Abstract
BackgroundSegmented body organizations are widely represented in the animal kingdom. Whether the last common bilaterian ancestor was already segmented is intensely debated. Annelids display broad morphological diversity but many species are among the most homonomous metameric animals. The front end (prostomium) and tail piece (pygidium) of annelids are classically described as non-segmental. However, the pygidium structure and development remain poorly studied.ResultsUsing different methods of microscopy, immunolabelling and a number of molecular markers, we describe the neural and mesodermal structures of the pygidium of Platynereis dumerilii. We establish that the pygidium possesses a complicated nervous system with a nerve ring and a pair of sensory ganglia, a complex intrinsic musculature, a large terminal circular blood sinus and an unusual unpaired torus-shaped coelomic cavity. We also describe some earlier steps of pygidial development and pygidial structure of mature animals after epitokous transformation.ConclusionsWe describe a much more complex organization of the pygidium of P. dumerilii than previously suggested. Many of the characteristics are strikingly similar to those found in the trunk segments, opening the debate on whether the pygidium and trunk segments derive from the same ancestral metameric unit. We analyze these scenarios in the context of two classical theories on the origin of segmentation: the cyclomeric/archicoelomate concept and the colonial theory. Both theories provide possible explanations for the partial or complete homology of trunk segments and pygidium.
Highlights
MethodsAnimals Platynereis specimens were obtained from laboratory cultures at Institute Jacques Monod, CNRS, France and SaintPetersburg University, Russia
Segmented body organizations are widely represented in the animal kingdom
We explore in depth the neural and mesodermal structures of the pygidium of P. dumerilii, with new descriptive data using electron microscopy and a number of molecular markers combined with confocal microscopy
Summary
Animals Platynereis specimens were obtained from laboratory cultures at Institute Jacques Monod, CNRS, France and SaintPetersburg University, Russia. Animals were dissected by razor blade and fixed in 4% paraformaldehyde in phosphate buffered saline (PBS) overnight at 4°C washed three times in PBS and stored in PBS, containing 0,1% NaN3 at 4°C. Semithin sections and electron microscopy For transmission electron microscopic studies animals were fixed 2 hours in 2,5% glutaraldehyde in 0.1 M cacodylate buffer, washed in cacodylate buffer (3 × 15 minutes) and postfixed in 0.1% OsO4 (2 h) in the same buffer. To normalize the osmotic pressure, corresponding amount of sucrose were added to all EM fixatives and washing buffers. Specimens were dehydrated in a graded series of ethanol and embedded in EMbed-812 (Electron Microscopy Sciences, 14120) or Araldite-EMbed 812 (Electron Microscopy Sciences, 13940)
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