Girstmair, Johannes; (2017) Building a light-sheet microscope to study the early development of the polyclad flatworm Maritigrella crozieri (Hyman, 1939). Doctoral thesis , UCL (University College London).

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Abstract

The Lophotrochozoa is an evolutionary interesting clade comprising many and mostly marine phyla. Despite their diverse morphology, these animals often have a biphasic life-cycle in form of a free-swimming, ciliated larval stage, the trochophore, and seemingly retained a highly conserved developmental pattern called spiral cleavage. This opens the door for comparative studies to better understand the shared mechanisms of the spiralian developmental program, its deviations and the evolution of lophotrochozoan body plans. Here I studied the early development and larva formation in the polyclad flatworm Maritigrella crozieri and compared it with other members of this evolutionarily diverse group of animals. In order to conduct this research, I first built a light-sheet microscope (OpenSPIM) that would allow me to follow the development of the polyclad embryo from the zygote into the larval stage and to acquire sophisticated 3d-reconstructions of fixed embryonic stages. I then used this and other techniques to characterise in detail the early development of M. crozieri and its conserved spiral cleavage pattern. Precise volume measurements of 3d-reconstructed early blastomeres and the investigations of associated cleavage patterns indicate that this polyclad worm may not follow a strictly equal spiral cleavage type, as was previously thought. I investigated the cleavage pattern and fate of micromere 4d, which in M. crozieri gives rise to mesoderm and generates bilaterally symmetric embryos at a cellular level. A first cell lineage analysis of this organism involved long-term live imaging recordings and point to a conserved fate of blastomeres that, like in other spiral cleavers, give rise to ectodermally derived structures, specifically the larva’s locomotion system (ciliary band) and a sensory organ at the apical point (apical organ). These findings strengthen the idea that these structures may be homologous to those found in other trochophore larvae. This work increased the current knowledge of the early development of the polyclad flatworm M. crozieri, which facilitates evolutionary comparisons of the development of different flatworms and lophotrochozoans more broadly and can contribute to addressing the homology of marine larvae.

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