QUICK SEARCH:   [advanced] Author: Keyword(s):
Home     Help     Feedback     Subscriptions     Archive     Search     Table of Contents

This Article Summary Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Bely, A. E. Articles by Wray, G. A. Search for Related Content PubMed PubMed Citation Articles by Bely, A. E. Articles by Wray, G. A.

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

View larger version (32K): [in a new window]   Fig. 1. Regeneration and paratomic fission in Pristina leidyi. Anterior is towards the left in this and all other figures. (A) Adult worms grow continuously by adding new segments from a subterminal posterior growth zone (pgz). Trunk segments bear bristle-like projections called chaetae (dc, dorsal chaetae; vc, ventral chaetae), which occur in paired dorsolateral and ventrolateral bundles roughly in the middle of each segment. The prostomium (pr) is the asegmental tissue in front of the mouth (greatly elongated in Pristina), the peristomium (pe) is the asegmental tissue around the mouth and the pygidium (py) is the posterior asegmental cap of tissue. (B) After posterior or anterior amputation, worms wound-heal, form a blastema (an undifferentiated mass of cells), and replace the missing structures through regeneration. (C) During paratomic fission, a worm forms a zone of cell proliferation (a fission zone) in the middle of its body. This zone splits into two proliferative zones, each forming new tissues anteriorly. A new head and tail are thus intercalated in the middle of the original worm’s body, forming a transiently linked chain of worms. Multiple fission zones may be present in a worm: younger fission zones form in progressively more anterior segments (see Fig. 9). In growth, regeneration and paratomic fission, new segments are added in an anterior to posterior direction. Blue, new posterior tissue; green, new anterior tissue; black, regions of cell proliferation (fission zone or posterior growth zone).

View larger version (14K): [in a new window]   Fig. 2. Homeodomain and domain EH5 of en-class genes. Percent amino acid identities to Pl-en are shown for the homeodomain in parentheses. Dots indicate residues identical to Pl-en. Sequences are from Wedeen et al. (Wedeen et al., 1991), Dick and Buss (Dick and Buss, 1994), Wray et al. (Wray et al., 1995), Webster and Mansour (Webster and Mansour, 1992), Poole et al. (Poole et al., 1985), Manzanares et al. (Manzanares et al., 1993), Holland et al. (Holland et al., 1997) and Logan et al. (Logan et al., 1992).

View larger version (43K): [in a new window]   Fig. 3. Homeodomain and C-terminal region of Otx-class and related genes. (A) Upper sequences are Otx-class genes; lower sequences are closely related, non-Otx-class genes. Percent amino acid identities to Pl-Otx1 (left column) and Pl-Otx2 (right column) are given in parentheses. Dots indicate residues identical to the top reference sequence; the arrowhead points to the lysine characteristic of Otx-class genes. (B) Amino acid motifs shared between deuterostomes (top) and protostomes (bottom) are in bold; the conserved domain is boxed; broken lines represent alignment gaps; asterisks represent termination codons. Sequences are from Bruce and Shankland (Bruce and Shankland, 1998), Arendt et al. (Arendt et al., 2001), Finkelstein et al. (Finkelstein et al., 1990), Umesono et al. (Umesono et al., 1999), Simeone et al. (Simeone et al., 1993), Smith et al. (Smith et al., 1999), Furukawa et al. (Furukawa et al., 1997), Frigerio et al. (Frigerio et al., 1986), Goriely et al. (Goriely et al., 1996), Simeone et al. (Simeone et al., 1994), Ueki et al. (Ueki et al., 1998), Wada et al. (Wada et al., 1996) and Li et al. (Li et al., 1996).

View larger version (67K): [in a new window]   Fig. 4. Pl-en expression during posterior growth. (A) lateral view; (B,C) ventral views. (A,B) In growing adults, Pl-en is expressed in dorsolateral cells (A, small arrow, positive cell slightly out of focus) near the anterior limit of the pygidium, as well as in bilaterally symmetrical ventral and ventrolateral cells (A,B, large arrows) of the posterior growth zone and young segments. (C) Visualization of nuclei of the specimen in B, as revealed by Hoechst 33258 staining, demonstrates that ventral Pl-en staining in young segments localizes primarily to the ventral nerve cord (C, white bracket). Nuclei of the ventral nerve cord form a ‘U’ in cross section; thus, in this ventral view, the outer margins of the nerve cord appear more densely nucleated. The black bracket in A,B marks a fully formed segment. Vertical scale bar: 50 µm.

View larger version (72K): [in a new window]   Fig. 5. Pl-en expression during posterior and anterior regeneration. (A,C) Lateral views; (B,D) ventral views. In diagrams of expression (left), two original (non-amputated) segments are included, broken line represents gut outline and color highlights the major expression domains. (A,B) Mid-stage posterior regeneration: Pl-en is expressed in a few cells (A, arrow) near the anterior limit of the regenerated pygidium, and in scattered ventral cells of the blastema (A,B, arrowheads) in regions where new segments are being formed. A is a composite of two images of the same specimen taken at slightly different focal planes. (C,D) Mid-stage anterior regeneration: Pl-en is similarly expressed in scattered ventral cells of the blastema. In this and all other figures, a white bar marks new posterior tissue and a black bar marks new anterior tissue. A bracket identifies the location of the original (non-regenerated) segment closest to the site of amputation. Vertical scale bar: 50 µm.

