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Proc Natl Acad Sci U S A. 2017 Oct 10;114(41):10936-10941. doi: 10.1073/pnas.1711238114. Epub 2017 Sep 25.

Assortative mating and persistent reproductive isolation in hybrids.

Schumer M1,2,3, Powell DL3,4,5, Delclós PJ3,4,5, Squire M3,4,5, Cui R3,6, Andolfatto P7,8, Rosenthal GG3,4,5.

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Harvard Society of Fellows, Harvard University, Cambridge, MA 02138;
Department of Biological Sciences, Columbia University, New York, NY 10027.
Centro de Investigaciones Científicas de las Huastecas "Aguazarca", Calnali 43230, Hidalgo, Mexico.
Department of Biology, Texas A&M University, College Station, TX 77843.
Program in Ecology and Evolutionary Biology, Texas A&M University, College Station, TX 77843.
Max Planck Institute for the Biology of Aging, 50931 Cologne, Germany.
Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544.
The Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, NJ 08544.


The emergence of new species is driven by the establishment of mechanisms that limit gene flow between populations. A major challenge is reconciling the theoretical and empirical importance of assortative mating in speciation with the ease with which it can fail. Swordtail fish have an evolutionary history of hybridization and fragile prezygotic isolating mechanisms. Hybridization between two swordtail species likely arose via pollution-mediated breakdown of assortative mating in the 1990s. Here we track unusual genetic patterns in one hybrid population over the past decade using whole-genome sequencing. Hybrids in this population formed separate genetic clusters by 2003, and maintained near-perfect isolation over 25 generations through strong ancestry-assortative mating. However, we also find that assortative mating was plastic, varying in strength over time and disappearing under manipulated conditions. In addition, a nearby population did not show evidence of assortative mating. Thus, our findings suggest that assortative mating may constitute an intermittent and unpredictable barrier to gene flow, but that variation in its strength can have a major effect on how hybrid populations evolve. Understanding how reproductive isolation varies across populations and through time is critical to understanding speciation and hybridization, as well as their dependence on disturbance.


assortative mating; hybridization; population structure; reproductive isolation

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