Chromosomal evolution among six mosquito species (Diptera: Culicidae) based on shared restriction fragment length polymorphisms

Morphological and genetic variability within Aedes aegypti in Niakhar, Senegal

Aedes aegypti (Linné, 1762) is a major vector of arboviruses such as Yellow Fever, Dengue and Chikungunya. In Africa, where the species exhibits major variations in morphology, ecology, behavior and vector competence, two subspecies have been described: a light form, named Ae. aegypti aegypti (Aaa) with highly domestic and anthropophilic habits and a cosmotropical distribution; and a dark form, referred to as Ae. aegypti formosus (Aaf), which is endemic to Africa and thrives in sylvan environments. In East Africa, both forms were described to occur in sympatry whereas only Aaf was reported from Central/West Africa. However, recent findings suggest Aaa was also common in Senegal. Here, we report on a longitudinal survey of morphological and genetic variability of Ae. aegypti sampled in the rural environment of Niakhar, Senegal. In agreement with recent findings, most of specimens we analyzed were classified as Aaa suggesting typical Aaf was scarce in the studied area. Among Aaa, significant temporal variations in abdominal pale scales pattern were detected. Depending on the season and the nature of larval breeding places, the specimens (particularly females) tend to segregate in two main morphological groups. Microsatellite-based estimates of genetic differentiation did not provide any clear evidence that the two groups were genetically distinct. Overall, these results improve our understanding of the diversity of Ae. aegypti in West Africa, where data are crucially lacking.

Genome-based polymorphic microsatellite development and validation in the mosquito Aedes aegypti and application to population genetics in Haiti

Background

Microsatellite markers have proven useful in genetic studies in many organisms, yet microsatellite-based studies of the dengue and yellow fever vector mosquito Aedes aegypti have been limited by the number of assayable and polymorphic loci available, despite multiple independent efforts to identify them. Here we present strategies for efficient identification and development of useful microsatellites with broad coverage across the Aedes aegypti genome, development of multiplex-ready PCR groups of microsatellite loci, and validation of their utility for population analysis with field collections from Haiti.

Results

From 79 putative microsatellite loci representing 31 motifs identified in 42 whole genome sequence supercontig assemblies in the Aedes aegypti genome, 33 microsatellites providing genome-wide coverage amplified as single copy sequences in four lab strains, with a range of 2-6 alleles per locus. The tri-nucleotide motifs represented the majority (51%) of the polymorphic single copy loci, and none of these was located within a putative open reading frame. Seven groups of 4-5 microsatellite loci each were developed for multiplex-ready PCR. Four multiplex-ready groups were used to investigate population genetics of Aedes aegypti populations sampled in Haiti. Of the 23 loci represented in these groups, 20 were polymorphic with a range of 3-24 alleles per locus (mean = 8.75). Allelic polymorphic information content varied from 0.171 to 0.867 (mean = 0.545). Most loci met Hardy-Weinberg expectations across populations and pairwise F_ST comparisons identified significant genetic differentiation between some populations. No evidence for genetic isolation by distance was observed.

Conclusion

Despite limited success in previous reports, we demonstrate that the Aedes aegypti genome is well-populated with single copy, polymorphic microsatellite loci that can be uncovered using the strategy developed here for rapid and efficient screening of genome supercontig assemblies. These loci are suitable for genetic and population studies using multiplex-PCR.

Transposable elements in mosquitoes

We describe the current state of knowledge about transposable elements (TEs) in different mosquito species. DNA-based elements (class II elements), non-LTR retrotransposons (class I elements), and MITEs (Miniature Inverted Repeat Transposable Elements) are found in the three genera, Anopheles, Aedes and Culex, whereas LTR retrotransposons (class I elements) are found only in Anopheles and Aedes. Mosquitoes were the first insects in which MITEs were reported; they have several LTR retrotransposons belonging to the Pao family, which is distinct from the Gypsy-Ty3 and Copia-Ty1 families. The number of TE copies shows huge variations between classes of TEs within a given species (from 1 to 1000), in sharp contrast to Drosophila, which shows only relatively minor differences in copy number between elements (from 1 to 100). The genomes of these insects therefore display major differences in the amount of TEs and therefore in their structure and global composition. We emphasize the need for more population genetic data about the activity of TEs, their distribution over chromosomes and their frequencies in natural populations of mosquitoes, to further the current attempts to develop a transgenic mosquito unable to transmit malaria that is intended to replace the natural populations.

