Quantitative genetic inheritance of morphological divergence in a lake-stream stickleback ecotype pair: implications for reproductive isolation

Ecological selection against hybrids between populations occupying differenthabitats might be an important component of reproductive isolation during theinitial stages of speciation. The strength and directionality of this barrier to geneflow depends on the genetic architecture underlying divergence in ecologicallyrelevant phenotypes. We here present line cross analyses of inheritance for twokey foraging-related morphological traits involved in adaptive divergencebetween stickleback ecotypes residing parapatrically in lake and stream habitatswithin the Misty Lake watershed (Vancouver Island, Canada). One mainfinding is the striking genetic dominance of the lake phenotype for body depth.Selection associated with this phenotype against first- and later-generationhybrids should therefore be asymmetric, hindering introgression from the laketo the stream population but not vice versa. Another main finding is thatdivergence in gill raker number is inherited additively and should thereforecontribute symmetrically to reproductive isolation. Our study suggests thattraits involved in adaptation might contribute to reproductive isolationqualitatively differently, depending on their mode of inheritance.