The malaria parasite Plasmodium falciparum prepares for life within the host cell by activating a complex gene network

The intraerythrocytic developmental cycle (IDC) during which the malaria parasite Plasmodium falciparum multiplies asexually initiates when parasite stages called merozoites are released into the bloodstream, following schizont rupture, and invade the host red blood cells. The merozoite is the less studied stage of the IDC, despite its importance for the establishment of infection. Here we uncovered the gene expression profile of the free merozoites, by doing single cell RNA-sequencing (scRNA-seq). We found that free merozoites express invasion and motility genes that are actively transcribed in schizonts and after merozoite egress. P. falciparum parasites undergo at least one IDC before sexually-committing and transforming into the sexual stages, the gametocytes, which are then transmitted to the mosquito vector. Our scRNA-seq dataset identified two early states of sexual commitment, preceding expression of the gametocytogenesis master regulator ap2-g. We hypothesize that early committed parasites are primed for sexual differentiation and definitive engagement into gametocytogenesis occurs when ap2-g is expressed. Our results contribute to a better understanding of how the parasite prepares for invasion and how it exits asexual growth and enters sexual differentiation. These transitions ensure survival of the parasite in the human host and transmission to the mosquito vector so the gene markers identified here can be therapeutically explored in the future to block malaria invasion or transmission. ImportanceThe malaria parasite Plasmodium falciparum, which causes the death of over half a million people each year, has a complex life cycle but symptoms only arise when the parasite invades and develops inside the host red blood cells. The merozoites are the parasite stages responsible for invading the human red blood cells and they are one of the few extracellular stages of the lifecycle, even if briefly. However, their gene expression profile had yet to be determined. Here we used single cell RNA-sequencing to establish the gene expression program of free merozoites. We also identified for the first time the gene signature of sexually-committed and sexual merozoites that will later give raise to gametocytes and ensure parasite transmission.