Quantitative genetics of vector competence for La Crosse virus and body size in Ochlerotatus hendersoni and Ochlerotatus triseriatus interspecific hybrids

La Crosse virus is a leading cause of pediatric encephalitis in the United States. The mosquito Ochlerotatus triseriatus is an efficient vector for La Crosse virus, whereas the closely related O. hendersoni transmits only at very low rates. Quantitative trait loci (QTL) affecting the ability to orally transmit this virus and adult body size were identified in 164 F(2) female individuals from interspecific crosses of O. hendersoni females and O. triseriatus males using a combination of composite interval mapping (CIM), interval mapping (IM) for binary traits, and single-marker mapping. For oral transmission (OT), no genome locations exceeded the 95% experimentwise threshold for declaring a QTL using IM, but single-marker analysis identified four independent regions significantly associated with OT that we considered as tentative QTL. With two QTL, an increase in OT was associated with alleles from the refractory vector, O. hendersoni, and likely reflect epistatic interactions between genes that were uncovered by our interspecific crosses. For body size, two QTL were identified using CIM and a third tentative QTL was identified using single-marker analysis. The genome regions associated with body size also contain three QTL controlling OT, suggesting that these regions contain either single genes with pleiotropic effects or multiple linked genes independently determining each trait.

Aedes aegypti genomics

The mosquito, Aedes aegypti, is the primary, worldwide arthropod vector for the yellow fever and dengue viruses. As it is also one of the most tractable mosquito species for laboratory studies, it has been and remains one of the most intensively studied arthropod species. This has resulted in the development of detailed genetic and physical maps for Ae. aegypti and considerable insight into its genome organization. The research community is well-advanced in developing important molecular tools that will facilitate a whole genome sequencing effort. This includes generation of BAC clone end sequences, physical mapping of selected BAC clones and generation of EST sequences. Whole genome sequence information for Ae. aegypti will provide important insight into mosquito chromosome evolution and allow for the identification of genes and gene function. These functions may be common to all mosquitoes or perhaps unique to individual species, possibly specific to host-seeking and blood-feeding behaviors, as well as the innate immune response to pathogens encountered during blood-feeding. This information will be invaluable to the global effort to develop novel strategies for preventing arthropod-borne disease transmission.

Quantitative trait loci conditioning transovarial transmission of La Crosse virus in the eastern treehole mosquito, Ochlerotatus triseriatus

Quantitative trait loci (QTL) affecting the ability of the Eastern treehole mosquito, Ochlerotatus triseriatus, to transovarially transmit (TOT) La Crosse virus (LAC) were mapped in an F1 intercross. The Holmen strain of O. triseriatus, previously selected for TOT refractoriness, was crossed to the AIDL strain that had been selected for TOT permissiveness. In P1 and F1 parents and 49 F2 individuals, regions of 10 cDNA loci were analysed with single strand conformation polymorphism (SSCP) analysis to identify and orientate linkage groups. Genotypes were also scored at fifty-six random amplified polymorphic DNA (RAPD)-SSCP loci. Twenty-eight F2 offspring were individually analysed for TOT. Three QTL for TOT were detected with standard interval mapping on chromosomes II and III. Alleles at the three loci contributed additively towards determining the overall TOT rate and cumulatively accounted for approximately 53% of the phenotypic variance in TOT.

The Anopheles genome and comparative insect genomics

The Anopheles gambiae genome sequence, coupled with the Drosophila melanogaster genome sequence, provides a better understanding of the insects, a group that contains our friends, foes, and competitors.

Linkage map organization of expressed sequence tags and sequence tagged sites in the mosquito, Aedes aegypti

A composite genetic linkage map for the yellow fever mosquito Aedes aegypti was constructed based on restriction fragment length polymorphism (RFLP), single nucleotide polymorphism (SNP) and single strand conformation polymorphism (SSCP) markers. The map consists of 146 marker loci distributed across 205 cM, and includes several morphological mutant marker loci. Most of the genetic markers are derived from random cDNAs or Ae. aegypti genes of known function. A number of markers are derived from random genomic DNAs, including several cloned RAPD-PCR fragments, and also several cDNAs from Drosophila melanogaster. Most of the random cDNAs (80.2%) have high BlastX sequence identities to known genes, with the majority of matches to genes from D. melanogaster. Access to sequence data for all markers will facilitate their continued development for use in high-throughput SNP marker analyses and also provides additional physical anchor points for an anticipated genome sequencing effort.