Systems-of-systems (SoS) represent a class of systems resulted from the interaction among independent systems that cooperate to form a larger and more complex system aiming at accomplishing global missions. An inherent characteristic of SoS is the high heterogeneity of their constituent systems, which are distributed, independent, and developed with different technologies. In addition, SoS are highly dynamic, so that their constituents are often partially known or even unknown at design time. As a consequence, these constituent systems need to be discovered, selected, and composed at runtime towards identifying the proper arrangements that contribute to the accomplishment of the global missions of the SoS. In this paper, we present the results of a systematic mapping aimed to investigate the existing approaches to discover and compose constituent systems within an SoS. Besides providing an overview of the state of the art on these topics, we shed light on important issues to be addressed by future research towards a more effective development of SoS.