Online discussion is essential for the communication and collaboration of online communities. The reciprocal exchange of messages between users that characterizes online discussion can be represented in many different ways. While some platforms display messages chronologically using a simple linear interface, others use a hierarchical (threaded) interface to represent more explicitly the structure of the discussion. Although the type of representation has been shown to affect communication, to the best of our knowledge, the impact of using either one or the other has not yet been investigated in a large and mature online community. In this work we analyze Meneame, a popular Spanish social news platform which recently transitioned from a linear to a hierarchical interface, becoming an ideal research opportunity for this purpose. Using interrupted time series analysis and regression discontinuity design, we observe an abrupt and significant increase in social reciprocity after the adoption of a threaded interface. We furthermore extend state-of-the-art generative models of discussion threads by including reciprocity, a fundamental feature to explain better the structure of the discussions, both before and after the change in the interface.