Genetic exchange in 2La inversion heterokaryotypes of Anopheles gambiae

In the malaria vector Anopheles gambiae, alternative arrangements of chromosome 2 (2La and 2L+^a) vary in relative frequency along clines of aridity, suggesting the action of natural selection on targets within the inversion. Our long term goal of detecting such targets depends in part on the level of genetic exchange between arrangements. Accordingly, we estimated recombination rates on 2L from the backcross progeny of 2La/+^a heterokaryotypes and as a control, from 2L+^a homokaryotypes. In homokaryotypes, the recombination rate was uniform at ∼2.0 centimorgans per megabase (cM/Mb). In heterokaryotypes, recombination within the rearranged region was reduced to < 0.5 cM/Mb, with slightly higher but nevertheless reduced levels (< 1.0 cM/Mb) flanking the rearrangement. Yet, gene exchange was recorded between nearly all markers, including those very near the distal inversion breakpoint. These results suggest that reduced recombination is a necessary but not sufficient mechanism for genetic isolation between alternative arrangements, and that the targets of natural selection can be identified against the different chromosomal backgrounds.