Aedes albopictus

The tiger mosquito (Aedes albopictus) is a species of nematoceran dipteran belonging to the Culicidae family. It is characterized by its black coloration with white ornamentation on the thorax and abdomen, black and white banded legs and a conspicuous central longitudinal white line on the thorax and head. It is about 5 to 10 mm (millimeters) long. Like other species of mosquitoes, the female has a thin and elongated proboscis, the proboscis, which, like a stylet, it uses to bite and extract blood from vertebrates, especially mammals and birds, which it uses to develop eggs as a proteins. It uses small filaments (whiskers) on the sides of the proboscis to detect carbon dioxide (CO₂) coming from animals (including humans) to stalk. In its bites it uses an anticoagulant substance to extract and store blood from its host. The males of the species, like other mosquitoes, feed on nectar. It is included in the list of the 100 most harmful invasive alien species in the world by the International Union for Conservation of Nature.

Biology and ecology

The larvae of this species develop in containers where there are small amounts of water, especially if they are in shady places, such as jars, buckets, vases, flowerpot plates and other objects containing water in gardens, patios and vacant. Their original larval breeding ground consists of water-filled cavities in trees. In the human environment it is a mainly urban mosquito that systematically takes advantage of water spots for its reproduction. Their bite, even through thin clothing such as socks, is very annoying, occurring mostly during daylight hours, during which some of the native species do not usually bite.

Genome characteristics

When the genome of the tiger mosquito was sequenced, it was observed that it was made up of 1967 Mb, which makes it the largest mosquito genome sequenced to date, since the size of the genomes of the species closest to Aedes albopictus vary between 174 Mb of Anopheles darlingi to 1376 Mb of Aedes aegypti. It was also possible to verify that between different individuals of the same species of Aedes albopictus there was a variation in the size of the genome.

A new sequencing of the genome of Aedes albopictus, which generated the reference genome AalbF2, determines that the average genome size of this insect is between 1190-1275 Mb, varying between individuals of the different populations, and that this species has three chromosomes. The difference in size with respect to that described in previous genomes is due to the fact that in this case massive third-generation sequencing techniques were used, allowing longer fragments to be sequenced and making it possible to better resolve the regions of repetitive DNA, which are abundant in this species.

It is believed that inter- and intraspecific variations in genome size among different mosquitoes are mainly due to changes in the amounts and organization of repetitive DNA. Therefore, it is believed that one of the reasons why Aedes albopictus has a longer genome than the rest is mainly due to the fact that it has a greater amount of repetitive DNA.

In an experiment, phylogenetic relationships were established between 5 different species of mosquitoes (Aedes albopictus, Aedes aegypti, Anopheles darlingi, Anopheles gambiae and Culex quinquefasciatus) and the vinegar fly (Drosophila melanogaster) from 2096 single-copy orthologous genes. It was confirmed that the species Aedes albopictus is part of the subfamily Culicinae and that the evolutionary divergence with the species Aedes aegypti occurred approximately 71 million years ago.

The 5 mosquito species shared a common ancestor with Drosophila melanogaster approximately 260 million years ago.

Repetitive DNA

The genome of Aedes albopictus contains a large number of transposons, 55% of the total genome is represented by these mobile elements. Repetitive sequences represent 70% of its genome consistent with its large size. In fact, the total length of these repetitive sequences is 40% longer than those found in Aedes aegypti, which is a member of the same subgenus, Stegomyia.

In the DNA of both species there are a large number of LINES sequences (long dispersed nuclear elements). LINES are sequences that are repeated thousands of times in the genome, but not in tandem, but in a dispersed manner. There are LINES sequences, such as the Duo sequence that are shared by both species, suggesting that this sequence was present in the common ancestor of both species. However, more than 20% of the Aedes albopictus LINES sequences do not present similarities with the Aedes aegypti sequences, leading to the hypothesis that it must have been produced a rapid expansion or increase in the number of repetitive DNA after the evolutionary divergence of species occurred, which is estimated to have occurred about 10 million years ago.

