Abstract
Within the Anopheles gambiae complex, the sibling species An. coluzzii and An. gambiae are undergoing sympatric speciation. These species are characterized by rare hybrids in most of their geographical distribution. A strong assortative mating mediated by spatial swarm segregation has been shown whereas no intrinsic post-zygotic barriers have been found in laboratory conditions. To test the role of the hybridisation in reproductive isolation in natural populations transplant experiment are therefore needed to establish the significance of post-zygotic barriers. Previous studies indicated that predation is one of the major forces driving ecological divergence between An. gambiae and An. coluzzii. Here we extended these studies to their hybrids. Parental species and their F1 hybrids from reciprocal crosses were generated by the forced-mating technique as follows: female An. coluzzii × male An. coluzzii; female An. coluzzii × male An. gambiae; female An. gambiae × male An. coluzzii and female An. gambiae × Male An. gambiae. First instar larvae of each group from the crossing (here after An. coluzzii, Hybrid COL/GAM, Hybrid GAM/COL and An. gambiae, respectively) were transplanted in a field experiment with predation effect. Emergence success, development time of larvae and body size of the newly emerging adults were estimated as fitness components and then compared between parental species and F1 hybrids in absence and in presence of predators. Our findings confirm that An. coluzzii had higher fitness than An. gambiae in presence of predators versus in absence of predators. Moreover, the fitness of the F1 hybrid COL/GAM whose female parent was An. coluzzii matched that of An. coluzzii while that of the F1 reciprocal hybrid GAM/COL was similar to An. gambiae.
Highlights
Speciation involves the evolution of barriers to gene flow between diverging populations
The larvae were composed of An. gambiae, An. coluzzii and the two reciprocal F1 hybrids (i.e the hybrid COL/GAM and the hybrid GAM/COL)
The mortality rate was significantly higher for An. gambiae than it was for An. coluzzii in presence of predator, but the reverse was true in absence of predators, thereby confirming that diverging responses to predators and the resulting genotypeenvironment interaction (G×E) may play an important role in niche partitioning and speciation
Summary
Speciation involves the evolution of barriers to gene flow between diverging populations. Understanding speciation implies two major tasks: determining which reproductive barriers are involved in the reduction in gene flow between populations and, understanding which evolutionary forces produce them. Dependent post-zygotic isolation within incipient species of the An. gambiae complex biological processes in the formation of new species, that acts on individuals to let only the mostly fit offspring survive and reproduce to their full potential. As an ecologically-dependent post-zygotic reproductive isolation occurs between species when hybrids are less efficient at exploiting parental environments and an intermediate environment is lacking [1, 2]. During the early phase of sympatric divergence, species may have been incompletely reproductively isolated populations, suggesting that ecotypes are excellent models for studying ecological speciation
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