Worm Breeder's Gazette 13(3): 72 (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.
Dominant ivermectin (IVM) resistance genes in C. elegans are very rare. After screening 1.2 million F1 genomes in an ems mutagenesis screen we identified no dominant mutations for ivermectin resistance at 5 ng/mL (in NGM agar); Wt worms being sensitive to 2.5 ng/mL but not 1.25 ng/mL. A much larger screen eventually provided us with 4 strains carrying dominant mutations (alleles nr272 , nr2344 , nr2389 & nr2477 ):heterozygotes and homozygotes are resistant to 10 ng/mL IVM. All four alleles are X-linked; nr2389 maps between dpy-7 and unc-6 ,whereas the other three map to the right of unc-18 .Although all four mutations have a Dyf phenotype as homozygotes, (Hunt & Grant, 1993 Worm Meeting Abstract p207 )this phenotype is dominant in nr2344 only (see table 1.). The Dyf phenotype is incompletely dominant in nr272 and nr2477 ,and dominance is increased by growth of heterozygotes on IVM. The Dyf phenotype is completely recessive in nr2389 and is not influenced by growth conditions.
Another mutagenesis screen has been conducted to identify suppressors of the IVM resistance conferred by nr2477 .Sixteen strains have been isolated by mutagenizing nr2477 and looking for reduced resistance in the F2 offspring. The ivermectin resistance status of the resultant strains which are nr24771s up-?doubles varies from slightly resistant (growth at 3ng/ml but not at 5) through Wt (growth at 1.25 ng/mL but not at 3) to super sensitive (no growth on agar containing 1.25 ng/mL IVM). All of the nr2477 /sup-?strains are Dyf, and many are Dpy. All of the sup alleles are recessive, as the sup/+, nr2477 /+animals are resistant to IVM at 5ng/mL.
In summary therefore we have four dominant resistance alleles at (at least) two loci on linkage group X. All four are Dyf as homozygotes but some are nonDyf (or are incompletely Dyf) as heterozygotes. Suppressors of varying strength have been generated for the nr2477 allele and although all of these affect IVM resistance, none affect dye-filling (i.e. sup/sup, nr2477 / nr2477 animals are Dyf). In addition some suppressors are Dpy.
Some of the questions we are pursuing further are:
1) Many Dyf mutations at previously defined loci impart IVM resistance (see article - this issue, and Worm meeting abstract, 1993). Newly generated mutations at these same loci, isolated in screens for resistance to IVM only, are sometimes nonDyf (see article - this issue). Similarly; resistant heterozygotes of some dominant resistance alleles are nonDyf; and conversely; non-IVM resistant, Dyf worms can be made using suppressors. What is the link between these phenotypes?
2) What is the mechanism behind suppression of IVM resistance by the Sup alleles we have generated, especially the Dpy sups.
3) These mutations are Daf-d and Che as well. What are the phenotypes of suppressed strains and heterozygotes with respect to these phenotypes and is there an effect of Daf-c mutations and other Daf-influencing genes (e.g. unc-104 , unc-3 , unc-31 ::see Vowels & Thomas WBG 12(2); Katsura et. al. WBG 13(2)) on IVM resistance??