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.