The papers that appear in this issue, describing the genome of the human malaria parasite Plasmodium falciparum, are published simultaneously with others in Science tackling the genome of the mosquito Anopheles gambiae. The connection is obvious: the parasite requires a mosquito to complete its complex life cycle and for transmission from one host to another. These two species are respectively the major parasite causing malaria and the major vector.
Plasmodium is taken up by mosquitoes in blood meals drawn from infected humans (see the life-cycle diagram on page 495). The parasite then undergoes several developmental stages, and crosses two mosquito cell layers that enclose the insect's midgut and salivary glands. Ultimately, Plasmodium is passed on when the mosquito bites a new human host, about two weeks after ingesting the first infected blood meal. For more than a century, an objective of malaria control programmes has been to block parasite transmission by mosquitoes. These approaches will clearly benefit from the improved understanding of mosquito biology and mosquito interactions with P. falciparum that the genome sequences will make possible.
The A. gambiae genome1 was sequenced by a collaboration between Celera Genomics, the French National Sequencing Centre (Genoscope) and The Institute for Genomics Research (TIGR), in association with several university laboratories. These groups used the same 'shotgun' strategy as that applied for sequencing the human, mouse and fruitfly (Drosophila melanogaster) genomes. Random fragments of genomic DNA were first cloned in bacteria, and sequenced, and the overlapping clones were then assembled into contiguous sequences. Unexpectedly, the high levels of genetic variation (polymorphisms) in the reference strain of A. gambiae used for sequencing — the PEST strain — made the genomic assembly step difficult. The genetic variation might be explained by the fact that two distinct populations of A. gambiae have contributed to the PEST strain, thereby creating a mosaic genome structure. This unprecedented situation required the development of new sequence-assembly strategies, and these will be a considerable asset for future genome projects — as with mosquitoes, not all organisms are available as inbred laboratory strains.
Comparison with the fruitfly
Much of the interest in the A. gambiae genome will centre on comparisons with that of D. melanogaster, which was published two years ago2. These two insects belong to the same taxonomic order, the Diptera, but inhabit distinct environments and have different lifestyles (Fig. 1). Drosophila melanogaster feeds on decaying organic matter, such as damaged or rotting fruit, where it also completes its life cycle, whereas A. gambiae feeds on sugar nectar and on the blood of vertebrate hosts. Blood meals are required for female mosquitoes to produce eggs; these are laid in water, where larvae develop and hatch. Blood feeding exposes the insect to viruses and parasites — like Plasmodium, these other pathogens exploit Anopheles as a vector for transmission.
Figure 1: The mosquito and the fruitfly in typical pose — Anopheles (top) on human skin, Drosophila on a banana.
![Figure 1 : The mosquito and the fruitfly in typical pose |[mdash]| Anopheles (top) on human skin, Drosophila on a banana. Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com](/nature/journal/v419/n6906/images/419496a-f1.0.jpg)
High resolution image and legend (140K)
One of the main differences between the two species is that, at 278 million base pairs, the A. gambiae genome is much bigger than that of D. melanogaster (estimated to be 180 million base pairs). But this difference is not reflected in the total number of genes, which, with 13,000–14,000 genes so far identified in both insects, is surprisingly similar. It seems that, in the course of evolution, Drosophila has experienced a progressive reduction both in the regions between genes and in the introns, the non-protein-coding stretches of DNA within genes.
Comparison of the coding sequences reveals that the genomes of Anopheles and Drosophila are less similar than would be expected for two species that diverged 'only' 250 million years ago. Only half of the genes in the two genomes can be interpreted as orthologues — genes in different species that have common ancestry, although their functions may differ. Anopheles and Drosophila orthologues show an average of about 56% identity in DNA sequence. As Zdobnov et al. point out in another of the papers in Science3, from the sequence standpoint, the two species differ more than do humans and pufferfish — species that diverged 450 million years ago. Some of the protein families present in both mosquito and fruitfly appear to have evolved from a common ancestral gene through independent gene-duplication in each species. The Anopheles genome shows several cases of such expansion which might reflect adaptation to its lifestyle. An example is the family of fibrinogen-like proteins (of which there are 58 in Anopheles and 13 in Drosophila), which in the mosquito are probably used as anticoagulant for the ingested blood meals.
Defence mechanisms
Insects have efficient immune systems for combating the various pathogens they encounter, and most of our knowledge in this area comes from genetic and molecular studies in Drosophila. Finding out how Anopheles responds to Plasmodium infection is essential for obtaining clues to controlling malaria. Christophides et al.4 analysed the gene families in A. gambiae that are linked to insect immunity, and show that they diverge widely from those in Drosophila. Good examples are the prophenoloxidase enzymes (nine in the mosquito, three in the fruitfly); these enzymes catalyse the synthesis of melanin, which is associated with several defence reactions in insects.
The study by Christophides et al. suggests that Anopheles employs the same general defence mechanisms as Drosophila, and uses similar pathogen-activated signal-transduction pathways, but that it has adapted recognition and effector immune genes to different types of aggressors. The best characterized effector system in insects consists of antimicrobial peptides, which display a wide spectrum of antibiotic activities. Interestingly, out of seven families of these peptides found in Drosophila, only two are also evident in Anopheles: five, then, are specific to Drosophila. Conversely, at least one mosquito-specific antimicrobial peptide has already been identified and others might be discovered by functional studies in the future. The expression profiles of some A. gambiae immune genes also suggest that, like the fruitfly, the mosquito mounts specific immune responses adapted to different types of pathogen4, 5.
The availability of the entire DNA sequence, together with tools such as DNA microarrays and targeted gene disruption6, 7, 8, will make Anopheles a powerful model system for studying insect biology. The genomic data will also help in developing strategies to combat malaria and other mosquito-borne human diseases, for example yellow fever, dengue, filariasis and encephalitis. Such strategies will include reducing the number and lifespan of infectious mosquitoes, analysing what attracts them to their human targets, and limiting the capacity of parasites to develop within the insect vector. Malaria is characterized by a highly complex set of interactions between the parasite, the vector and the host. Now that the genomes of all three players have been fully sequenced, the post-genomic era in combating this dreadful disease can really begin.
