Examining questions of homology in nematodes using three-dimensional reconstructions of morphology based on ultrastructural data.

摘要

Nematodes are perhaps the most biologically diverse group of metazoa, yet we know relatively little about their comparative anatomy. As the use of nematodes as model organisms increases, it becomes more important to address fundamental questions of homology in the group. Advanced electron microscopy techniques, paired with well-resolved phylogenies, facilitate more sophisticated studies of anatomy. We address questions of homology in two nematode organ systems the nose epidermal syncytia and the array of sensory structures surrounding the mouth opening. The epidermal cells are relevant to understanding the evolutionary origins of the stomatostylet in plant parasitic nematodes, and the sensory anatomy is compared to the model organism Caenorhabditis elegans.Several improvements in methodology were applied. High pressure freezing and freeze substitution were used in order to substantially improve specimen fixation, and specialized chambers were developed in order to reduce specimen loss during processing. Images were acquired digitally, eliminating time-consuming dark room work. Advanced software for image alignment, volume segmentation and visualization were used for representation of the data.For both the epidermis and the sensory systems there are identifiable homology between A. complexus and C. elegans for nearly all cells examined. The high level of conservation for cell identity suggests that it may be possible to understand homology in the highly derived nose of the plant parasitic Tylenchida, which have are more closely related to A. complexus than to C. elegans.Though cell identity is highly conserved, there are striking differences in the details of anatomy between C. elegans and A. complexus. One notable observation is the previously undocumented high degree of subcellular specialization in many of the epidermal cells of A. complexus. For example, several types of processes with unknown function branch from the somal extensions of the posterior arcade cell. Also interesting are the highly elaborate dendritic termini of the internal sensory receptors of A. complexus that imply substantial shifts in function. Other details of sensory anatomy differ in interesting ways between the two taxa and may prove to be important for both developmental and taxonomic questions.