LEECH SEGMENTATION - A MOLECULAR PERSPECTIVE

A variety of leech homeobox genes have been identified by homology wit h genes that are known to bring about the regionalization and segmenta tion of the anteroposterior body axis in other organisms. Embryonic ex pression patterns suggest a number of interphyletic similarities in th e way that these genes are utilized. However, several interesting diff erences have also been observed. In particular, transplantation experi ments in the leech embryo have shown that axially aligned patterns of homeobox gene expression are not specified by a global pattern of posi tional cues. Rather, the leech independently establishes anteroposteri or patterns of gene expression in each of five discrete stem cell line ages, and these patterns are brought into their final alignment throug h a process of morphogenetic assembly.