Annelids shed light on the evolution of spiralian development

Spiralian development is characterized by stereotypic cell geometry and spindle orientation in early cleavage-stage embryos, and of ultimate fates of descendent clones. Diverse taxa such as molluscs, annelids, flatworms and nemerteans exhibit spiralian development, but it is a mystery how such a conserved developmental program gives rise to such diverse body plans. This review highlights examples of variation during early development among spiralians, emphasizing recent experimental studies in the annelid Capitella teleta(Blake, 2009). Intracellular fate mapping studies in C. teleta reveal that many of its cellsâ fates are shared among spiralians, but it also has a previously undescribed novel origin for trunk mesoderm (3c and 3d micromeres). Studies have identified an inductive signal in spiralians that has â organizing activityâ , and influences cell fates in the surrounding embryo. C. teleta also has an organizing activity, however, surprisingly, it is localized to a different cell, it signals at a different developmental stage, and likely utilizes a distinct molecular signaling pathway compared with that in molluscs. A model is presented to provide a mechanistic explanation of evolutionary changes in the cellular identity of the organizer. Detailed experimental investigations in spiralian embryos demonstrate variation in developmental features that may influence the evolution of novel forms.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author