Polymorphic DNA markers in the genome of parasitic nematodes

Polymorphic molecular markers are being identified to characterize the geno mes of parasitic nematodes. The aim is to construct a map with markers even ly spread over the six chromosomes. With such a map, regions can be identif ied that are under selection pressure when attempts are being made to eradi cate worms, be it by drugs, vaccines or genetic resistance in the sheep. Se veral types of markers have been developed, microsatellites, transposon-ass ociated markers, amplified fragment length polymorphism (AFLP) and expresse d sequence tag (EST) markers. Linkage groups can be constructed using sever al genetic crosses between inbred and drug resistant strains. EST markers w ill be especially important for comparative mapping with the genome of Caen orhabditis elegans, and therefore localization of the linkage group on a ch romosome. It will then be possible to identify functional genes close to ma rkers that have changed allele frequencies under selection pressure and ide ntify the mechanisms of resistance to parasite control.