Hox cluster genomics in the horn shark, Heterodontus francisci

Reconstructing the evolutionary history of Hox duster origins will lead to insights into the developmental and evolutionary significance of Hox gene c lusters in vertebrate phylogeny and to their role in the origins of various vertebrate body plans. We have isolated two Hox clusters from the horn sha rk, Heterodontus francisci, These have been sequenced and compared with one another and with other chordate Hox clusters. The results show that one of the horn shark clusters (HoxM) is orthologous to the mammalian HoxA cluste r and shows a structural similarity to the amphioxus cluster, whereas the o ther shark cluster (HoxN) is orthologous to the mammalian HoxD cluster base d on cluster organization and a comparison with noncoding and Hox gene-codi ng sequences. The persistence of an identifiable HoxA cluster over an 800-m illion-year divergence time demonstrates that the Hox gene clusters are hig hly integrated and structured genetic entities. The data presented herein i dentify many noncoding sequence motifs conserved over 800 million years tha t may function as genetic control motifs essential to the developmental pro cess.