Currently, software systems have become increasingly large and complex, often resulted by the integration of several operationally independent systems, resulting in a new class of systems: the Systems of Systems (SoS). In another perspective, software architectures play a major role in determining system quality, since they form the backbone of any successful software-intensive system. Attention given to the software architectures of SoS is also certainly fundamental to the success of such systems. However, it is observed that there is a lack of works that present a wide and, at the same time, a detailed panorama about how SoS architectures have been treated. In this scenario, the main contribution of this paper is to present the state of the art on software architectures of SoS, mainly regarding their development, representation, evaluation, and evolution. This work also contributes with future research topics on SoS architectures that should be still investigated. Besides that, we intend this paper opens new perspectives of research in the software architecture area, intending to contribute to the success of SoS.