View larger version (81K): [in a new window]   Fig. 6. Pl-en expression during fission. (A) lateral view; (B,C) ventral views. In diagrams of expression (left), two original segments are shown on either side of the fission zone. (A) Pl-en expression is first detected in young fission zones in dorsolateral (large arrow) and ventrolateral (small arrow) cells near the anterior margin of the developing pygidium. The positive dorsolateral cells persist through late stages of fission, becoming the anterior pygidium expression seen during posterior growth and regeneration. A is a composite of two images of the same specimen taken at slightly different focal planes. (B,C) During mid (B) and late (C) stages of fission, Pl-en is expressed in scattered ventral cells (black arrowheads) and ventrolateral cells (small arrow) of developing anterior and posterior segments, often in a recognizably segmentally iterated pattern. The ventral expression is localized primarily to the ventral nerve cord. Expression fades below detectable levels in older (more anterior) segments of the fission zone, once these have produced chaetae (white arrowheads). Brackets mark one original (parental) segment on either side of the fission zone. A broken line marks the fission plane, along which the anterior and posterior worms will eventually separate. Vertical scale bar: 50 µm.

View larger version (66K): [in a new window]   Fig. 7. Pl-Otx2 expression during posterior growth and Pl-Otx1 expression in fully formed mid-body segments. (A) Lateral view; (B,C) ventral views. See text for explicit comparisons of Pl-Otx1 and Pl-Otx2 expression. (A,B) During growth, Pl-Otx2 is expressed ventrally in young segments, primarily in the developing ventral nerve cord (arrows). (C) Pl-Otx1 is expressed in a single, unpaired cell of the ventral ganglion of several contiguous midbody segments, typically beginning in segment VI (left arrow). A bracket marks a fully formed segment in each panel. Vertical scale bar: 50 µm.

View larger version (71K): [in a new window]   Fig. 8. Pl-Otx2 expression during posterior and anterior regeneration. (A,C,E-G) Lateral views; (B) ventral view; (D) dorsal view. See Fig. 5 for explanation of expression diagrams (left). (A,B) Mid-stage posterior regeneration: Pl-Otx2 is expressed in ventral cells of the blastema (arrows), primarily in regions where the ventral nerve cord of new segments is forming. (C,D) Early anterior regeneration: Pl-Otx2 is expressed intensely in lateral crescents on each side of the developing blastema (arrows). (E) Mid-stage anterior regeneration: deep cells of the developing foregut strongly express Pl-Otx2 (arrowhead), and a lateral cluster of unidentified cells near the anterior margin of the blastema transiently expresses Pl-Otx2 (small arrow). (F,G) Late anterior regeneration: foregut expression (F, arrowhead) persists, localizing specifically to the pharynx. In addition, Pl-Otx2 becomes expressed in scattered ventral cells of the blastema (F, arrow), mostly in the developing ventral nerve cord, and in a bilaterally symmetrical cluster of dorsolateral cells (G, arrow), possibly associated with an eyespot (see text). Vertical scale bar: 50 µm.

View larger version (66K): [in a new window]   Fig. 9. Pl-Otx1 and Pl-Otx2 expression during fission. (B) Pl-Otx1 expression; (A,C-F) Pl-Otx2 expression. (A,C,D,F) Lateral views; (B,E) dorsal views. See Fig. 6 for explanation of expression diagrams (left). (A,B) Early fission: Pl-Otx2 is expressed in the body wall in bilaterally symmetrical crescents (A,B, large arrows). Expression occurs just behind the plane of fission (evident in B by the constriction in the body wall), i.e. at the anterior limit of the new developing head. Worms in A,B possess multiple fission zones at different stages of development: the leftmost arrows in A,B indicate very recently initiated fission zones, in which expression is limited to a short strip of only a few cells, and the right-most arrow in A indicates the oldest fission zone in the panel, with more extensive staining. (C) Mid-stage fission: the lateral expression of Pl-Otx2 extends dorsally to form a dorsal horseshoe of expression, which then splits dorsally into a posterior strip (arrow) and an anterior strip (not visible in this plane of focus). The prostomium emerges dorsally between these two strips of expression. At this stage of fission, Pl-Otx2 is also expressed in the gut of the developing head (arrowhead). The staining in the left-most region of C is associated with a different, more anterior fission zone. (D-F) Late fission: the dorsally emerging prostomium (p) continues to be encircled by Pl-Otx2 expression (D,E, large arrows). Foregut expression (D, arrowhead) persists, becoming localized to the pharynx. In addition, Pl-Otx2 becomes expressed in scattered ventral/ventrolateral cells (including cells of the ventral nerve cord) in the developing anterior and posterior segments (D, small arrows), and in possible ‘eyespots’ (see text) in the developing head (F, arrow). Vertical scale bar: 50 µm.