Worm Breeder's Gazette 13(4): 33 (October 1, 1994)

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

18S Ribosomal RNA Gene Phylogeny For Some Rhabditidae.

David H.A. Fitch1, Bozena Bugaj-Gaweda2, Scott W. Emmons2

1 Department of Biology, New York University
2 Department of Molecular Genetics, Albert Einstein College of Medicine

Figure 1

  We have investigated the molecular evolution of the nucleotide sequences of 18S
ribosomal RNA genes (18S rDNA) from part of our collection of nematodes in the family
Rhabditidae. Our aim was to evaluate the usefulness of this gene for molecular
systematics of this family, as well as to establish phylogenetic relationships within a
group that has potential for comparative studies of the relationship between
development and evolution. We determined the 18S rDNA sequences of 9 species of
nematodes representing 6 genera within this family: Caenorhabditis briggsae (Cbr),
Caenorhabditis vulgaris (Cvu), Caenorhabditis remanei (Cre), Rhabditis blumi (Rbl),
Rhabditis sp. br (Rbr), Rhabdilella axei (Rax), Pellioditis typica (Pty), Teratorhabditis
palmarum (Tpa) and Pelodera strongyloides dermatitica (Pst). Using hypothetical
models for secondary structure as well as nucleotide similarity, these sequences were
aligned with the 18S rDNA sequence published for C. elegans (Ellis et al. 1986, Nucl.
Acids Res. 14:2345-2364) and with the partial sequences published for 8 ascaridoid
species (Nadler 1992, Mol. Biol. Evol. 9:932-944).
  We found that 18S rDNA is likely to be a useful tool to resolve relationships at the
intrafamilial level. However, 18S rDNA sequences cannot be used (by themselves) to
resolve relationships between taxa as closely related as the Caenorhabditis species.
Parsimony, minimum evolution and maximum likelihood methods strongly reject one
proposed phylogenetic classification based on adult morphological characters
(Andrassy 1983, tree V below), but support that of Sudhaus (1976, tree IV) as one
alternative of a few possible phylogenies (trees I-IV) that were significantly better than
all other topologies tested. When variation in substitution rate over different sites is
modeled by a gamma distribution, there is stronger support for monophyly of
Rhabditis than when rate differences are ignored. When transversions are weighted
as phylogenetically more informative than transitions, or only conserved positions are
considered (2/3 of the total data), support jumps markedly for monophyly of the
"Eurhabditis" clade ( Rhabditis species, Rhabditella and Pellioditis ). Based on these
trends in the data, we predict that 18S data from additional taxa will support a tree (i.e.,
tree IV below) very similar to that proposed by Sudhaus (1976).
  Distances between 18S rDNA sequences of different Caenorhabditis species
(0.8-1.8%) are comparable to distances between 18S rDNA of different orders in class
Mammalia. Distances between genera in Rhabditidae are about 8 times as great as
distances between tetrapod classes, suggesting rapid rates of substitution, ancient
divergence, or both. (see figure)
 Andrassy, 1983. A taxonomic review of the suborder Rhabditina (Nematoda:Secernentia).
 Sudhaus, 1976. Vergleichende Untersuchungen zur Phylogenie, Systematik... Zoologica 43(125):

Figure 1