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First published online 26 November 2003
doi: 10.1242/dev.00919

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A distinct patterning mechanism of O and P cell fates in the development of the rostral segments of the leech Helobdella robusta: implications for the evolutionary dissociation of developmental pathway and morphological outcome

Graduate Program in Zoology and Section of Molecular Cell and Developmental Biology, Institute of Cellular and Molecular Biology, University of Texas at Austin, Austin, Texas 78712, USA

View larger version (37K): [in a new window]   Fig. 1. Early development of the OP lineage. (A) The OP proteloblast undergoes four asymmetric cell divisions to form the four primary op blast cells. The op4 blast cell is labeled red. The OP proteloblast undergoes a symmetric cell division to form the two O/P teloblasts. Each O/P teloblast then resumes asymmetric cell divisions to form the o and p bandlets. Blue arrows indicate the axis of cell division. (B) Divisions of the op4 blast cell. (C) Fluoromicrograph showing the four tertiary blast cells of the op4 clone (clonal age is 44 hours). The OP proteloblast was injected with rhodamine dextran immediately before the birth of the op4 blast cell. The specimen was counterstained with Hoechst 33258, and is shown with anterior toward the top. The boundary between the op4 clone and the o and p bandlets is marked by a yellow line. Note that the posterior edge of the op4.pp cell retains a unique chevron shape which borders the o bandlet to the future ventral side (left) and the p bandlet to the future dorsal side (right). Scale bar: 10 µm.

View larger version (94K): [in a new window]   Fig. 2. The OP lineage of rostral segments is serially homologous to the sum of the O and P lineages in more posterior segments. This embryo was fixed and dissected at stage 9, by which time the primary blast cells have given rise to their segmental clones of differentiated descendants. The op4 clone is labeled with rhodamine, and the more posterior O and P lineages are labeled with rhodamine and fluorescein (see Materials and methods). The preparation shown here is oriented with anterior to the top, ventral midline to the left and dorsal to the right. The segmental boundaries are marked on the left side of each panel. (A) The morphological pattern of differentiated descendants in the op4 blast cell clone (red) is overtly similar to one segmental repeat of the O and P lineages combined (yellow). (B) The op4-derived pattern elements in the ventral nerve cord are homologous to the segmental pattern elements derived from the O and P lineages. The ventral nerve cord is visualized with Hoechst 33258 counterstaining (blue). Scale bar: 20 µm.

View larger version (27K): [in a new window]   Fig. 4. Schemetic representation of the serial homology in cell identity between OP sublineages and O and P sublineages. (A) Comparison of an op blast cell clone (top) and the paired o and p blast cell clones (bottom) during embryonic stage 7. At the stage shown, the op blast cell has undergone two rounds of division (see Fig. 1B), and the o and p blast cells have undergone their second cell division in only the anterior half of the clone. Each homologous pair of cells gives rise to the same set of pattern elements, and their nuclei are labeled here with the same color. The positional relationship between the OP, O and P sublineages is indicated with anterior to the top, ventral to the left and dorsal to the right. (B) Descendant pattern elements arising from one op blast cell, or a consegmental pair of o and p blast cells, in the stage 9 embryo. The clonal origin of the pattern elements is represented by the same color scheme used in Fig. 4A. See Fig. 3 for labeling of pattern elements.

View larger version (86K): [in a new window]   Fig. 3. Serial homology in cell identity between specific OP sublineages and O and P sublineages is shown by injection of rhodamine dextran lineage tracer. The labeled embryos were fixed and dissected at stage 9. The clonal origin of the labeled cells is shown at the lower right corner of each panel. The pattern elements derived from the op.aa cell (A) in the rostral segments are homologous to the pattern elements derived from the o.aa cell (B) and the p.aa cell (C) in more posterior segments. The pattern elements derived from the op.ap cell (D) in the rostral segments are homologous to the pattern elements derived from the p.ap cell (E) in more posterior segments. The pattern elements derived from the op.pa cell (F) in the rostral segments are homologous to the pattern elements derived from the o.ap cell (G) in more posterior segments. The pattern elements derived from the op.pp cell (H) in the rostral segments are homologous to the pattern elements derived from the p.p cell (I) in more posterior segments. See text for detail. CR, crescent neuron cluster; PV, posterior ventral neuron cluster; AD, anterior dorsal neuron cluster; WE, wedge-shaped neuron cluster; mpg, medial packet glia; c.f.3, cell floret 3; t., tubular structure homologous to the nephridial tubule (n.t.) in midbody segments (see text for detail); epi., epidermis. Orientation of the specimen is the same as in Fig. 2. Scale bar: 20 µm.

View larger version (51K): [in a new window]   Fig. 5. `Isolated' O-type cells show no sign of developmental regulation. (A) The `isolated' op4.aa cell consistently gives rise to its normal fate (Fig. 3A). (B) The `isolated' op4.pa cell consistently gives rise to its normal fate (Fig. 3F). Pattern element labels are given in Fig. 3. Scale bar: 20 µm.

View larger version (96K): [in a new window]   Fig. 6. Developmental regulation in `isolated' P-type cells. (A) An example of an `isolated' op4.ap cell clone. (B) An example of an `isolated' op4.pp blast cell clone. In both cases, the `isolated' P-type cell gives rise to pattern elements that are normally derived from the ablated sublineages (marked by parentheses) and unidentifiable cell clusters (marked by asterisks). Note that `isolated' P-type cells give rise to a variable combination of pattern elements (see Table 1), and differing combinations were seen in different embryos. Scale bar: 20 µm.

View larger version (66K): [in a new window]   Fig. 7. A correlation between bandlet slippage and developmental regulation in the op.pp lineage following ablation of the op.pa cell. The OP lineage was labeled with rhodamine lineage tracer and the op4.pa cell ablated during stage 7. The operated embryos were fixed at stage 9 and counterstained with Hoechst 33258. (A,B) In those embryos in which bandlet slippage did not take place, the PV and AD clusters are usually missing from the op4 clone. The normal sites of the missing op4 pattern elements are marked by the white arrowhead and asterisk. (C,D) In embryos that experience slippage, a gap forms between the portion of the blast cell clone immediately anterior to the ablated cell and the portion immediately posterior to the ablated op4.pa cell. Within the gap, no fluorescently labeled cells are observed, and the ganglia in the gap show a reduced size. A PV cluster (yellow arrowhead) and an AD cluster (yellow asterisk) are seen immediately posterior to the gap, and are therefore likely to be derived by regulation of the op4.pp cell. Orientation of the specimen is the same as in Fig. 2. Yellow, op4 descendants; light blue, op3 descendants; green, the descendants of the anteriormost o and p blast cells. Arrowhead indicates PV cluster; asterisk indicates AD clusters. See Fig. 3 for labeling of additional pattern elements. Scale bar: 20 µm.