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Abstract:
Molecular phylogenies often challenge traditional views that nevertheless seem to be based on plausible interpretations of morphological evolution. These include classical light microscope-based hypotheses of transformation from the muscular pharyngeal basal bulb of microbivores to the glandular basal bulb associated with plant parasitism (Tylenchomorpha). Congruent with this hypothesis is evolution from the open stoma of microbivores through several intermediates to a protrusible stylet of Tylenchomorpha. However, molecular-based phylogenetics, contradicting these hypotheses, instead point to a counterintuitive morphological transformation to plant parasitism evolving within microbivorous Cephalobomorpha. Whereas superficially this extreme divergence appears unlikely, TEM reconstruction in representatives of Rhabditomorpha, Cephalobomorpha, Tylenchomorpha, and outgroups demonstrates congruence of this transformation with evolution of the glandular basal bulb. We have also employed TEM including 3D computer reconstruction from serial micrographs to demonstrate new hypotheses of homology and a plausible hypothesis for stylet evolution that is congruent with molecular-based insight. The cuticle that forms the lining of the open channel stoma (Cephalobomorpha) and the stomatostylet (Tylenchomorpha) are both produced by a suite of distinctive hypodermal, arcade, and pharyngeal cells. Whereas these cells and their relative positions are conserved, details of their expression effect highly divergent phenotypic expression of feeding structures. In Tylenchomorpha, hypodermal cells (cheilostom) are positioned to form the lining, including the framework, vestibule, and vestibule extension, that guides the stylet. Arcade cells (gymnostom) are expressed between molts as reduced syncytia that form the cone and shaft of the stylet. The anteriormost pharyngeal radial muscle cells (stegostom) are homologs of stylet protractors and form stylet knobs. Examples we have studied underscore that more slowly evolving cellular patterns may underlie dramatic phenotypic change and, in fact, the more conserved underlying patterns may be most directly phylogentically informative. It is often molecular phylogenies that direct us to the most interesting and tractable questions of morphological evolution.
