Larval habitat segregation between the molecular forms of the mosquito Anopheles gambiae in a rice field area of Burkina Faso, West Africa

Abstract

In West Africa, lineage splitting between the M and S molecular forms of the major Afro-tropical malaria mosquito, Anopheles gambiae (Diptera: Culicidae), is thought to be driven by ecological divergence, occurring mainly at the larval stage. Here, we present evidence for habitat segregation between the two molecular forms in and around irrigated rice fields located within the humid savannahs of western Burkina Faso. Longitudinal sampling of adult mosquitoes emerging from a range of breeding sites distributed along a transect extending from the heart of the rice field area into the surrounding savannah was conducted from June to November 2009. Analysis revealed that the two molecular forms and their sibling species Anopheles arabiensis are not randomly distributed in the area. A major ecological gradient was extracted in relation to the perimeter of the rice fields. The M form was associated with larger breeding sites mostly consisting of rice paddies, whereas the S form and An. arabiensis were found to depend upon temporary, rain-filled breeding sites. These results support hypotheses about larval habitat segregation and confirm the suggestion that the forms have different larval habitat requirements. Segregation appears to be clearly linked to anthropogenic permanent habitats and the community structure they support.

Figure 1

Relative frequency and distribution of members of the An. gambiae complex emerging from breeding sites in the area of Bama, Western Burkina Faso. Above: position of Burkina Faso in Africa and localization of the area of Bama (blue dot) in Burkina Faso (Source NDVI: http://free.vgt.vito.be/index.php). Below: the area of Bama (Wellens et al., 2008) showing the rice-fields perimeter (red) and the 80 breeding sites (yellow dots) monitored throughout the 2009 rainy season. Pie charts show the relative frequencies of the members of the An. gambiae s.l. complex emerging from each larval development site (data pooled across collections dates). The M form is in black, the S form is in white and An. arabiensis is in grey.

Figure 2

Logistic plot of relative frequencies of the M and S form of An. gambiae and An. arabiensis in relation to distance to the rice-fields perimeter.

Figure 3

Multifactorial analysis of the distribution of An. gambiae complex mosquitoes in relation to environmental variables. Principal component analysis (PCA) diagram is shown in A with environmental variables passively plotted. Ordination diagram from redundancy analysis (RDA) is shown in B and species-environment biplot from partial redundancy analysis (RDA) in C. For each graph the proportion of total variance in species data explained by each axis is given in brackets. Blue triangles represent nominal environmental factors and red arrows represent continuous ones. Mosquito taxa are represented by black arrows and collection months by brown crosses.