Malaria parasites are capable of modulating the diversion of resources from
asexual growth to the production of stages infective to mosquitoes (gameto
cytes). Increased rates of gametocytogenesis appear to be a general respons
e to stress, both naturally encountered and novel. We have previously repor
ted earlier and greater gametocytogenesis in response to subcurative antima
larial chemotherapy in the rodent malaria, Plasmodium chabaudi, in vivo. Us
ing an immunofluorescent assay to detect parasites that had invaded red blo
od cell monolayers, we demonstrate a 5-fold increase in gametocytogenesis i
n the human malaria, P. falciparum, in vitro, in response to treatment with
the antimalarial drug chloroquine. In all clones used, gametocytogenesis i
ncreased with increasing inhibition of asexual growth by chloroquine. Furth
ermore, there were clone differences in the relationship between stress and
gametocyte production, implying the response was genetically variable. Thi
s was not, however, associated with chloroquine resistance. The epidemiolog
ical significance of these results is discussed.