Worm Breeder's Gazette 13(3): 70 (June 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.
We have been using C. elegans as a model for studying ivermectin (IVM) resistance in parasitic nematodes of livestock.
Previous work led to the identification of at least 25 loci mutable for ivermectin resistance at low levels of drug (< 25 ng/mL); comparable levels to those present in bodily fluids of IVM treated hosts.
During a mutagenesis screen, we observed that IVM sensitive worms were disorientated on NGM media containing IVM. We also observed that resistant worms sometimes wandered away from the bacterial lawn even in the absence of IVM. To investigate this further, Che strains (and some other strains) were obtained and tested for growth on 5 ng/mL IVM. The results are tabulated opposite (Table 1.). All strains exhibiting resistance to the drug had amphid neuron defects. These defects result in unusual or impaired staining of amphid neuron cell bodies with DiO. Next we attempted to stain resistant strains isolated in our labs and found that most of the low-level resistance strains were Dyf.
One of the most mutable loci for IVM resistance was found to be avr-1 . From its map position on LG1 ,we thought it was possible that avr-1 could be analogous to either che-3 or che-13 .Complementation testing has since revealed that avr-1 ( nr5 )fails to complement che-3 ( e1124 )(Hunt & Grant, 1993 Worm Meeting Abstract p207 ).Subsequent molecular work has reinforced that avr-1 is che-3 . The second most mutable locus for IVM resistance was found to be avr-5 and is allelic with osm-3 .
Because of the high frequency with which avr-1 / che-3 alleles are detected in a mutagenesis screen, we have a large collection of strains carrying mutations at this locus. On DiO staining of the whole collection, we found several strains which are nonDyf but IVM resistant. These results complement other findings that there is much between-allele variability in the Dyf phenotype of mutations at some dyf loci (e.g. osm-3 Shakir, Miwa & Siddiqui, 1993). Furthermore, heterozygotes of some dominant avr alleles are Avr nonDyf (homozygotes are AvrDyf; see Hunt and Grant, this issue) and strains of one DOM avr allele which carry a suppressor of the Avr phenotype remain Dyf i.e. are nonDyf. The relationship between Dyf and Avr is, therefore, unclear but IVM may be an additional tool in the analysis of amphid neuron structure and function.
In order to further investigate the mechanism of resistance to IVM in the low-level resistance alleles discussed above, we intend to address the following questions:
1) What is the functional link (if any) between defects in amphid neuron dye filling and IVM resistance?
2) What are the functional/structural differences between Dyf and nonDyf IVM resistant worms?