D quadrant specification in the leech Helobdella: Actomyosin contractility controls the unequal cleavage of the CD blastomere

The unequal division of the CD blastomere at second cleavage is critical in establishing the second embryonic axis in the leech Helobdella, as in other unequally cleaving spiralians. When CD divides, the larger D and smaller C blastomeres arise invariantly on the left and right sides of the embryo,...

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Bibliographic Details
Published in:Developmental biology 2009-10, Vol.334 (1), p.46-58
Main Authors: Lyons, Deirdre C., Weisblat, David A.
Format: Article
Language:English
Subjects:
Actomyosin
Actomyosin - physiology
Animals
Annelida
Blastomeres - metabolism
Chirality
Cleavage Stage, Ovum - physiology
D quadrant specification
Embryo, Nonmammalian - metabolism
Embryonic Development
Helobdella
Hirudinea
Leeches - embryology
Lophotrochozoa
Spindle orientation
Spiral cleavage
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Summary:The unequal division of the CD blastomere at second cleavage is critical in establishing the second embryonic axis in the leech Helobdella, as in other unequally cleaving spiralians. When CD divides, the larger D and smaller C blastomeres arise invariantly on the left and right sides of the embryo, respectively. Here we show that stereotyped cellular dynamics, including the formation of an intercellular blastocoel, culminate in a morphological left–right asymmetry in the 2-cell embryo, which precedes cytokinesis and predicts the chirality of the second cleavage. In contrast to the unequal first cleavage, the unequal second cleavage does not result from down-regulation of one centrosome, nor from an asymmetry within the spindle itself. Instead, the unequal cleavage of the CD cell entails a symmetric mitotic apparatus moving and anisotropically growing rightward in an actomyosin-dependent process. Our data reveal that mechanisms controlling the establishment of the D quadrant differ fundamentally even among the monophyletic clitellate annelids. Thus, while the homologous spiral cleavage pattern is highly conserved in this clade, it has diverged significantly at the level of cell biological mechanisms. This combination of operational conservation and mechanistic divergence begins to explain how the spiral cleavage program has remained so refractory to change while, paradoxically, accommodating numerous modifications throughout evolution.
ISSN:0012-1606
1095-564X
DOI:10.1016/j.ydbio.2009.07.007