Anopheles gambiae complex along The Gambia river, with particular reference to the molecular forms of An. gambiae s.s

Abstract

Background: The geographic and temporal distribution of M and S molecular forms of the major Afrotropical malaria vector species Anopheles gambiae s.s. at the western extreme of their range of distribution has never been investigated in detail.

Materials and methods: Collections of indoor-resting An. gambiae s.l. females were carried out along a ca. 400 km west to east transect following the River Gambia from the western coastal region of The Gambia to south-eastern Senegal during 2005 end of rainy season/early dry season and the 2006 rainy season. Specimens were identified to species and molecular forms by PCR-RFLP and the origin of blood-meal of fed females was determined by ELISA test.

Results: Over 4,000 An. gambiae s.l. adult females were collected and identified, 1,041 and 3,038 in 2005 and 2006, respectively. M-form was mainly found in sympatry with Anopheles melas and S-form in the western part of the transect, and with Anopheles arabiensis in the central part. S-form was found to prevail in rural Sudan-Guinean savannah areas of Eastern Senegal, in sympatry with An. arabiensis. Anopheles melas and An. arabiensis relative frequencies were generally lower in the rainy season samples, when An. gambiae s.s. was prevailing. No large seasonal fluctuations were observed for M and S-forms. In areas where both M and S were recorded, the frequency of hybrids between them ranged from to 0.6% to 7%.

Discussion: The observed pattern of taxa distribution supports the hypothesis of a better adaptation of M-form to areas characterized by water-retaining alluvial deposits along the Gambia River, characterized by marshy vegetation, mangrove woods and rice cultivations. In contrast, the S-form seems to be better adapted to free-draining soil, covered with open woodland savannah or farmland, rich in temporary larval breeding sites characterizing mainly the eastern part of the transect, where the environmental impact of the Gambia River is much less profound and agricultural activities are mainly rain-dependent. Very interestingly, the observed frequency of hybridization between the molecular forms along the whole transect was much higher than has been reported so far for other areas.

Conclusion: The results support a bionomic divergence between the M and S-forms, and suggest that the western extreme of An. gambiae s.s. geographical distribution may represent an area of higher-than-expected hybridization between the two molecular forms.

Figure 1

Distribution of species and molecular forms of Anopheles gambiae complex in the study area. Numbers (and sample sizes) above pie-charts refer to samples, as listed in Table 2. Dashed lines indicate the boundaries of the arbitrarily defined areas and sub-areas (see text for details): Lower River Area, Western (LRA-W); Lower River Area, South Bank (LRA-S); Lower River Area, North Bank (LRA-N); Central River Area (CRA); Upper River Area (URA); Eastern Area (EA). ERS-2005 = late rainy-early dry season 2005 (below); RS-2006 = rainy season 2006 (above).

Figure 2

Distribution of molecular forms of Anopheles gambiae s.s. in the study area. Numbers (and sample sizes) above pie-charts refer to samples with N>10, as listed in Table 3. Dashed lines indicate the boundaries of the arbitrarily defined areas and sub-areas (see text for details): Lower River Area, Western (LRA-W); Lower River Area, South Bank (LRA-S); Lower River Area, North Bank (LRA-N); Central River Area (CRA); Upper River Area (URA); Eastern Area (EA). ERS-2005 = late rainy-early dry season 2005 (below); RS-2006 = rainy season 2006 (above).