Résumé
Explaining how and why reproductive isolation evolves and determining
which forms of reproductive isolation have the largest impact on the
process of population divergence are major goals in the study of
speciation. By studying recent adaptive radiations in incompletely
isolated taxa, it is possible to identify barriers involved at early
divergence before other confounding barriers emerge after speciation is
complete. Sibling species of the Anopheles gambiae complex offer
opportunities to provide insights into speciation mechanisms. Here we
studied patterns of reproductive isolation among three taxa, An. coluzzii,
An. gambiae s.s. and An. arabiensis, to compare its strength at different
spatial scales, to dissect the relative contribution of pre- versus
post-mating isolation, and to infer the involvement of ecological
divergence on hybridization. Because F1 hybrids are viable, fertile, and
not uncommon, understanding the dynamics of hybridization in this trio of
major malaria vectors has important implications for how adaptations arise
and spread across the group, and in planning studies of the safety and
efficacy of gene drive as a means of malaria control. We first performed a
systematic review and meta-analysis of published surveys reporting on
hybrid prevalence, showing strong reproductive isolation at a continental
scale despite geographically restricted exceptions. Second, we exploited
our own extensive field datasets collected at a regional scale in two
contrasting environmental settings, in order to assess: i) levels of
pre-mating isolation; ii) spatio/temporal and frequency-dependent dynamics
of hybridization, iii) relationship between reproductive isolation and
ecological divergence, and iv) hybrid viability penalty. Results are in
accordance with ecological speciation theory predicting a positive
association between the strength of reproductive isolation and degree
ecological divergence, and indicate that post-mating isolation does
contribute to reproductive isolation among these species. Specifically,
only post-mating isolation was positively associated with ecological
divergence, whereas pre-mating isolation was correlated with phylogenetic
distance.