Development

Six3 repression of Wnt signaling in the anterior neuroectoderm is essential for vertebrate forebrain development. Lagutin, O. V. et al. Genes Dev. 17, 368–379 (2003)

From Zebrafish and Xenopus data we know that Wnt signalling must be inhibited at the anterior end of the neural plate for normal development of the future forebrain. Using transgenic mice and gene misexpression in chick and zebrafish, Lagutin et al. now show that the binding of Six3, a homeobox transcription factor, to the Wnt1 promoter is required for this inhibition.

Cytogenetics

Reciprocal chromosome painting between human, aardvark, and elephant (superorder Afrotheria) reveals the likely eutherian ancestral karyotype. Yang, F. et al. Proc. Natl Acad. Sci. USA 100, 1062–1066 (2003)

By painting aardvark and elephant chromosomes with human probes, and vice versa, Yang et al. were able to infer that the eutherian ancestral diploid karyotype comprised 44 chromosomes. The evolutionary breakpoints identified could be studied in future to identify the motifs promoting chromosome breakage and to investigate the evolution of mammalian genome.

Mouse Models

Genetics of dark skin in mice. Fitch, K. et al. Genes Dev. 17, 214–228 (2003)

Programmes to chemically mutagenize the mouse genome are producing many new mutants with which to study molecular pathways that are relevant to human disease. In one of the largest studies of its kind, Fitch et al. focus on a new group of dark-skin phenotypes. They map and clone the genes responsible for the increased skin-pigmentation, demonstrating that a phenotype-driven mutagenesis approach that focuses on a specific biological process can be an effective method of identifying new candidate genes and mouse models.

Evolutionary

Generation of a bacterium with a 21 amino acid genetic code. Mehl, R. et al. J. Am. Chem. Soc. 125, 935–939 (2003)

Could the genetic code have evolved to specify more than 20 amino acids? Mehl et al. engineer an Escherichia coli that encodes 21 amino acids: p-aminophenylalanine (pAF) is the addition to the standard set of 20. This completely autonomous bacterium can synthesize pAF from simple carbon sources, and incorporate it into proteins with high fidelity. The pAF bacterium will allow intriguing evolutionary experiments and could lead to the production of proteins with enhanced or new abilities.