow the new delineation of pallial and subpallial domains in the developing frog telencephalon, which are compared with mouse subdivisions. In the pallium, the mouse combinatorial expression of LIM-hd is notably richer than in the frog, again possibly reflecting evolutionary changes in cortical connectivity.