STRAIN EVOLUTION IN CAENORHABDITIS-ELEGANS - TRANSPOSABLE ELEMENTS ASMARKERS OF INTERSTRAIN EVOLUTIONARY HISTORY

Evolutionary relationships across taxa can be deduced from sequence di vergence of proteins, RNA, or DNA; sequences which diverge rapidly, su ch as those of mitochondrial genes, have been especially useful for co mparisons of closely related species, and-within limits-of strains wit hin a species. We have utilized the transposable element Tc1 as a poly morphic marker to evaluate the evolutionary relationships among nine C aenorhabditis elegans strains. For five low-Tc1-copy strains, we compa red patterns of restriction fragments hybridizing to a cloned Tc1 prob e. Twenty of the 40 Tc1 insertion sites thus characterized were common to all five strains, and so presumably preceded strain divergence; th e 20 differential bands were used to construct a maximum-parsimony tre e 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 ''clustere d'' with respect to the other elements tested. These data imply a clos e evolutionary relationship among the high-copy strains, such that two of these strains appear to have been derived from the highest-copy-nu mber lineage (represented by two stocks) through crossing with a low-T c1 strain. Abundances of Tc1 elements were also estimated for the four high-copy-number stocks, at similar to 200-500 copies per haploid gen ome, by quantitative dot-blot hybridization relative to two low-copy s trains. Annealing with P-32-labeled probes corresponding to full-lengt h Tc1, an oligonucleotide within the Tc1 terminal inverted repeats, an d an internal Tc1 oligonucleotide, gave essentially identical results- indicating that Tc1 termini exist in the genome primarily as component s of full-length Tc1 elements. A composite evolutionary tree is propos ed, based on the locations and numbers of Tc1 elements in these strain s, which is consistent with a four-branch intraspecific tree deduced p reviously by maximum-parsimony analyses of mitochondrial sequence chan ges; it also serves to elucidate the evolutionary history of transposo n mobility.