Homologous tails? Or tales of homology?

Classical mutations at the mouse Brachyury (T) locus were discovered becaus e they lead to shortened tails in heterozygous newborns, no tail (ntl) muta nts in the zebrafish, as their name suggests, show a similar phenotype. In Drosophila, mutants in the brachyenteron (byn) gene disrupt hindgut formati on. These genes all encode T-box proteins, a class of sequence-specific DNA binding proteins and transcription factors. Mutations in the C. elegans ma b-9 gene cause massive defects in the male tail because of failed fate deci sions in two tail progenitor cells. In a recent paper, Woollard and Hodgkin ((1)) have cloned the mab-9 gene and found that it too encodes a T-box prot ein, similar to Brachyury in vertebrates and brachyenteron in Drosophila. T he authors suggest that their results support models for an evolutionarily ancient role for these genes in hindgut formation. We will discuss this pro posal and try to decide whether the gene sequences, gene interactions and g ene expression patterns allow any conclusions to be made about the rear end of the ancestral metazoan. BioEssays 22:781-785, 2000. (C) 2000 John Wiley & Sons, Inc.