Viral elements

A large number of nrEVEs (non-retroviral endogenous viral elements) can be located in the genome of Aedes albopictus. These genetic elements are inserted into the genome of Aedes albopictus due to its continuous contact with different types of viruses. Let us not forget that this species is in contact with mammalian blood and, therefore, with its pathogens. The different populations of this insect can be differentiated by observing their viral sequences since they present homology with the characteristic viruses of various areas. The function of nrEVEs could be related to immunity against different viruses.

Distribution and structure of piRNA clusters

RNAs that interact with PIWI proteins (piRNA) have the function, among others, of directing immunity against transposons in the germ line, this has been proven mainly in Drosophila melanogaster, The activity of these piRNAs is based on the recognition of their complementary genetic material and subsequent inactivation. Most piRNAs derive from genetic regions called piRNA clusters; these regions have memory of transposon invasions and confer protection against these agents. In the case of Aedes albopictus the piRNA clusters are highly enriched with viral sequences, therefore, it may be that in Aedes albopictus the piRNAs direct immunity against different types of viruses.

Sexual determination

In Aedes albopictus the sex of the individual is controlled by a locus (M locus) that resides on chromosome 1 and that when it presents certain active genes (nix and myo-sex) generates male individuals. The Nix factor discovered in the species Aedes aegypti, is the main responsible for the development of male characters in Ae. albopictus,this factor is expressed by the nix gene composed of two exons separated by a small intron. The myo-sex gene located near the nix gene at the M locus in Aedes aegypti is also important in sexual determination as it expresses the myosin heavy chain that allows males to fly. A myo-sex homologue has been described in the species Aedes albopictus but its exact position has not been defined.

Genes involved in diapause and detoxification

Diapause occurs in insects and consists of a dynamic state of low metabolic activity and growth that is mediated by neurohormones. Insects use this diapause to survive in environments with extreme temperature and humidity and can be specific to a stage of development, which is genetically predetermined. In mosquitoes, diapause can occur in both the embryonic and larval or adult stages. When a study of the Aedes albopictus genome was carried out, it was observed that there were about 211 genes that belonged to expansion families. Of these 211, about 96 genes were related to stress response, lipid metabolism, regulation of gene expression, hormonal signaling, etc. Therefore, it is believed that the expansion of these genes helps the mosquito tolerate heterogeneous environments and that is what allows it to invade several continents and transmit these diseases.

In Aedes albopictus several genes involved in detoxification have been found, such as the genes that code for the cytochrome oxidase CYP4G. In addition, genes involved in the synthesis of the enzyme carboxyl cholinesterase (CCE) have been found, such as the CCEae 3A gene, which is involved in resistance to temephos, which is a larvicide used to control mosquito pests. It occurs as two genes duplicated in tandem.

It also has genes involved in resistance to insecticides, such as the gene that codes for the protein glutathione S-transferase (GST), or those that code for ABC transport proteins, which are involved in resistance to several types of drugs.. Specifically, in the case of the ABCG protein, it was found that the gene was duplicated up to 6 times in both Aedes albopictus and Aedes aegypti and is involved in lipid transport..

Dissemination

The tiger mosquito is a diurnal invasive species native to Southeast Asia, having spread throughout Africa, America and Europe since 1979 and later throughout the Pacific area. It represents a growing threat to public health around the world due to its rapid and aggressive expansion from its native distribution area, since in just 40 years it managed to spread to all continents of the planet except Antarctica. Its arrival can occur during the transport of people, or goods with remains of accumulated water such as used tires, or ornamental plants such as the lucky bamboo (Dracaena sanderiana). Its eggs are resistant to desiccation, so it can also enter imported used rubber that has its dry eggs attached internally. Once they come into contact with water, they hatch and the first stage larvae emerge (of four possible).

