Hexamethyldisilazane for scanning electron microscopy of Gastrotricha.

We evaluated treatment with hexamethyldisilazane (HMDS) as an alternative to critical-point drying (CPD) for preparing microscopic Gastrotricha for scanning electron microscopy (SEM). We prepared large marine (2 mm) and small freshwater (100 microm) gastrotrichs using HMDS as the primary dehydration solvent and compared the results to earlier investigations using CPD. The results of HMDS dehydration are similar to or better than CPD for resolution of two important taxonomic features: cuticular ornamentation and patterns of ciliation. The body wall of both sculpted (Lepidodermella) and smooth (Dolichodasys) gastrotrichs retained excellent morphology as did the delicate sensory and locomotory cilia. The only unfavorable result of HMDS dehydration was an occasional coagulation of gold residue when the solvent had not fully evaporated before sputter-coating. We consider HMDS an effective alternative for preparing of gastrotrichs for SEM because it saves time and expense compared to CPD.

Phylogeny of Gastrotricha: a morphology-based framework of gastrotrich relationships.

Currently, the phylum Gastrotricha is divided into the orders Macrodasyida and Chaetonotida, with the structure of the myoepithelial pharynx being an important distinguishing feature. Macrodasyida currently has six recognized families, and Chaetonotida comprises seven families. However, within-group relationships are poorly understood. To arrive at a better understanding of gastrotrich systematics and phylogeny, we performed the first cladistic analysis of nearly all known gastrotrich genera using 71 morphological characters. Results suggest that the Gastrotricha is a monophyletic group (supported by 82% of bootstrap replications) with its most primitive taxa distributed among the families Dactylopodolidae and Neodasyidae. Monophyly of Macrodasyida and Chaetonotida was supported by 90% and 52% bootstrap replications, respectively. Within the Macrodasyida, the families Dactylopodolidae, Turbanellidae, Macrodasyidae, and Thaumastodermatidae all formed monophyletic clades. The families Planodasyidae and Lepidodasyidae were paraphyletic. Among the Chaetonotida, the marine family Xenotrichulidae was monophyletic, supported by 51% of bootstrap replications. A second clade containing all freshwater families was supported by 62% bootstrap values. However, Chaetonotidae were paraphyletic. Using this analysis as a framework, we now can explore possible patterns of evolution within it, and arrive at a consensus of the gastrotrich ground pattern. Moreover, in future molecular studies of metazoan phylogeny, we will be able to select gastrotrich species that are more appropriate representatives of the phylum.

A reappraisal of the systematics of the monocelididae (Platyhelminthes, Proseriata): inferences from rDNA sequences.

The current classification system for the Monocelididae which is based on the character "presence or absence of an accessory prostatoid organ" divides the family into two subfamilies, namely the Minoninae and the Monocelidinae. However, other characters relating to the structure of the male copulatory bulb and to karyotypes do not support this division. Monocelidid male copulatory bulbs can be either of the simplex or the duplex-type, and if this character is mapped onto the current classification, then both subfamilies contain species with either type of copulatory bulb. We therefore decided to construct an independent phylogeny for the Monocelididae using nucleotide-sequence data of the gene coding for the 26/28S rDNA. Distance-and parsimony-based analyses resulted in phylogenetic trees that strongly supported a division of the Monocelididae based on characters of the male copulatory bulb and not on the accessory prostatoid organ. Thus, all species possessing a simplex-type copulatory bulb cluster together into one monophyletic group, the Monocelidinae (sensu Midelburg), whereas species characterized by a duplex-type copulatory bulb constitute a paraphyletic assemblage.