Microanatomy of the trophosome region of <Emphasis Type="Italic">Paracatenula</Emphasis> cf. <Emphasis Type="Italic">polyhymnia</Emphasis> (Catenulida,Platyhelminthes) and its intracellular symbionts |
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Authors: | Nikolaus Leisch Ulrich Dirks Harald R Gruber-Vodicka Markus Schmid Wolfgang Sterrer Jörg A Ott |
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Institution: | 1.Department of Marine Biology,University of Vienna,Vienna,Austria;2.Department of Microbial Ecology,University of Vienna,Vienna,Austria;3.Bermuda Aquarium, Natural History Museum and Zoo (BAMZ),Flatts Village,Bermuda |
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Abstract: | Marine catenulid platyhelminths of the genus Paracatenula lack mouth, pharynx and gut. They live in a symbiosis with intracellular bacteria which are restricted to the body region
posterior to the brain. The symbiont-housing cells (bacteriocytes) collectively form the trophosome tissue, which functionally
replaces the digestive tract. It constitutes the largest part of the body and is the most important synapomorphy of this group.
While some other features of the Paracatenula anatomy have already been analyzed, an in-depth analysis of the trophosome region was missing. Here, we identify and characterize
the composition of the trophosome and its surrounding tissue by analyzing series of ultra-thin cross-sections of the species
Paracatenula cf. polyhymnia. For the first time, a protonephridium is detected in a Paracatenula species, but it is morphologically reduced and most likely not functional. Cells containing needle-like inclusions in the
reference species Paracatenula polyhymnia Sterrer and Rieger, 1974 were thought to be sperm, and the inclusions interpreted as the sperm nucleus. Our analysis of similar cells and their inclusions
by EDX and Raman microspectroscopy documents an inorganic spicule consisting of a unique magnesium–phosphate compound. Furthermore,
we identify the neoblast stem cells located underneath the epidermis. Except for the modifications due to the symbiotic lifestyle
and the enigmatic spicule cells, the organization of Paracatenula cf. polyhymnia conforms to that of the Catenulida in all studied aspects. Therefore, this species represents an excellent model system for
further studies of host adaptation to an obligate symbiotic lifestyle. |
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