Malaria is a serious disease caused by a eukaryotic microbe in the Plasmodium genus and leads to more fatalities than all other parasitic illnesses combined. For the parasite to move from a human host to an Anopheles mosquito, it must transition to its sexual stage, known as the gametocyte stage. Unlike mammals, where sex is determined at the chromosome level, the mechanisms behind how this single-celled parasite develops into male or female forms remain unclear. Recent studies from Stockholm University have utilized advanced genomic technologies to chart the wide array of genes involved in gametocyte development towards male or female sex characteristics.
The research, featured in Nature Communications, highlights the genes that are activated in Plasmodium falciparum—the most lethal of the malaria parasites—from the earliest stages of sexual development to their maturity. At maturity, the male and female gametocytes become viable for uptake by female Anopheles mosquitoes, triggering the ongoing cycle of transmission.
“We have integrated cutting-edge single-cell genomics with a unique computational approach to characterize the expression of crucial genetic regulators throughout the developmental journey of male and female gametocytes,” explains Johan Ankarklev, Associate Professor in the Molecular Biosciences department at the Wenner Gren Institute, and the study’s lead author.
The collaboration at Stockholm University, along with Dr. Johan Henriksson from MIMS — Umeå University and the Microbial Single Cell Genomics facility at SciLifeLab, significantly enhances our comprehension of the genetic factors that underpin malaria transmission. A highly conserved group of transcription factors known as ApiAP2 has emerged as essential regulators of gene expression during the differentiation and development stages of Plasmodium.
“Thanks to our high-resolution data, we were able to computationally associate several of these ApiAP2 genes with either the male or female lineage, suggesting their role in sexual cell fate determination. Furthermore, we identified a substantial set of novel candidate ‘driver’ genes influencing male and female development, which we are currently investigating further in the lab using CRISPR technology,” Ankarklev adds.
The study offers vital insights not only to the malaria research community but to the broader scientific field:
- From a clinical standpoint, traditional treatment approaches have mainly focused on the symptomatic, asexual blood stage of infection with varying success, and current therapies do not prevent malaria transmission. This study introduces important genetic markers that could lead to the future development of transmission-blocking treatments, which are essential for controlling the spread of malaria.
- From an evolutionary viewpoint, given that Plasmodium is an old microbial eukaryote that produces both male and female forms, the findings provide new insights and hints regarding the evolution of sex in eukaryotes.
Our understanding of malaria’s sexual reproduction remains limited
Most eukaryotic organisms engage in sexual reproduction to promote diversity and enhance fitness. Typically, in animals, sex determination revolves around males and females. However, among the various eukaryotic microbes, the mechanisms of sex determination are exceptionally diverse and often not clearly understood. The malaria parasite, Plasmodium spp., is part of the Apicomplexan phylum, a category of obligate invasive, unicellular parasites that generate male and female gametes. Notably, Alphonse Laveran, a French scientist, first identified the crescent-shaped malaria gametocyte in 1880, and two decades later, British physician Robert Ross discovered the role of mosquitoes in malaria transmission. Despite these key findings, substantial progress in understanding the biology of malaria transmission stages has only recently been made, thanks to advancements in biotechnology.
Innovative genomic tools advance malaria research
Single-cell transcriptome profiling provides a snapshot of the multitude of genes expressed within a single cell, specifically, one malaria parasite at a certain stage of development. By analyzing thousands of single-cell transcriptomes, researchers have a powerful method for identifying genetic pathways and developmental decisions, crucial for tracing lineages.
“Utilizing computational tools like Pseudotime and RNA Velocity, we arranged several thousand cells along a branched pseudo-time axis and employed RNA velocity estimates to understand transcriptional splicing dynamics throughout the developmental path. This approach allowed us to predict a broad spectrum of potential ‘driver genes’ linked to male and female development, many of which had not been previously annotated,” explains Mubasher Mohammed, a former PhD student at the Ankarklev Lab and the study’s lead author.
Researchers with personal experiences
Mubasher grew up in Sudan and witnessed the harmful impacts of malaria firsthand.
“It’s an exciting time to be a scientist, as emerging technologies allow us to make significant strides in understanding various diseases affecting humanity,” shares Mubasher Mohammed.
The transmission stage is particularly vulnerable
The transmission stage of malaria is characterized by a notable decline in parasite populations, making it a prime target for control interventions.
“During such a population bottleneck, the parasites become more susceptible to drugs and environmental challenges. By clarifying the molecular pathways involved in gametocyte development, we can focus on these pathways to create effective strategies for blocking transmission—crucial for efforts aimed at eradicating malaria,” states Alexis Dziedziech, a former postdoc at the Ankarklev Lab and co-author of the study.
Key facts about malaria
- Malaria continues to be a significant global health issue, with around 230 million infections and over 600,000 fatalities annually, predominantly among children in Sub-Saharan Africa. There are more than a hundred Plasmodium species, five of which infect humans. Plasmodium falciparum accounts for most severe cases and deaths.
- Being a vector-borne illness, malaria spreads from person to person through the Anopheles mosquito. Symptoms typically manifest 10-15 days after being bitten and include fever, headaches, and chills. If untreated, it can lead to severe complications like cerebral malaria, severe anemia, or respiratory distress.
- Current treatments exist, with artemisinin-based combination therapy considered the best option for P. falciparum infection. However, malaria parasites can quickly develop resistance to all existing medications.
Key facts about the Apicomplexa
- The Apicomplexan phylum encompasses a diverse range of eukaryotic microbes, all sharing certain structures and organelles known as the apical complex, which are necessary for penetrating host cells.
- Apicomplexa possess complex lifecycles with multiple developmental stages, and all are obligate parasites during some phases of their lifecycle. Some even require two different hosts for their asexual and sexual stages, as seen with malaria parasites.
- Diseases caused by Apicomplexan organisms include malaria, toxoplasmosis, cryptosporidiosis, babesiosis, and cyclosporiasis.
