Résumé
Chromosome inversions have long been thought to be involved in speciation
and local adaptation. We have little quantitative information, however,
about the effects that inversion polymorphisms have on reproductive
isolation and viability. Here we provide the first estimates from any
organism for the total amount of reproductive isolation associated with an
inversion segregating in natural populations. We sampled chromosomes from
751 mosquitoes of the malaria vector Anopheles funestus along a 1421 km
transect in Cameroon that traverses savannah, highland, and rainforest
ecological zones. We then developed a series of population genetic models
that account for selection, migration, and assortative mating, and fit the
models to the data using likelihood. Results from the best-fit models
suggest there is strong local adaptation, with relative viabilities of
homozygotes ranging from 25% to 130% compared to heterozygotes.
Viabilities vary qualitatively between regions: the inversion is
underdominant in the savannah, while in the highlands it is overdominant.
The inversion is also implicated in strong assortative mating. In the
savannah, the two homozygote forms show 92% reproductive isolation,
suggesting that this one inversion can generate most of the genetic
barriers needed for speciation.