Size variation in chromosomes from independent cultured isolates of Plasmodium falciparum

The complexity of the life cycle of the protozoan malaria parasite Plasmodium falciparum has hindered genetic analysis¹; even the number of chromosomes in P. falciparum is uncertain². The blood stages of rodent malaria parasites are haploid^1,3 and hybridization with cloned complementary DNAs similarly suggests a haploid genome in P. falciparum blood stages (ref. 4 and our unpublished results). A novel approach to karyoptic and linkage analysis in P. falciparum has been provided recently by the technique of pulsed-field gradient (PFG) gel electrophoresis⁵, which allows the fractionation of DNA molecules of 30-3,000 kilobases (kb), a range including the sizes of intact chromosomal DNA molecules from eukaryotes such as yeast ⁵ and trypanosomatids⁶. We describe here the fractionation by PFG electrophoresis of chromosomal DNA molecules from P. falciparum into at least seven discrete species which vary in size by up to 20% between different isolates. Several genes for P. faciparum antigens which contain repetitive sequences are located on different chromosomes. Surprisingly, two of the chromosomes seem to contain the same sequences.