Ultrastructure of the protonephridial system in Neodasys chaetonotoideus (Gastrotricha: Chaetonotida) and in the ground pattern of Gastrotricha

Abstract

The taxon Neodasys has a basal position within Gastrotricha. This makes it very interesting for phylogenetic considerations in this group. To complete the reconstruction of the nephridial system in the stem species of Gastrotricha started earlier, we have studied the whole protonephridial system of Neodasys chaetonotoideus by means of complete sets of ultrathin sections and TEM. In many characters, protonephridia of N. chaetonotoideus resemble those of macrodasyidan gastrotrich species. For example, each of the six protonephridia, arranged in three pairs, consists of three distinct cells that constitute the continuous protonephridial lumen. Especially, the terminal cell of the protonephridia of N. chaetonotoideus shows a striking pattern: The perforation of the filter region is a meandering cleft that is continuous with the seam of the enfolded lumen of that cell. With the results presented here and that of former TEM studies, we give a comprehensive idea of the excretory organs in the ground pattern of Gastrotricha. Moreover, we can elaborate on the hypothesized protonephridial system in the stem species of Bilateria. We suggest that a meandering filtration cleft is a feature of the ground pattern of the Bilateria.

The nervous system of Neodasys chaetonotoideus (Gastrotricha: Neodasys) revealed by combining confocal laserscanning and transmission electron microscopy: evolutionary comparison of neuroanatomy within the Gastrotricha and basal Protostomia

“…Reconstructions of ground pattern features of Gastrotricha based on a comprehensive species-character matrix and using the criterion of parsimony have by now been carried out for the myoepithelial pharynx (Ruppert 1982), the body musculature (Hochberg and Litvaitis 2001;Hochberg 2005), the protonepridial system (Kieneke et al 2007) and the reproductive organs (Kieneke et al 2009). This by degrees improves our knowledge of the last common ancestor of the phylum.…”

Novel implications for the basal internal relationships of Gastrotricha revealed by an analysis of morphological characters

“…Character 131 is only applied if character 130 is coded with state 1 (caudal organ present). X‐organ — existence : (0) absent; (1) present — type Arenotus strixinoi . Protonephridia — existence : (0) absent; (1) present. The pure existence of one or several pairs of protonephridia is known for many species of Gastrotricha (see Kieneke et al . 2007, 2008a and references therein). External cilia covered with cuticle — existence : (0) absent — type Praeconvoluta minor ; (1) present — type Neodasys chaetonotoideus .…”

“…Protonephridia — existence : (0) absent; (1) present. The pure existence of one or several pairs of protonephridia is known for many species of Gastrotricha (see Kieneke et al . 2007, 2008a and references therein).…”

“…But such data, that are normally obtainable for a small number of species only, can be mapped onto tree topologies in order to infer the possible evolution of different organ systems and their ancestral states. Such approaches have been applied, for example, for the muscular system (Hochberg & Litvaitis 2001b) and for the protonephridial system (Kieneke et al 2007(Kieneke et al , 2008a. These phylogenetic assessments of certain organ systems will expand our idea of the gastrotrich ancestor and will help find the actual phylogenetic position of Gastrotricha.…”

“…We suggest that a more comprehensive understanding of the basal relationships within Bilateria depends upon a better knowledge of the character patterns of the stem species (ground pattern) of its monophyletic subtaxa. We have started this reconstruction for some organ systems (protonephridia and reproductive organs) of the Gastrotricha based on our own ultrastructural investigations in combination with former results and recent hypotheses on the internal phylogeny of Gastrotricha (Kieneke et al . 2007, 2008a,b).…”