The palpal ratio method compared with PCR to distinguish between Anopheles gambiae s.s. and A. melas from Guinea Bissau, West Africa

Palp ratio as a field identification tool for two members of the Anopheles gambiae complex in Ghana (A. melas and A. gambiae)

Background

The Anopheles gambiae Giles complex is the most widely studied and the most important insect vector group. We explored the use of the palp ratio method as a field tool to identify A. melas and A. gambiae in Ghana.

Methods

Human landing catches were conducted to collect mosquitoes in the coastal area of Western Region of Ghana. Palps were removed and segments 3 and 4 + 5 measured using a compound microscope. DNA extraction and downstream PCR for species identification was carried out using the legs and wings. Known A. gambiae collected from the Ashanti Region of Ghana were used for comparison.

Results

A total of 2120 A. gambiae were collected. Lengths of segments 3 and 4 + 5 were significantly correlated in samples from both regions. Using a palp ratio of 0.81 as the cut-off value, 14.9 % outliers (≥0.81) from our study area were confirmed by PCR as A. melas. PCR also confirmed outliers from the Ashanti Region with palp ratio < 0.81 (10.2 %) as A. gambiae.

Conclusion

The palp ratio method proved to be a useful tool to identify populations of salt and freshwater A. melas and A. gambiae.

First report of an exophilic Anopheles arabiensis population in Bissau City, Guinea-Bissau: recent introduction or sampling bias?

Background

The malaria vector Anopheles arabiensis exhibits greater behavioural and ecological plasticity than the other major vectors of the Anopheles gambiae complex, which presents challenges for major control methods. This study reports for the first time the presence of An. arabiensis in Antula, a suburb of Bissau city, the capital of Guinea Bissau, where high levels of hybridization between Anopheles coluzzii and An. gambiae have been reported. Given that previous surveys in the area, based on indoor collections, did not sample An. arabiensis, the possibility of a recently introduced exophilic population was investigated.

Methods

Larval and adult mosquito collections were carried out in Antula at the end of the rainy season of 2010. Anopheles gambiae species composition, determined by rDNA-IGS and SINE200X6.1 markers, was compared with four previously collected samples dating back to 1993. Analysis of ten microsatellites was used to estimate levels of genetic diversity, relatedness and to investigate demographic stability.

Results

Anopheles arabiensis comprised 54.0% of larvae and 25.6% of adults collected in 2010, but was absent in all previous collections, a highly unlikely observation by chance if the population was stable. This species had the lowest levels of genetic diversity, highest relatedness and, along with An. gambiae, exhibited evidence of a recent population expansion.

Conclusions

Results point to the presence of a previously undetected outdoor population of An. arabiensis in Antula, which appears to have expanded recently, highlighting the importance of complementing indoor-based mosquito collections with sampling methods targeting outdoor adults and immature stages for a more complete assessment of mosquito biodiversity. A change in temporal dynamics in the species complex composition was also detected. Coupled with previous evidence of asymmetric introgression from An. coluzzii to An. gambiae, this suggests that the study area may be subject to ecological changes with a potential impact on both the genetics of these species and on malaria transmission.

Electronic supplementary material

The online version of this article (doi:10.1186/1475-2875-13-423) contains supplementary material, which is available to authorized users.

High levels of hybridization between molecular forms of Anopheles gambiae from Guinea Bissau

In the malaria vector Anopheles gambiae Giles sensu stricto, two molecular forms denoted M and S are considered units of incipient speciation within this species. Very low hybrid frequencies and significant genetic differentiation have been found in sympatric M- and S-form populations. We studied the molecular form composition and the degree of genetic differentiation at 15 microsatellites in two samples of An. gambiae collected in two consecutive years from Bissau, Guinea Bissau. High frequencies of M/S hybrids (19-24%) were found in this area. Coincidently, very low levels of genetic differentiation were detected between forms when analysis involved microsatellites mapped at chromosome-3 (mean Fst, 0.000-0.002). The single exception was the X-linked AGXH678, for which high differentiation was measured (Fst, 0.158-0.301). This locus maps near the centromere of chromosome X, a low recombination region in which selection is likely to promote divergence between M and S forms. These results strongly suggest that the degree of isolation between M and S forms, considered the units of incipient speciation within An. gambiae, is not homogenous throughout the species distribution range.

Endophilic Anopheles mosquitoes in Guinea Bissau, west Africa, in relation to human housing conditions

Environmental risk factors associated with increased malaria mosquito (Anopheles) abundance indoors were studied in a suburban area, Antula, of Guinea Bissau, during the rainy seasons of 1993-1995. All bedrooms in 30 houses were searched for resting mosquitoes three times each year. The most abundant mosquito was An. gambiae s.s. Significantly greater numbers of resting mosquitoes were present in rooms with open eaves and in houses with a well on the compound. Pigs were the most common domestic animals in Antula. Presence of pigs in a house was associated with increased mosquito abundance in the bedrooms of the same house. The abundance of female mosquitoes also increased with increasing human biomass per square meter of bedroom area.