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Evolution of regeneration and fission in annelids: insights from engrailed- and orthodenticle-class gene expression

Alexandra E. Bely^*, and Gregory A. Wray^¶

Department of Ecology and Evolution, State University of New York at Stony Brook, Stony Brook, NY 11794-5245, USA
Present address: Department of Molecular and Cell Biology, 385 LSA, University of California at Berkeley, Berkeley, CA 94720-3200 USA
^¶ Present address: Department of Biology, Duke University, Box 90338, Durham, NC 27708-0338, USA

*Author for correspondence (e-mail: abely{at}uclink.berkeley.edu)

Accepted April 9, 2001

The recent explosion of information on the role of regulatory genes in embryogenesis provides an excellent opportunity to study how these genes participate in post-embryonic developmental processes. We present a detailed comparison of regulatory gene expression during regeneration and asexual reproduction (by fission) in the segmented worm Pristina leidyi (Annelida: Oligochaeta). We isolated three genes from Pristina, one homolog of engrailed and two homologs of orthodenticle, and characterized their expression in different developmental contexts. In situ hybridization studies on worms undergoing normal growth, regeneration and fission demonstrate that in all three processes, Pl-en is expressed primarily in the developing nervous system, and Pl-Otx1 and Pl-Otx2 are expressed primarily in the anterior body wall, foregut and developing nervous system. Our data reveal extensive similarities between expression during regeneration and fission, consistent with the idea that similar developmental processes underlie these two types of development. Thus, we argue that in these annelids fission may have evolved by recruitment of regenerative processes. Furthermore, by comparing our data to existing data from leech embryos, we find evidence that embryonic processes are re-deployed during regeneration and fission.

Key words: Regeneration, Fission, Post-embryonic development, Annelid, Gene expression, Evolution, Homeobox gene, orthodenticle, engrailed, Pristina leidyi

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