EFFECT OF NEMATOCIDAL BACILLUS-THURINGIENSIS STRAINS ON FREE-LIVING NEMATODES .1. LIGHT-MICROSCOPIC OBSERVATIONS, SPECIES AND BIOLOGICAL STAGE SPECIFICITY AND IDENTIFICATION OF RESISTANT MUTANTS OF CAENORHABDITIS-ELEGANS

Light-microscopic observations of the toxic action of Bacillus thuring iensis spore/crystals reveals that, in Caenorhabditis elegans, the int estine is destroyed in two stages over a period of 24 h. The anterior ring of four cells is the first and foremost target. Observations indi cate that the intestine is the only tissue being destroyed. Screening of fourteen additional rhabditid nematode species against three nemati cidal B. thuringiensis strains active against C. elegans, resulted in only one additional sensitive nematode species, and indicates a high s pecies specificity of the nematicidal factor. However, in contrast to insect-specific B. thuringiensis toxins, the nematicidal toxin exhibit s low developmental stage specificity against C. elegans; all developm ental stages, including adult nematodes are sensitive. Moreover, sensi tivity increases as development proceeds. Using ethyl methyl sulfonate induced mutagenesis two mutants of C. elegans have been recovered, ex hibiting reduced sensitivity of up to 50 % against one of the nematici dal strains. Moreover, one of the mutants exhibited cross-resistance t o a second nematicidal B. thuringiensis strain against which it was no t screened. Preliminary data indicate that the reduced sensitivity in the mutants is not due to reduced pharyngeal pumping activity.