Using cloned lines of Plasmodium berghei producing mixed asexual and sexual (clone 234L) and purely asexual (clone 233L) parasitaemias, the courses of parasitaemia, gametocytogenesis, exflagellation, ookinete production in vitro and mosquito infectivity have been followed. For clone 234L mosquito infectivity is maximal at day 3 and has ceased by day 6 post-infection. Conversely, gametocytogenesis, exflagellation and ookinete production are at minimal levels at day 3 and rise to peaks between days 10 and 15 of infection (in TO mice infected with blood at mechanical blood passage 3). Sexual potential declines progressively with sustained mechanical passage (up to P14). Gametocyte conversion is highest early in infection and declines exponentially; however, upon each mechanical passage conversion is again raised but decreases more rapidly with succeeding passages. For practical mosquito transmission in the laboratory we consider P8 to be the useful limit for mechanical transmission for this parasite clone. Asexual parasite growth (virulence) is more rapid with increased mechanical passage inducing a rapid fall in haematocrit. By P14 the course of infection closely parallels that of the purely asexual clone 233L.

Blood-stage malaria parasites in the vertebrate host can develop either into the asexual, multiplying forms, called schizonts, or into gametocytes, the sexual stages of the parasite. In the present work we studied the differentiation into asexual parasites or gametocytes of the progeny of single, isolated schizonts of the clone 3D7A of Plasinodium falciparum, using monoclonal antibodies specific for the sexual or asexual stages of the parasite. We observed that schizonts obtained from a continuous culture undergoing serial cycles of growth and dilution with fresh red blood cells produced either only gametocytes or only asexual parasites, showing a high degree of commitment to one or the other developmental pathway.

The relative proportion of schizonts which produced gametocytes was very low at low parasite densities in culture, while at high parasite densities a much greater proportion of schizonts produced gametocytes. Nevertheless, at both low and high parasite densities individual schizonts were almost always fully committed to producing only gametocytes or only asexual parasites.

Commitment to the production of female and male gametocytes was studied in the NF54 line of the human malaria parasite Plasmodium falciparum. The development of sibling parasites derived from individual schizonts was followed, and 2 antisera against the female gametocyte-specific protein Pfg377 and the male gametocyte-specific protein α-tubulin II were used to determine the sex of sibling gametocytes. The experiment showed that individual cohorts of sibling gametocytes were stained in a mutually exclusive fashion by only one or the other antiserum, indicating that individual schizonts committed to yield sexual parasite progeny produce gametocytes of the same sex. This work suggests that in P. falciparum commitment to sexual differentiation occurs prior to schizont maturation, at the same moment when the sex of the resulting gametocytes is determined.

The differentiation of the two sexes in the gametocytogenesis of Plasmodium falciparum was investigated using a plaque assay and antibodies specific for various stages and sexes of gametocytes. Immunofluorescence assays on plaques of cultured parasites grown in monolayers of erythrocytes revealed that the merozoites released from a single sexually-committed schizont became either all male or all female gametocytes. Thus, the commitment of this species to differentiate into one sex or the other is likely to occur prior to the nuclear division of the sexually-committed schizont. The characteristic female-biased gametocyte sex ratio observed for many Plasmodium species is manifested in P. falciparum by a greater percentage of schizonts that produce female gametocytes (67–71%) than those that yield males. From the plaque assay, it was determined that the number of gametocytes produced per sexually-committed schizont was similar for both sexes, indicating that allocation of parasite resources was equal for each sex of gametocyte. The timing of sexual differentiation and features of the gametocyte sex ratio is discussed in relation to previous observations on P. falciparum and related malaria parasites.

Malaria parasites are capable of modulating the diversion of resources from asexual growth to the production of stages infective to mosquitoes (gametocytes). Increased rates of gametocytogenesis appear to be a general response to stress, both naturally encountered and novel. We have previously reported earlier and greater gametocytogenesis in response to subcurative antimalarial chemotherapy in the rodent malaria, Plasmodium chabaudi, in vivo. Using an immunofluorescent assay to detect parasites that had invaded red blood cell monolayers, we demonstrate a 5-fold increase in gametocytogenesis in the human malaria, P. falciparum, in vitro, in response to treatment with the antimalarial drug chloroquine. In all clones used, gametocytogenesis increased with increasing inhibition of asexual growth by chloroquine. Furthermore, there were clone differences in the relationship between stress and gametocyte production, implying the response was genetically variable. This was not, however, associated with chloroquine resistance. The epidemiological significance of these results is discussed.