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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.