Scientists decode the complete genome of two vectors of leishmaniasis

An international consortium of scientists, with the participation of the Oswaldo Cruz Foundation (Fiocruz), decoded the complete genome of two species of sandflies, vectors of leishmaniasis. The insects Lutzomyia longipalpis  and Phlebotomus papatasi had their DNA sequenced for the first time, with the characterization of genes associated with important behaviors in disease transmission. The results were published in the scientific journal Plos Neglected Tropical Diseases.

Phlebotomine insects are popularly known as straw mosquito, asa-dura, tatuquiras or birigui (photo: Josué Damacena/IOC/Fiocruz)

Leishmaniasis is a group of diseases caused by parasites of the genus Leishmania, which are transmitted to people through the bite of sandflies. These insects are popularly known as straw mosquitoes, asa-dura, tatuquiras or birigui, among other names. The infection can cause lesions on the skin, mucous membranes (such as the mouth and nose) or internal organs (such as the liver and kidneys), which can be fatal if not properly treated. 

Brazil is one of the countries most affected by leishmaniasis in the world. Every year, 21,000 cases of tegumentary leishmaniasis (which affects the skin or mucous membranes) and 3,500 cases of visceral leishmaniasis (which affects internal organs) are registered, according to data from the Ministry of Health.

According to the authors of the study, sequencing the genome of vectors opens doors to expanding research, including the search for new vector control strategies. 

From the left, Márcio Pavan, Rafaela Bruno, Fernando Genta, Yara Traub-Cseko and Rayane de Freitas: IOC researchers highlighted the importance of scientific cooperation (photo: Ricardo Schmidt/IOC/Fiocruz) 

“For control and surveillance actions, you need to know your target well. Genome sequencing opens up an 'avenue' of investigations into the biology of sandflies and provides a database to search for specific and effective tools against these vectors,” said the head of the Laboratory of Biochemistry and Physiology of Insects at the Oswaldo Cruz Institute (IOC /Fiocruz), Fernando Genta.

“For many years, knowledge about insect genetics was based on Drosofila melanogaster, the fruit fly. Afterward, we had complete genome sequencing for mosquitoes, for kissing bugs, but not yet for sandflies, which are vectors of great epidemiological importance. This project shows that there are conserved sequences, but also important differences between the studied species”, pointed out Rayane Teles de Freitas, PhD in the Postgraduate Program in Cellular and Molecular Biology at the IOC.

The effort to sequence the insect genomes brought together more than 70 researchers from 13 countries. The group had the participation of 15 scientists from the Oswaldo Cruz Institute (IOC/Fiocruz), from six of the laboratories: Molecular Biology of Insects; Molecular Biology of Parasites and Vectors; Biochemistry and Physiology of Insects; Parasitic Diseases; Interdisciplinary Entomological Surveillance in Diptera and Hemiptera; and Hematozoan Transmitting Mosquitoes. At Fiocruz, researchers from the Gonçalo Moniz Institute (Fiocruz Bahia) and the René Rachou Institute (Fiocruz Minas) also participated.

Mapped genes

From conception to publishing the results, the project took more than 15 years to complete. Among the challenges, the researchers had to deal with the diversity of sandflies. Insects of the genus Lutzomyia transmit the infection in the Americas, while vectors of the genus Phlebotomus spread the disease in Europe, Asia and Africa. Altogether, there are dozens of species with a recognized role in the transmission of leishmaniasis.

In this scenario, the scientists selected one important species from each genus for research. the insect Lu. Longipalpis is found in Central and South America, including Brazil, where it transmits Leishmania infantum, which causes visceral leishmaniasis. The species Ph. papatasi is present in Europe, the Middle East and North Africa, transmitting the parasite Leishmania major, which causes cutaneous leishmaniasis.

Analyzing the complete genome of the vectors, the researchers characterized genes associated with behaviors that interfere with the ability to transmit the infection and can be targeted in vector control strategies. As an example, the authors cite genes that regulate the circadian rhythm and the immune response of vectors.

“Rhythm genes regulate vector behavior in response to daily variations in light and temperature. This is related to vector competence because it influences, for example, the search for breeding sites and the timing of bites”, explained the head of the Laboratory of Molecular Biology of Insects, Rafaela Bruno. 

“The annotation of these genes forms a database that can be explored in other research”, completed the researcher at the Laboratory of Mosquitoes Transmitting Hematozoa, Márcio Pavan.

“When they are infected by Leishmania, sandflies develop an immune response, just like humans. Knowing the insect's immunity genes is important, for example, to identify targets to block infection in the vector", said the researcher from the Laboratory of Molecular Biology of Parasites and Vectors, Yara Traub-Cseko, citing ongoing projects at the laboratory that seek to develop vaccines capable of blocking the transmission of leishmaniasis. 

In the different groups of characterized genes, the analyzes revealed similarities and differences between the two sand fly species studied. 

“We saw that the enzyme machinery that the insect uses to digest proteins has many molecules conserved in both species. In sugar digestion, however, there is more variation. Lu. longipalpis has a more diverse arsenal of sugar-digesting enzymes, which may be associated with exposure to a greater diversity of plants in the Americas. This may have been one of the evolutionary strengths of this genome”, evaluated Fernando. 

The study also included analyzes of populations of each vector found in different locations with the aim of mapping the internal genetic variability of each species. Rafaela highlighted the importance of this investigation since several data point to the characterization of Lu. longipalpis as a species complex, bringing together vectors that look very similar but can be classified as distinct species because of behavioral and genetic differences.

“We carried out collections in different places in Brazil where apparently different species of the complex are found. The genetic analysis of these populations showed important differences, which reinforce that Lu. longipalpis is a complex of species”, said the researcher. 

About sequencing

The genome of sand flies (like that of other insects, animals and even humans) is made up of millions of small molecules, called nucleotides, which are identified by the letters A, C, T and G. When we say that the genome complete sequence of vectors has been sequenced, this means that scientists have discovered the order of the molecules that make up the DNA of insects. 

However, this is just the beginning of the work. Genes are stretches of DNA that guide the production of proteins and other molecules that make up the organism and regulate behavior. If DNA is a code, identifying genes corresponds to decoding the message inscribed in the genome.

In the newly published study, researchers found that the DNA of Lu. longipalpis is composed of about 150 million pairs of nucleotides, containing more than ten thousand genes. in the DNA of Ph. papatasi, approximately 350 million pairs of nucleotides were detected and more than 11,000 genes were annotated. 

The large difference in the size of the genomes was explained by the presence of many repetitive stretches in the DNA of the Old World vector. The presence of these segments, which do not correspond to genes, is associated with the pattern of evolution of the species.

“The sequencing of the complete genome forms a database. If a researcher decides to study something new, that no one has studied before, he has this source to make comparisons and identify new genes or variations in sequences, to deepen what he didn't know, "said Yara.