article published yes Sean Stankowski author 43161670-5719-11EA-8025-FABC3DDC885E Anja M Westram author 3C147470-F248-11E8-B48F-1D18A9856A870000-0003-1050-4969 Zuzanna B. Zagrodzka author Isobel Eyres author Thomas Broquet author Kerstin Johannesson author Roger K. Butlin author NiBa department The evolution of strong reproductive isolation (RI) is fundamental to the origins and maintenance of biological diversity, especially in situations where geographical distributions of taxa broadly overlap. But what is the history behind strong barriers currently acting in sympatry? Using whole-genome sequencing and single nucleotide polymorphism genotyping, we inferred (i) the evolutionary relationships, (ii) the strength of RI, and (iii) the demographic history of divergence between two broadly sympatric taxa of intertidal snail. Despite being cryptic, based on external morphology, Littorina arcana and Littorina saxatilis differ in their mode of female reproduction (egg-laying versus brooding), which may generate a strong post-zygotic barrier. We show that egg-laying and brooding snails are closely related, but genetically distinct. Genotyping of 3092 snails from three locations failed to recover any recent hybrid or backcrossed individuals, confirming that RI is strong. There was, however, evidence for a very low level of asymmetrical introgression, suggesting that isolation remains incomplete. The presence of strong, asymmetrical RI was further supported by demographic analysis of these populations. Although the taxa are currently broadly sympatric, demographic modelling suggests that they initially diverged during a short period of geographical separation involving very low gene flow. Our study suggests that some geographical separation may kick-start the evolution of strong RI, facilitating subsequent coexistence of taxa in sympatry. The strength of RI needed to achieve sympatry and the subsequent effect of sympatry on RI remain open questions. The Royal Society2020 eng 1471-2970 32654639 00055266210001410.1098/rstb.2019.0545 3751806 Stankowski, Sean, Anja M Westram, Zuzanna B. Zagrodzka, Isobel Eyres, Thomas Broquet, Kerstin Johannesson, and Roger K. Butlin. “The Evolution of Strong Reproductive Isolation between Sympatric Intertidal Snails.” <i>Philosophical Transactions of the Royal Society. Series B: Biological Sciences</i>. The Royal Society, 2020. <a href="https://doi.org/10.1098/rstb.2019.0545">https://doi.org/10.1098/rstb.2019.0545</a>. Stankowski, S., Westram, A. M., Zagrodzka, Z. B., Eyres, I., Broquet, T., Johannesson, K., &#38; Butlin, R. K. (2020). The evolution of strong reproductive isolation between sympatric intertidal snails. <i>Philosophical Transactions of the Royal Society. Series B: Biological Sciences</i>. The Royal Society. <a href="https://doi.org/10.1098/rstb.2019.0545">https://doi.org/10.1098/rstb.2019.0545</a> Stankowski S, Westram AM, Zagrodzka ZB, et al. The evolution of strong reproductive isolation between sympatric intertidal snails. <i>Philosophical Transactions of the Royal Society Series B: Biological Sciences</i>. 2020;375(1806). doi:<a href="https://doi.org/10.1098/rstb.2019.0545">10.1098/rstb.2019.0545</a> S. Stankowski, A.M. Westram, Z.B. Zagrodzka, I. Eyres, T. Broquet, K. Johannesson, R.K. Butlin, Philosophical Transactions of the Royal Society. Series B: Biological Sciences 375 (2020). Stankowski, Sean, et al. “The Evolution of Strong Reproductive Isolation between Sympatric Intertidal Snails.” <i>Philosophical Transactions of the Royal Society. Series B: Biological Sciences</i>, vol. 375, no. 1806, 20190545, The Royal Society, 2020, doi:<a href="https://doi.org/10.1098/rstb.2019.0545">10.1098/rstb.2019.0545</a>. S. Stankowski <i>et al.</i>, “The evolution of strong reproductive isolation between sympatric intertidal snails,” <i>Philosophical Transactions of the Royal Society. Series B: Biological Sciences</i>, vol. 375, no. 1806. The Royal Society, 2020. Stankowski S, Westram AM, Zagrodzka ZB, Eyres I, Broquet T, Johannesson K, Butlin RK. 2020. The evolution of strong reproductive isolation between sympatric intertidal snails. Philosophical Transactions of the Royal Society. Series B: Biological Sciences. 375(1806), 20190545. 81672020-07-26T22:01:01Z2023-08-22T08:22:13Z