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The evolutionary pathway that has led to male tails of diverse morphology
among species of the nematode family Rhabditidae was reconstructed. This
family includes the well-studied model species Caenorhabditis elegans. By
relating the steps of male tail morphological evolution to the phenotypic
changes brought about by developmental mutations induced experimentally in
C. elegans, the goal is to identify genes responsible for morphological
evolution. The varying morphological characters of the male tails of
several rhabditid species have been described previously (Fitch and
Emmons, 1995, Dev. Biol. 170:564--582). The developmental events preceding
differentiation of the adult structures have also been analyzed; in many
cases the origins of varying adult morphological characters were traced to
differences during ontogeny. In the present work, the evolutionary changes
producing these differences were reconstructed in the context of the four
possible phylogenies supported independently by sequences of 18S ribosomal
RNA genes (rDNA). Two or more alternative states were defined for 36
developmental and adult morphological characters. These characters alone
do not provide sufficient data to resolve most species relationships;
however, when combined with the rDNA characters, they provide stronger
support for one of the four rDNA phylogenies. Assuming a model of ordered
transformations for multistate developmental characters generally results
in greater resolution. Transformations between character states can be
assigned unequivocally by parsimony to unambiguous branches for most of
the characters. Correlations are thereby revealed for some of the
developmental characters, indicating a probability of a shared
developmental or genetic regulatory pathway. Four of the unequivocal
character state changes on unambiguously supported branches closely
resemble the phenotypic changes brought about by known mutations in C.
elegans. These mutations define genes that are known to act in genetic
regulatory hierarchies controlling pattern formation, differentiation, and
morphogenesis. Although these studies are still at an early stage, these
results strongly suggest that parallel studies of developmental mutants in
C. elegans and of morphological and developmental evolution among related
nematodes will help define genetic changes underlying the evolution of
form.
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