Deciphering the onychophoran ‘segmentation gene cascade’: inferences about the evolution of segmentation

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• Ralf Janssen (Uppsala University)
• Wednesday 13 November 2013, 13:00-14:00
• Part II Lecture Theatre, Department of Zoology.

If you have a question about this talk, please contact Jeremy Solly.

The question of the origin of segmentation has troubled scientists’ minds for far more than a century, and the question still remains open. Three animal phyla are usually described as being unambiguously segmented. These are the vertebrates, to which we belong, the annelids and the arthropods. That means that we find ‘segmented’ animals in each of the main groups of animals, i.e. deuterostomes, lophotrochozoan protostomes, and ecdysozoan protostomes. At the same time, however, most other animal phyla are clearly non-segmented, or only show very superficial signs of ‘segmentation’.

One approach to shed light on the origin of segmentation was believed to lie in the investigation of developmental mechanisms. The idea was that if segmentation had evolved only once or twice, conserved mechanisms should be found that might indicate this. Conversely, if segmentation had evolved independently in the different groups, different molecular mechanisms would be expected to underlie their segment formation.

A first step in the investigation of such molecular mechanisms is typically the analysis of gene expression patterns, and this standard method allows a quick first glance at what role a certain gene may have during development. During the last few decades of gene expression pattern analysis, remarkable discoveries have been made, such as the involvement of Notch/Delta signaling in arthropod segmentation and the strong conservation of the segment-polarity gene network in arthropods and annelids. Such findings fuelled a vigorous discussion about the origin of segmentation and suggested a common origin to some. However, as we believe we know today, many of these patterns may be the result of convergent evolution, or in other words, may have evolved (and/or have been recruited) independently.

In order to understand the level of conservation of arthropod segmentation, I first analyzed segmentation genes (as known from Drosophila) in the myriapod Glomeris marginata, and later expanded this approach towards the onychophorans. The latter represent the likely sister-group to the arthropods, and are therefore of special interest for the understanding of arthropod segmentation. Data from onychophorans allow for the polarization of characters found in arthropods, and at the same time offer a deeper insight into the origin of conserved arthropod segmentation mechanisms.

In my talk I will present my current work on onychophoran segmentation genes and discuss my findings in terms of what they may mean (or not mean) for the origin of segmentation.

This talk is part of the Evolution and Development Seminar Series series.

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