GENOMIC DRIFT OF TOXOPLASMA-GONDII

We review herein studies concerning the genomic polymorphism of Toxopl asma gondii including 3 clones (1 linked with mouse pathogenicity), 5 zymodemes, and 13 schizodemes. Because mutations occur with some frequ ency and several allelic configurations are present in isolates grown in the same environment, we conclude that many of the mutations may no t be affected by selection pressure. However, the gametocyte-forming a bility is under selection pressure from the host and depends on the de velopment of bradyzoites in tissue cysts. After prolonged multiplicati on exclusively in the tachyzoite stage in mice and, possibly, in patie nts the gametocyte-forming ability may be lost. To avoid this genomic change, isolates should be passed in the laboratory, permitting bradyz oite and tissue-cyst formation. Mouse pathogenicity is selected for du ring mouse passage. We find no major genomic instability justifying sp ecies or subspecies distinctions.