UNIDIRECTIONAL DOMINANCE OF CYTOPLASMIC INHERITANCE IN 2 GENETIC CROSSES OF PLASMODIUM-FALCIPARUM

Malarial parasites have two highly conserved cytoplasmic DNA molecules : a 6-kb tandemly arrayed DNA that has characteristics of a mitochondr ial genome, and a 35-kb circular DNA that encodes functions commonly f ound in chloroplasts. We examined the inheritance pattern of these ele ments in two genetic crosses of Plasmodium falciparum clones. Parent-s pecific oligonucleotide probes and single-strand conformation polymorp hism analysis identified single nucleotide changes that distinguished the parental 6- and 35-kb DNA molecules in the progeny. In all 16 inde pendent recombinant progeny of a cross between a Central American clon e, HB3, and a Southeast Asian clone, Dd2, the 6- and 35-kb DNAs were i nherited from the Dd2 parent. In all nine independent recombinant prog eny of a cross between clone HB3 and a likely African clone, 3D7, the 6-kb DNA was inherited from the 3D7 parent. Inheritance of cytoplasmic genomes of the Dd2 and 3D7 parents was, therefore, dominant over that of the HB3 parent. Cytoplasmic DNA molecules were found almost exclus ively in the female gametes of malarial parasites; hence, clone HB3 di d not appear to have served as a maternal parent for the progeny of tw o crosses. Defective differentiation into male gametes by clone Dd2 is likely to be a reason for the cytoplasmic inheritance pattern seen in the HB3 x Dd2 cross. However, incompetence of male or female gametes is unlikely to explain the uniparental dominance in recombinant progen y of the HB3 x 3D7 cross, since both parents readily self-fertilized a nd completed the malaria life cycle on their own. Instead, the data su ggest unidirectional parental incompatibility in cross-fertilization o f these malarial parasites, where a usually cosexual parental clone ca n participate only as a male or as a female. Such an incompatibility m ay be speculated as indicating an early phase of reproductive isolatio n of P. falciparum clones from different geographical regions.