title: Strain evolution in Caenorhabditis elegans: transposable elements as markers of interstrain evolutionary history.
abstract: Evolutionary relationships across taxa can be deduced from sequence divergence of proteins, RNA, or DNA; sequences which diverge rapidly, such as those of mitochondrial genes, have been especially useful for comparisons of closely related species, and--within limits--of strains within a species. We have utilized the transposable element Tc1 as a polymorphic marker to evaluate the evolutionary relationships among nine Caenorhabditis elegans strains. For five low-Tc1-copy strains, we compared patterns of restriction fragments hybridizing to a cloned Tc1 probe. Twenty of the 40 Tc1 insertion sites thus characterized were common to all five strains, and so presumably preceded strain divergence; the 20 differential bands were used to construct a maximum-parsimony tree relating these strains. In four high-copy-number stocks (three wild-type strains and a subline), we determined occupancy of 35 individual Tc1 insertion sites by a polymerase chain reaction assay. Surprisingly, the high-copy strains share a common subset of these Tc1 insertions, and the chromosomal distribution of conserved Tc1 sites is "clustered" with respect to the other elements tested. These data imply a close evolutionary relationship among the high-copy strains, such that two of these strains appear to have been derived from the highest-copy-number lineage (represented by two stocks) through crossing with a low-Tc1 strain. Abundances of Tc1 elements were also estimated for the four high-copy-number stocks, at approximately 200-500 copies per haploid genome, by quantitative dot-blot hybridization relative to two low-copy strains. Annealing with 32P-labeled probes corresponding to full-length Tc1, an oligonucleotide within the Tc1 terminal inverted repeats, and an internal Tc1 oligonucleotide, gave essentially identical results--indicating that Tc1 termini exist in the genome primarily as components of full-length Tc1 elements. A composite evolutionary tree is proposed, based on the locations and numbers of Tc1 elements in these strains, which is consistent with a four-branch intraspecific tree deduced previously by maximum-parsimony analyses of mitochondrial sequence changes; it also serves to elucidate the evolutionary history of transposon mobility.
authors.affiliation: Department of Medicine, University of Arkansas for Medical Sciences, Little Rock, USA.
authors.forename: N K
authors.initials: NK
authors.lastname: Egilmez
authors.forename: R H
authors.initials: RH
authors.lastname: Ebert
authors.forename: R J
authors.initials: RJ
authors.lastname: Shmookler Reis
mesh_terms: D000818:Animals; D001483:Base Sequence; D017173:Caenorhabditis elegans / Q000235:genetics*; D004251:DNA Transposable Elements / Q000235:genetics*; D018509:DNA, Helminth / Q000235:genetics; D005819:Genetic Markers; D008969:Molecular Sequence Data; D010802:Phylogeny; D016133:Polymerase Chain Reaction
chemical_list: D004251:DNA Transposable Elements; D018509:DNA, Helminth; D005819:Genetic Markers