Expression patterns of anterior Hox genes in the polychaete Chaetopterus: Correlation with morphological boundaries

Expression patterns for five Hox genes were examined by whole-mount in situ hybridization in larvae of Chaetopterus, a polychaete annelid with a tagma tized axial body plan. Phylogenetic analysis demonstrates that these genes are orthologs of the Drosophila genes labial, proboscipedia, zen, Deformed, and Sex combs reduced and are termed CH-Hox1, CH-Hox2, CH-Hox3, CH-Hox4, a nd CH-Hox5, respectively. Expression studies reveal a biphasic expression p attern. In early larval stages, well before any indications of segmental or ganization exist, a novel pattern of expression in bilateral posterior prol iferating cell populations, corresponding to the teloblasts, was detected f or each of the genes, with CH-Hox1 and CH-Hox2 expressed before the remaini ng three. In middle larval stages, all five genes are expressed in bilatera l strips along the ventral midline, corresponding with the developing ventr al nerve cord. In addition, CH-Hox1 and CH-Hox2 show strong expression at t he foregut-midgut boundary. By late larval stages the expression is general ly confined to the ventral CNS and ectoderm of the anterior parapodia. Ante rior boundaries of expression are "colinear," at later larval stages, with CH-Hox2 expressed most rostrally, in the first segment, and anterior expres sion boundaries for CH-Hox1, CH-Hox3, CH-Hox4, and CH-Hox5 in segments 2, 3 , 4, and 5, respectively. Like vertebrates and spiders, but unlike insects, CH-Hox3 participates in this colinear axial expression pattern. CH-Hox1 an d CH-Hox2 have distinct posterior boundaries of expression in the ninth seg ment, which corresponds to a major morphological boundary, and may reflect a reorganization of Hox gene regulation related to the evolutionary reorgan ization of the Chaetopterus body plan. (C) 2000 Academic Press.