Effects of lithium chloride and retinoic acid on the expression of genes from the Xenopus laevis Hox 2 complex.

We show that Xenopus laevis has a Hox 2 complex, and that this complex is strongly conserved with the mammalian one, both in structure and in the rules of spatial and temporal sequential expression of its genes during the early stages of development. Lithium chloride and retinoic acid, two reagents known to alter axial patterning of the body when applied to Xenopus embryos, produce, respectively, embryos with reduced posterior but exaggerated anterior structures and embryos with truncation of anterior structures. We report here on the effect of these reagents on the expression of Hox 2 genes in the Xenopus embryos. LiCl has a dramatic effect on Hox genes, suppressing the expression of these genes during gastrulation and early neurulation. However, later on expression of these genes reaches significant levels, suggesting the existence of two phases in the control of Hox gene expression. Retinoic acid increases the steady state level of transcripts from Hox genes with the greatest effect on Hox 2.7, the most anterior of the genes studied. This suggests that the results obtained in EC cells (Simeone et al., 1990, 1991) reflect what occurs in vivo. Neither LiCl nor RA change the sequential order of the onset of expression of the genes, showing that these reagents do not perturb the molecular mechanisms used to establish the sequential activation of the genes of the Hox complexes.