In continental America, the first introduction was in the United States in 1985 and in Brazil around 1986. Subsequently, it invaded Mexico where it was detected in 1988, being the first record of Aedes albopictus in this country. naturally infected with DEN-1 and DEN-3 on the mainland. The invasion route has included Central America, in Guatemala, Honduras, El Salvador, Nicaragua and Panama (Cuellar-Jiménez et al, 2007) and in the Caribbean in the Cayman Islands, Dominican Republic, Cuba and Trinidad and Tobago, while in America from the South, in Brazil it spread through 20 of 27 states and where 2 genotypes seem to be present. In Colombia it has been noted for: Leticia in 1998, Buenaventura in 2001, Cali in 2007 and the Colombian Caribbean, mainly Barranquilla and Cartagena in 2014, and in Argentina in 1998 to the north (Misiones), an area in which its distribution is maintained. and more recently (2009) in Caracas, Venezuela.

In Europe, until 2006, it had been detected in Albania, Italy, Spain, France, Belgium, Switzerland, Hungary, Montenegro, Holland and Greece. The first detection of the presence of the mosquito in Spain was reported in August 2004 in the town of San Cugat del Vallés (Catalonia) by the Mosquito Control Service of the Comarcal Council of Bajo Llobregat, having spread since then to many other municipalities in the Spanish Levant, without causing disease transmission in this area. Favored by climate change, with increasingly milder winters, two decades later its presence had reached the north and west of the country.

Disease transmission

In endemic areas, the tiger mosquito is a vector for the transmission of diseases such as dengue in Central America, South America and the Pacific area, yellow fever and, although much less frequently than the very common Culex pipiens,can be a vector in the transmission of West Nile virus.

In the summer of 2007, an epidemic outbreak of chikungunya epidemic arthritis occurred in Ravenna (Italy) caused by the CHIKV virus and which was spread by the bite of the tiger mosquito. Until then, this disease had only affected tropical countries, which raised alarms about the globalization of this type of infections.

In the summer of 2010, tiger mosquitoes infected by the West Nile virus were detected in northern Greece, causing the death of some people who were bitten by them.

In 2014, an epidemic of chikungunya epidemic arthritis broke out, which left several fatalities in South America due to the bite of this mosquito. The countries most affected by this virus were the Dominican Republic and Venezuela.

Eradication

In the countries where it has spread, the fight to eradicate it is difficult and very expensive, which is why it is advisable to act as early as possible by raising awareness among institutions and citizens to prevent this spread. Among the actions proposed are:

• Manage properly the storage, transport and recycling processes of used tyres. This seems to be one of the fundamental actions to limit its intercontinental expansion, since the pathways of the mosquito have been linked to those of the transport of this commodity worldwide.
• Avoid possible reservoirs for mosquito breeding as can be containers, pots, cans or any other utensil in which water cannot be replaced at least every week.

Genetic polymorphism and lineage imbalance

The level of genetic variability between the populations of a species is the substrate of evolution, in the case of Aedes albopictus this population variability is generated by processes of adaptation to new ecosystems, selection of alleles resistance to insecticides and coevolution with local pathogens. These biological data are important to estimate the epidemiological danger of each population and to generate control strategies based on genetic data. The rapid spread of this mosquito, which began about 50 years ago, has been possible thanks to the rapid change in climatic conditions, but also thanks to its great power of adaptation. Large variations can be observed between populations, making it more difficult to develop global control strategies.

Invasive species

In Spain

The first detection of the presence of the mosquito in Spain was reported in August 2004 in the town of San Cugat del Vallés (Catalonia) by the Mosquito Control Service of the Comarcal Council of Bajo Llobregat, having since spread to many other municipalities in eastern Spain, without causing disease transmission in this area.

Due to its colonizing potential and constituting a serious threat to native species, habitats or ecosystems, this species has been included in the Spanish Catalog of Invasive Exotic Species, approved by Royal Decree 630/2013, of August 2.

In Spain, it is believed that its spread has been through the transport of goods and private vehicles, since Aedes albopictus, on its own, throughout its life is usually present in a 500 meter radius.^{[citation required]}

In Argentina

The presence of Aedes albopictus in Argentina had not been detected until the beginning of 1998. In April of that same year, 33 breeding sites were registered, with few specimens in each one, in 25 of the 161 homes inspected in the city of El Dorado, Province of Misiones. The homogeneous spatial distribution of the proliferation foci detected during the study period suggests the existence of a widespread infestation in this locality.