Baguñà J: Hox and parahox genes in flatworms: characterization and expression

SYNOPSIS. Flatworms (phylum Platyhelminthes) are favourite organisms in De-velopmental Biology and Zoology because of their extraordinary powers of regen-eration and because they may hold a pivotal place in the origin and evolution of the Bilateria. Hox genes play key roles in both processes: setting up the new an-teroposterior pattern in the former, and as qualitative markers of phylogenetic affinities among bilaterian phyla in the latter. We have searched for Hox and ParaHox genes in several flatworm groups spanning from freshwater triclads to marine polyclads and, more recently, in the acoels, the likely earliest extant bila-terian. We have isolated and sequenced eight Hox genes from the freshwater tri-clad Girardia tigrina and three Hox and two ParaHox genes from the polyclad Discocelis tigrina. Data from the acoels Paratomella rubra and Convoluta roscof-fensis is also reported. Flatworm Hox sequences and 18S rDNA sequence data support clear affinities of Platyhelminthes to spiralian lophotrochozoans. The basal position of acoel flatworms supported from recent 18S rDNA data, remains still uncertain. Expression of Hox genes in intact and regenerating adult organisms show nested patterns with graded anterior expression boundaries, or ubiquitous expression. New approaches to study the function of Hox genes in flatworms, such as RNA interference are briefly discussed