The Internet of Things (IoT) has emerged as a paradigm in which smart things actively collaborate among them and with other physical and virtual objects available in the Web in order to perform high level tasks. These things can be engaged in complex relationships including the composition and collaboration with other independent and heterogeneous systems in order to provide new functionalities, thus leading to the so-called systems-of-systems (SoS). In the context of integrating IoT-based systems in order to compose complex, large-scale SoS, this paper presents a systematic mapping aimed to discuss current scenarios and approaches in the development of IoT-based SoS, as well as some challenges and research opportunities in this context.