title: Sexual isolation promotes divergence between parapatric lake and stream stickleback.
abstract: Speciation can be initiated by adaptive divergence between populations in ecologically different habitats, but how sexually based reproductive barriers contribute to this process is less well understood. We here test for sexual isolation between ecotypes of threespine stickleback fish residing in adjacent lake and stream habitats in the Lake Constance basin, Central Europe. Mating trials exposing females to pairings of territorial lake and stream males in outdoor mesocosms allowing for natural reproductive behaviour reveal that mating occurs preferentially between partners of the same ecotype. Compared to random mating, this sexual barrier reduces gene flow between the ecotypes by some 36%. This relatively modest strength of sexual isolation is surprising because comparing the males between the two ecotypes shows striking differentiation in traits generally considered relevant to reproductive behaviour (body size, breeding coloration, nest size). Analysing size differences among the individuals in the mating trials further indicates that assortative mating is not related to ecotype differences in body size. Overall, we demonstrate that sexually based reproductive isolation promotes divergence in lake-stream stickleback along with other known reproductive barriers, but we also caution against inferring strong sexual isolation from the observation of strong population divergence in sexually relevant traits.
authors.affiliation: Zoological Institute, University of Basel, Basel, Switzerland.
authors.forename: D
authors.initials: D
authors.lastname: Berner
authors.affiliation: Zoological Institute, University of Basel, Basel, Switzerland.
authors.forename: M
authors.initials: M
authors.lastname: Ammann
authors.affiliation: Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Southampton, UK.
authors.forename: E
authors.initials: E
authors.lastname: Spencer
authors.affiliation: Zoological Institute, University of Basel, Basel, Switzerland.
authors.forename: A
authors.initials: A
authors.lastname: Rüegg
authors.affiliation: Zoological Institute, University of Basel, Basel, Switzerland.
authors.forename: D
authors.initials: D
authors.lastname: Lüscher
authors.affiliation: Zoological Institute, University of Basel, Basel, Switzerland.
authors.forename: D
authors.initials: D
authors.lastname: Moser
mesh_terms: D000818:Animals; D005075:Biological Evolution; D005060:Europe; D005260:Female; D051456:Gene Flow; D060106:Lakes; D008297:Male; D060047:Reproductive Isolation; D045483:Rivers; D023701:Smegmamorpha
keywords: 
  Gasterosteus aculeatus
  ; assortative mating; body size; ecological speciation; mate choice; mesocosm; nest; nuptial coloration; reproductive isolation