The rift valley complex as a barrier to gene flow for Anopheles gambiae inKenya

Recent studies of Anopheles gambiae, the principal mosquito vector of malar ia in Africa, suggested that the eastern Rift Valley and its surrounding ar eas act as a barrier to gene flow. To quantify the unique effect of these a reas on gene flow, we measured genetic variation within and between populat ions from each side of the Rift. Low differentiation was measured between p opulations on each side of the Rift (mean F-ST < 0.008, mean R-ST < 0.002), However, high differentiation was measured across the Rift (mean F-ST = 0. 104; mean R-ST = 0.032), Genetic diversity within populations was lower in eastern populations, suggesting that the effective population sizes (N-e) o f these populations were lower than those of western populations. We partit ioned the overall differentiation across the Rift into three factors: varia tion in N-e between populations contributed 7-20%; distance contributed 10- 30%, and the remainder, corresponding to the unique effect of the Rift was 50-80%, The Rift's effect was highly significant based on F-ST. The greater sensitivity of F-ST in measuring differentiation indicated that drift and not mutation generated the differences between populations. Restricted gene exchange across several hundred kilometers on the face of intense human tr ansportation implies that active mosquito dispersal is the major form of mi gration, and that migration is a multistep process, where step length is re latively short.