Mapping variation in chromosome homologues of different Giardia strains

A landmark physical map of the 2-Mb chromosome of the Giardia duodenalis cloned line WB-1B, constructed using randomly cloned, chromosome specific markers, was used to compare the organisation and map order of the equivalent chromosome in other strains. A representative marker from each of the 13 NotI segments of the 2-Mb chromosome was hybridized to NotI cleavages of whole chromosomes of the other strains. Two strains, one isolated from a human, and one from a cat, had the same chromosome hybridization patterns as WB-1B. A strain isolated from a sheep, had one NotI chromosome 5 segment larger than WB-1B. Two additional strains isolated from a calf and a human had significantly different NotI cleavage patterns from the previous strains and shared no similar-sized chromosome NotI segment from their 2Mb chromosome homologues and only one in common with WB-1B. In one strain, two markers from the same WB-1B NotI segment did not hybridize suggesting deletion events have occurred. The order of some NotI segments within the 2Mb chromosome homologue was maintained, as determined from partial NotI chromosome cleavages, while in the most divergent of strains internal chromosome rearrangements and deletions were evident. All but one of the 2Mb WB-1B chromosome markers examined hybridized to a single chromosome band in all strains. Thus, while Giardia chromosomes vary in size, copy number and organisation, some linkage of markers is apparently maintained in isolates from disparate hosts and localities. We have therefore generated a genetic analysis system for Giardia with landmark maps using representative markers to replace the paucity of classical genetic markers and mutants. This approach is being extended to the complete genome.