Plasmodium berghei: gametocyte production, DNA content, and chromosome-size polymorphisms during asexual multiplication in vivo.

In this study the DNA content and the karyotype of clones of Plasmodium berghei, which differed in the capability to produce gametocytes, were determined. The DNA content per haploid genome was established by cytofluorometric methods after staining of the haploid merozoites with DNA-specific fluorescent dyes. Field inversion gel electrophoresis was used to establish the number and size of the chromosomes. Parasites of a high gametocyte producer clone (original HP) and a low producer clone (original LP) contained 13 to 14 chromosomes in the size range of 0.5-3.8 megabase. In four independent experiments parasites of the original HP clone were maintained in mice and were mechanically transmitted for prolonged periods of time (up to 90 weeks). During the transmission period the capability to produce gametocytes decreased in all four lines. After mosquito transmission of parasites from these low producer lines, the gametocyte production returned to the level of the original HP clone. The total DNA content per haploid genome of low producer parasites was not significantly different from that of HP parasites. During prolonged periods of asexual multiplication of the HP clone in vivo, changes in the relative size of several chromosomes were detected. Mosquito transmission of the original HP clone did not result in a change of the karyotype. However, novel karyotypes were readily observed after mosquito transmission of parasites of the low producer lines. The decrease of the capability to produce gametocytes did not necessarily involve detectable changes in DNA content or in karyotype.