Genomic organization of the Hoxa4-Hoxa10 region from Morone saxatilis: Implications for Hox gene evolution among vertebrates

The physical mapping of Hox gene clusters from a Limited number of vertebra tes has shown an overall conservation in gene organization in which major e volutionary changes appear to be primarily restricted to the deletion of on e or more genes, with the exception of the amplification;of additional clus ters as postulated from zebrafish. We have sequenced a 31 kb region of the HoxA cluster from the teleost Morone saxatilis (striped bass), both to prov ide a detailed physical map of this region and to better understand the nat ure of Hox cluster evolution among vertebrate taxa. We identified five link ed Hox genes: Hoxa4, Hoxa5, Hoxa7, Hoxa9, and Hoxa10, which are organized s imilarly to those of other vertebrates. Furthermore, we have documented the absence of the Hoxa6 and Hoxa8 genes within the 31 kb contig. Comparison o f our results to those published for other vertebrates suggests that the ab sence of Hoxa6 is a common characteristic of teleosts, whereas the absence of Hoxa8 is common to vertebrates in general, with the possible exception o f zebrafish. Further comparisons between the HoxA genes from Morone with th ose from the pufferfish, Fugu, rubripes, revealed the Likely presence of a previously unreported Hoxa7 gene, or gene fragment, in the Fugu genome, whi ch suggests that the Hoxa7 gene, unlike Hoxa6 or Hoxa8, is present in teleo sts. In addition to these differences in vertebrate Hox cluster structure, we also observed a marked reduction in the length of the Hoxa4-a10 region b etween vertebrate lineages representative of teleosts and mammals. Comparat ive analysis of HoxA cluster organization among teleosts and mammals sugges ts that cluster length reduction and lineage-specific gene loss events are hallmarks of Hox cluster evolution. (C) 1999 Wiley-Liss, Inc.