MOLECULAR VARIATION IN TRYPANOSOMES

Several species of the genus Trypanosoma cause parasitic diseases of c onsiderable medical and veterinary importance throughout Africa, Asia and the Americas. These parasites exhibit considerable intra-species g enetic diversity and variation, which has complicated their taxonomic classification. This diversity and variation can be defined at the lev el of both the genome and of individual genes. The nuclear genome show s considerable inter- and intra-species plasticity in terms of chromos ome number and size (molecular karyotype). The mitochondrial (kDNA) ge nome also varies considerably between species, especially in terms of minicircle size and organization. There is also considerable intra-spe cific sequence diversity in minicircles and within the Variable Region of the maxicircle. Restriction enzyme analysis of this diversity has lead to the concept of 'schizodemes'. At the gene level, isoenzyme ana lysis has proven very useful for strain and isolate identification, wi th the classification into numerous 'zymodemes'. Considerable antigeni c diversity has also been identified in T. cruzi and T. brucei, with t he development of 'serodemes' in the latter. In addition to this inter -strain diversity, African trypanosomes (T. brucei, T. congolense, and T. vivax) exhibit the phenomenon of antigenic variation, where indivi dual parasites are able to express any one of hundreds of different co pies of the Variant Surface Glycoprotein gene at any particular time. The molecular mechanisms underlying antigenic variation are now unders tood in considerable detail. The implication of this molecular diversi ty and variation are discussed in terms of trypanosome taxonomy and di sease control.