Molecular and cellular differentiation during the early shell field development in Lymnaea stagnalis

by Jennifer Hohagen
Doctoral thesis
Date of Examination:2013-12-16
Date of issue:2014-02-13
Advisor:Prof. Dr. Daniel John Jackson
Referee:Prof. Dr. Daniel John Jackson
Referee:Prof. Dr. Joachim Reitner
Persistent Address: http://dx.doi.org/10.53846/goediss-4358

 

 

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Abstract

English

Since the evolutionary emergence of the molluscan shell in the Pre-Cambrian, diverse shell forms have evolved, which are adapted to various ecological environ-ments. The innovation of the shell facilitated the vast morphological and ecological diversification of the Mollusca, but its formation during the early embryogenesis is poorly understood. Many ontogenetic events associated with larval shell formation are evolutionarily conserved. Particularly, the initial specification of the responsible organ, the molluscan shell field, often coincides with a direct cell-cell contact between the prospective shell field cells and underlying endodermal cells. This observation raised the hypothesis of a contact-mediated induction event that specifies the molluscan shell field. Until now, this remains the canonical theory of molluscan shell field specification. In this thesis, I evaluate the role of an ancient intercellular signalling pathway − the Notch pathway − in the shell field specification of the derived gastropod Lymnaea stagnalis. The cellular arrangements and differentiation events during its early shell field developments are shown to be in agreement with a specification via contact-mediated induction. The spatial expres-sion of core components of the Notch pathway does not support a deployment of Notch signalling in this process. Rather, Notch signalling seems to function during the early neurogenesis. Also, the pharmacological inhibition of Notch signalling during the presumably specifying contact event does not alter the shell field appearance, but prevents the correct cellular differentiation of the archenteron. This thesis represents the first set of investigations into the molecular regulators of shell field specification. Knowledge on the molecular basis of shell field development in L. stagnalis represents a first step towards further studies on a range of molluscan representatives. Such comparative studies will allow for inferences about the ancestral structure of the molecular framework that underlies shell development, and thus the genetic conditions under which the molluscan shell first arose.
Keywords: shell; Mollusca; evolution; Notch; development; specification
 
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