Abstract
BackgroundWidely distributed species with populations adapted to different environmental conditions can provide valuable opportunities for tracing the onset of reproductive incompatibilities and their role in the speciation process. Drosophila montana, a D. virilis group species found in high latitude boreal forests in Nearctic and Palearctic regions around the globe, could be an excellent model system for studying the early stages of speciation, as a wealth of information concerning this species' ecology, mating system, life history, genetics and phylogeography is available. However, reproductive barriers between populations have hereto not been investigated.ResultsWe report both pre- and postmating barriers to reproduction between flies from European (Finnish) and North American (Canadian) populations of Drosophila montana. Using a series of mate-choice designs, we show that flies from these two populations mate assortatively (i.e., exhibit significant sexual isolation) while emphasizing the importance of experimental design in these kinds of studies. We also assessed potential postmating isolation by quantifying egg and progeny production in intra- and interpopulation crosses and show a significant one-way reduction in progeny production, affecting both male and female offspring equally.ConclusionWe provide evidence that allopatric D. montana populations exhibit reproductive isolation and we discuss the potential mechanisms involved. Our data emphasize the importance of experimental design in studies on premating isolation between recently diverged taxa and suggest that postmating barriers may be due to postcopulatory-prezygotic mechanisms. D. montana populations seem to be evolving multiple barriers to gene flow in allopatry and our study lays the groundwork for future investigations of the genetic and phenotypic mechanisms underlying these barriers.
Highlights
Distributed species with populations adapted to different environmental conditions can provide valuable opportunities for tracing the onset of reproductive incompatibilities and their role in the speciation process
Allopatric populations in the initial stages of divergence, can provide valuable opportunities to study the onset of reproductive barriers and fertile grounds for testing hypotheses concerning the roles of environmental adaptation, sexual selection and random genetic divergence in generating reproductive barriers among populations
In the present study we measured the strengths of reproductive barriers, and gained some information on the mechanisms underlying these barriers, between two geographically isolated D. montana populations adapted to different kinds of environmental conditions
Summary
Distributed species with populations adapted to different environmental conditions can provide valuable opportunities for tracing the onset of reproductive incompatibilities and their role in the speciation process. In non-ecological speciation, genetic divergence of populations occurs through the fixation of different advantageous mutations in each population, even though they are adapting to similar environmental conditions (mutation-order model; [6]) In this case, even though the same alleles may be favored in both populations due to the similarity of their environments, they may not exhibit the same mutations or fix them in the same order, such that when the populations come back into contact, incompatible alleles may interact negatively in hybrids creating pre- and/or postzygotic reproductive barriers
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