AbstractAchievement of malaria elimination requires development of novel strategies interfering with parasite transmission, including targeting the parasite sexual stages (gametocytes). The formation of Plasmodium falciparum gametocytes in the human host takes several days during which immature gametocyte-infected erythrocytes (GIEs) sequester in host tissues. Only mature stage GIEs circulate in the peripheral blood, available to uptake by the Anopheles vector. Mechanisms underlying GIE sequestration and release in circulation are virtually unknown. We show here that mature GIEs are more deformable than immature stages using ektacytometry and microsphiltration methods, and that a switch in cellular deformability in the transition from immature to mature gametocytes is accompanied by the deassociation of parasite-derived STEVOR proteins from the infected erythrocyte membrane. We hypothesize that mechanical retention contributes to sequestration of immature GIEs and that regained deformability of mature gametocytes is associated with their release in the bloodstream and ability to circulate. These processes are proposed to play a key role in P falciparum gametocyte development in the host and to represent novel and unconventional targets for interfering with parasite transmission.

Population subgroups of the African malaria vector Anopheles gambiae have not been comprehensively characterized owing to the lack of unbiased sampling methods. In the arid savanna zone of West Africa, where potential oviposition sites are scarce, widespread collection from larval pools in the peridomestic human habitat yielded a comprehensive genetic survey of local A. gambiae population subgroups, independent of adult resting behavior and ecological preference. A previously unknown subgroup of exophilic A. gambiae is sympatric with the known endophilic A. gambiae in this region. The exophilic subgroup is abundant, lacks differentiation into M and S molecular forms, and is highly susceptible to infection with wild Plasmodium falciparum. These findings might have implications for the epidemiology of malaria transmission and control.Much of the current genetic knowledge of African malaria vector populations is based on collection and analysis of indoor house-resting (endophilic) mosquitoes. Indoor capture of mosquitoes is often regarded as an unbiased collection method for epidemiologically important vectors of human malaria, including A. gambiae sensu stricto (ss), because they are thought to be 'naturally endophilic' (1). Outdoor-resting (exophilic) mosquitoes can contribute to malaria transmission but are underrepresented or absent from indoor collections, particularly if they also bite outdoors (2-7). Collection methods for exophilic mosquitoes are much less efficient than for indoor-resting mosquitoes, involving variants of i) artificial resting sites such as pits, boxes or pots, ii) manual aspiration from vegetation and holes, or iii) capture of mosquitoes landing on animal or human bait, now largely proscribed due to the risk of infection of human collectors (2). Further highlighting the challenge in sampling exophilic mosquitoes, resting sites thought to harbor large A. gambiae populations during the dry season have resisted detection for decades (8)(9)(10) Genetic division of A. gambiae populations into subgroups allows fine ecological partitioning by the species, mediating the expansion of malaria transmission spatially and temporally. Chromosome inversion polymorphisms define certain subgroups (10,12). Frequency of the 2La inversion follows a geographic cline from the humid Central African forest, where the wild-type 2La+ allele is fixed, north to the arid West African savanna, where the inverted 2La allele is fixed (12). One likely phenotype of the 2La inversion is adaptation to aridity. Another example of niche expansion of A. gambiae by genetic subdivision is represented by two genetically diverged molecular forms, termed M and S (13). These molecular forms are detected by assays for fixed nucleotide differences on the X chromosome (Molecular Form Diagnostic SNPs, MFDS), and display other fixed SNPs in genomic 'speciation islands ' (14). The M form dominates in marginal and disturbed habitats where S is less competitive (15).We collected mosquito larvae from natural breeding pools in an arid...

Arboviruses are transmitted by mosquitoes and other arthropods to humans and animals. The risk associated with these viruses is increasing worldwide, including new emergence in Europe and the Americas. Anopheline mosquitoes are vectors of human malaria but are believed to transmit one known arbovirus, o’nyong-nyong virus, whereas Aedes mosquitoes transmit many. Anopheles interactions with viruses have been little studied, and the initial antiviral response in the midgut has not been examined. Here, we determine the antiviral immune pathways of the Anopheles gambiae midgut, the initial site of viral infection after an infective blood meal. We compare them with the responses of the post-midgut systemic compartment, which is the site of the subsequent disseminated viral infection. Normal viral infection of the midgut requires bacterial flora and is inhibited by the activities of immune deficiency (Imd), JAK/STAT, and Leu-rich repeat immune factors. We show that the exogenous siRNA pathway, thought of as the canonical mosquito antiviral pathway, plays no detectable role in antiviral defense in the midgut but only protects later in the systemic compartment. These results alter the prevailing antiviral paradigm by describing distinct protective mechanisms in different body compartments and infection stages. Importantly, the presence of the midgut bacterial flora is required for full viral infectivity to Anopheles, in contrast to malaria infection, where the presence of the midgut bacterial flora is required for protection against infection. Thus, the enteric flora controls a reciprocal protection tradeoff in the vector for resistance to different human pathogens.