1. Hyperparasitism

All living organisms can take part in parasitic relationships, either as parasites or as hosts (Combes, 2001; Krasylenko et al., 2021). Interactions between parasites and their hosts are typically regarded as closed one-to-one systems. In reality, however, these relationships involve complex multitrophic interactions (Kiss, 2001). The term “hyperparasite” refers to an organism that parasitizes another parasitic organism (Fig. 1). Hyperparasitism has been well documented for many groups of organisms, mainly insect parasitoids associated with parasitoid hosts, viruses that parasitize disease-causing protozoans, and parasitic flowering plants (Grybchuk et al., 2018; Krasylenko et al., 2021; Sullivan, 1987). Hyperparasitism by fungi is poorly studied, even though it is thought to be rather widespread in nature (Haelewaters et al., 2018a, 2021a; Parratt and Laine, 2016; Sun et al., 2019). As fungi are able to parasitize organisms from different kingdoms (Moore et al., 2020), this chapter focuses on fungal hyperparasites parasitic on both fungal and non-fungal hosts.