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Biology

Enlarged sequencing

The presence of jumping genes increases the size of the genome of Aedes aegypti, the dengue and yellow fever mosquito

James Gathany/CDCAedes aegypti right after feeding on blood: jumping genes account for 47% of the genomeJames Gathany/CDC

The publication, in the May 18 edition of the journal Science, of the first ever version of the Aedes aegypti mosquito genome, the result of an international effort by scientists from seven countries, including Brazil, came as a great surprise: the genetic material from the agent that transmits dengue fever and yellow fever is five times bigger than the material of Anopheles gambiae, the vector of the malaria protozoa, and almost seven times greater than that of Drosophila melanogaster, the fruit fly, until then the two only species of mosquito whose genomes had been sequenced. The DNA of Aedes comprises almost 1.4 billion base pairs, the chemical units that comprise the genetic code, while that of Anopheles has 280 million base pairs and Drosophila, 180 million.

Despite the enormous difference in size the genomes of the dengue fever and malaria mosquitoes, just focusing on the two insects that transmit diseases, have more or less the same number of genes, around 15,000. The larger size of the genome of Aedes is due to a peculiarity also found to the same degree in human DNA: almost half of its entire sequence, more precisely 47% of it, consists of so-called transposition elements, or more simply, transposons, the popularly named (by people who work with genetics) jumping genes. This is a very high percentage. In Anopheles, which in evolution terms derived from types of mosquitoes of the Aedes and Culex types 150 million years ago, the transposition elements account for less than 25% of the genome.

Transposons are DNA strands that can change position within a genome, or even move from the genome of one species to that of another, by generating copies of its original sequence or simply by moving between two regions. More than 1,000 different types of transposons have been identified in the genome of the dengue fever mosquito. One of them, called Feilai-B, has nearly 50,000 copies spread throughout the insect’s chromosomes. In the case of Aedes, these movable strands of DNA can be used as tools for studying the interaction between the mosquito and the dengue fever virus. They may be the key to revealing a way of interfering in the transmission of the disease from insect to man. “From transposons we may get a clue as to why Aedes can pass the virus to man while Anopheles transmits a protozoa”, says Sergio Verjovski-Almeida, from USP’s Institute of Chemistry, who coordinated Brazil’s participation in the international consortium that sequenced the DNA of the dengue fever mosquito. It was the responsibility of the national team, which also includes scientists from the Butantan Institute, to sequence the genes of the mosquito larvae, and on a smaller scale, from their salivary glands. The contribution of the Brazilians came via a partnership with FAPESP and the Pasteur Institute in Paris, which financed the work of the team coordinated by Verjovski.

Transposons have already been considered part of the so-called trash DNA, as the pieces of genome that appeared not to perform any function whatsoever are disparagingly called. Now, however, it is becoming increasingly clear that inserting these jumping genes – or at least some of them – into a genome may alter or regulate the action of other genes. Therefore, these movable and repetitive sequences, which some believe are the genetic remains of material of a viral origin, may be responsible for mutations and contribute to the occurrence of some diseases in human beings, such as Duchene’s muscular dystrophy and hemophilia, and cause changes in physical features. A single cob of Indian corn, for example, may have grains of three different colors (white, yellow and reddish) because of the presence of jumping genes. “The genome of some types of corn may consist of as much as 80% transposons”, says Marie-Anne Van Sluys, an academic working with transposons at USP’s Biosciences Institute. In fact, it was when working with the genetics of corn in the 1940’s that Barbara McClintock, from the USA, started to discover the influence of these transposition elements, a pioneering feat that  earned her a Nobel prize in 1983.

The Project
FAPESP Aedes aegypti cdna project in partnership with the Institute Pasteur – Amsud network (nº 04/10136-8); Modality: Regular line of support for research – Genome Project; Coordinator: Sergio Verjovski-Almeida – IQ/USP; Investment: R$ 690,882.36 (FAPESP/Instituto Pasteur)

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