Mechanisms controlling the asymmetric second clevage in the Helobdella embryo
Deirdre Colleen Lyons, University of California, Berkeley (Degree granting institution)
Spiral cleavage is a stereotyped cell division program seen in the embryos of animals as diverse as annelids, molluscs and flatworms. Comparing spiral cleavage in extant species provides insight into how changes in lineage-driven development have influenced body plan evolution. In this respect, conservation and modification of the spiral cleavage program are of equal interest. Among the monophyletic clitellate annelids many steps of development are undoubtedly homologous. In the leech Helobdella and the oligochaete Tubifex the unequal first and second divisions are critical for establishing bilateral symmetry by segregating developmental determinants (teloplasm) exclusively to the D cell at the 4-cell stage. I investigated the mechanisms underlying asymmetric second cleavage of the teloplasm-containing CD cell in Helobdella. At metaphase a symmetric bi-astral spindle attaches via both asters to the blastocoel wall cortex. The spindle then shifts towards the right side of the cell inducing an eccentrically located cytokinetic furrow. These data reveal D-quadrant specification divergence between Helobdella and Tubifex
Thesis, Dissertation, English, 2008
Academic Dissertation
1 online resource (2, ii, 126 leaves : illustrations)
892838280
Electronic reproduction, [Place of publication not identified], UMI
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