if there is no alleles of a gene, how can I get a mutant of this gene?I would start by reading the chapter on reverse genetics in Wormbook. It’s a Methods chapter, intended to be a source of protocols, but the introductory text for each section should explain some of the principles, and the references cited can enlighten you further.
In particular, you should note the following passage from the section on deletion mutants:
The time, manpower, space, and financial resources required to use this procedure are modest enough that this technology is suitable for even a small laboratory that needs to generate several knockout mutations in several genes. However, we do not recommend it for labs that only need to knockout a single or just a few genes. For such laboratories, requests can be made to two C. elegans gene knockout consortiums. Information about such knockout requests can be found online at: http://celeganskoconsortium.omrf.org/ or National BioResource Project (NBRP)::C.elegans
You should check whether either group might have an allele of your gene that’s not listed in Wormbase, and you should look into whether and how to request a deletion from them.
After that, you may want to venture further. The Wormbook chapter is not comprehensive nor completely up to date. In particular, it mentions nothing about the available options using transposons. You can mobilize endogenous transposons, or you can use a heterologous transposon derived from Mariner; the latter system has been used to generate a collection of strains with known transposon insertion sites. It also doesn’t mention the new technologies that use engineered nucleases to generate site-specific double-strand breaks; such breaks, whether generated by nuclease activity or by transposon excision, can be used for targeted gene replacement and other similar genome engineering.