2.1. Hyperparasitism, mycoparasitism, and fungicolous fungi

The term hyperparasitism was introduced by Boosalis (1964) as an alternative for mycoparasitism and used in reference to the phenomenon of one fungus parasitic on another fungus. Although similar, these terms imply two different things. “Mycoparasitism” is a phenomenon in which one fungus (the mycoparasite) parasitizes another fungus (the host), regardless of whether the host is a saprotroph, mutualist, parasite, or commensalist (Karlsson et al., 2018; Moore et al., 2020). Moreover, mycoparasitism typically involves cell wall degradation and, in most cases, penetration of the host cells, e.g., as in the mycoparasitic activity of Trichoderma harzianum against Rhizoctonia solani (Altomare et al., 1999; Atanasova et al., 2013; Sun et al., 2019).
In contrast, “hyperparasitism” occurs only if the host is also a parasite (Bermúdez-Cova et al., 2022; Faticov et al., 2022; Haelewaters et al., 2018a; Piepenbring, 2015). It is important to note that hyperparasitic fungi use different methods to interact with their hosts (Boosalis, 1964; Jeffries, 1995), from hyphae or haustoria that penetrate host tissues to hyphal contact without penetration, to buffer cells that may facilitate flow of nutrients from host to parasite (Barnett and Lilly, 1958). For a fungus – or any other organism – to be considered a hyperparasite, it needs to negatively impact host fitness, otherwise it would be referred to as a “hypermutualist” or “hypercommensal” (Kaishian et al., 2023; Northrup et al., 2021). A hyperparasitic interaction consists of at least three trophic levels (Fig. 2): a primary host, which is parasitized by a primary parasite, which serves as secondary host to a secondary parasite or hyperparasite.