Host repertoire: Community structure shaped by host evolution
The evolutionary history of the hosts can have a significant impact on the community structure in host-parasite networks (e.g. Mouillotet al. 2008a; Braga et al. 2014, 2020). Here, we show that the realised host repertoire of most species of Cichlidogyrus is determined more by the host evolutionary history than the host environment. First, host species differed more regarding the ecological niche than the phylogenetic relationships as evidenced by a reduction of the mean functional-phylogenetic distances (FPDist) with increasing phylogenetic weight (a → 1) (Fig. 4). Second, FPDist estimates rarely differed from the null distribution for increasing functional weight (a → 0) (but see C. sp. ‘nyanza in Fig. 4). Third, estimates outside the null distribution were always underdispersed (clustered), i.e. lower than expected at random at both the ancient and recent evolutionary scale (measured as MPD and MNTD respectively). The strong phylogenetic influence and underdispersion have previously been associated with co-divergent evolution (Clark & Clegg 2017). Co-divergence assumes that host and parasite phylogenies are phylogenetically congruent (Page 2003; Hoyal Cuthill & Charleston 2012), a pattern that has already been observed for species ofCichlidogyrus (Vanhove et al. 2015). In fact, host repertoires observed here frequently coincide with related groups of host species. According to the Stockholm Paradigm , congruence might arise especially in younger lineages that have experienced a phase of isolation resulting in co-diverging host and parasite lineages (Agosta & Brooks 2020), e.g. species of Cichlidogyrus infecting tropheine cichlids (Vanhove et al. 2015), which arose 7–11 MYA (Schedel et al. 2019).
Another explanation for the important role of the host phylogeny on the host range of the parasites might be phylogenetic constraints. Within the same ecosystem, parasites cannot simply infect any host but are often limited to compatible host lineages. For instance, phylogenetic relationships are reportedly determinants of neotropical (Braga et al. 2014) or Mediterranean (Desdevises et al. 2002) monogenean-fish communities but also in plant-pollinator, plant-frugivore (Rezende et al. 2007), plant-mycorrhiza (Jacquemyn et al. 2011), and other host-parasite networks (Mouillot et al. 2008). Yet natural (Birgi & Euzet 1983; Birgi & Lambert 1986) and invasion-induced (Jiménez-Garcia et al.2001; Šimková et al. 2019; Jorissen et al. 2020) host switches of species of Cichlidogyrus highlight that fundamental host repertoires might differ considerably from the realised host repertoires estimated here. Monogeneans have the capacity to infect new hosts and adapt to new environments (Braga et al. 2014). For instance, the attachment organ, as the main physical connection of the parasite to its host, is often used as a proxy for evolutionary processes in monogenean flatworms (e.g. Jarkovský et al. 2004; Vignon et al. 2011). In some species of Cichlidogyrus that underwent recent host switches (Messu Mandeng et al. 2015), the organ’s morphology diverges considerably from those of close relatives suggesting that several species were able to expand their host ranges and make subsequent morphological changes. Consequently, phylogenetic constraints might play less of a role in cichlid-Cichlidogyrusthan the observed host repertoires suggest at the first glance.