Segmentation in Leeches

David A. Weisblat and Christopher ). Winchell

CONTENTS

• 7.1 Introduction..................................................................................................153
• 7.2 Segmentation in the Helobdella Body Plan..................................................155
• 7.3 Boundary-Driven versus Lineage-Driven Segmentation..............................155
• 7.4 An Axial Posterior Growth Zone (PGZ) Originates from the

D Quadrant in Helobdella and Other Clitellate Annelids............................158

7.5 The Leech PGZ Provides a Highly Simplified and Experimentally

Accessible Example of Axial Growth and Patterning..................................159

• 7.6 Evolutionary Antiquity of the M Teloblasts.................................................161
• 7.7 Kinship Groups Are Well Conserved within Each Lineage, But Heterogeneous in Terms of Cell-Type Composition and

Spatial Distribution.......................................................................................162

• 7.8 Kinship Groups Are Not Clones...................................................................162
• 7.9 The О-P Equivalence Group........................................................................164
• 7.10 Grandparental Stem Cell Lineages...............................................................165
• 7.11 Expression Patterns of Drosophila Segmentation Gene Homologs.............167
• 7.12 Spatiotemporal Registration of Blast Cell Clones........................................168
• 7.13 Segment Identities and the Paradoxical Specification of Regional

Differences along the Anterior-Posterior Axis............................................170

• 7.14 The “Leech Perspective” on the Counting Problem in Segmentation..........172
• 7.15 Genesis of Endoderm from a Syncytial Yolk Cell.......................................173
• 7.16 Mesoderm As a Primary Driver of Segmental Patterning in Leeches.........174
• 7.17 Summary and Conclusions...........................................................................175

References..............................................................................................................176