Analysis of parasite host-switching: limitations on the use of phylogenies

Abstract

SUMMARYEven the most generalist parasites usually occur in only a subset of potential host species, a tendency which reflects overriding environmental constraints on their distributions in nature. The periodic shifting of these limitations represented by host-switches may have been an important process in the evolution of many host-parasite assemblages. To study such events, however, it must first be established where and when they have occurred. Past host-switches within a group of parasites are usually inferred from a comparison of the parasite phylogeny with that of the hosts. Congruence between the phylogenies is often attributed to a history of association by descent with cospeciation, and incongruence to host-switching or extinction in ‘duplicated’ parasite lineages (which diverged without a corresponding branching of the host tree). The inference of host-switching from incongrucnt patterns is discussed. Difficulties arise because incongruence can frequently be explained by different combinations of biologically distinct events whose relative probabilities are uncertain. Also, the models of host parasite relationships implicit in historical reconstructions may often not allow for plausible sources of incongruence other than host-switching or duplication/extinction, or for the possibility that colonization could, in some circumstances, be disguised by ‘false’ congruence.

The interplay between host community structure and pathogen life‐history constraints in driving the evolution of host‐range shifts

“…By contrast, when there is a trade‐off between virulence and burst size, host switching can evolve when interspecific competition experienced by the novel host is severe (upper right corner of Figures c,d and c,d). These results could be tested in a factorial analysis exploring host‐range diversification among pathogen lineages that vary in the key constraints, and that may also differ on whether they (or their ancestors) inhabited communities where host competition is either primarily symmetric or asymmetric (but see Jackson, for the inherent limitations of such an approach). A comparative approach examining host shifts among communities of potentially competing host species that comprise the sylvatic cycle of several emerging pathogens (e.g.…”

Evolution of complex life cycles in trophically transmitted helminths. I. Host incorporation and trophic ascent

“…Host switching is a general term for colonization of new host taxa, occurring when a parasite lineage establishes in, and evolves with, a previously unexploited host lineage, abandoning the original host lineage completely or leaving a sister parasite lineage within it (Blair et al, 2001). The sister lineage may or may not then speciate if the host switch involves a definitive host (Jackson, 1999;Johnson et al, 2003). Host switching is commonplace, attracting much study in relation to adaptive radiation, speciation and phylogeny, and generalism vs. specialism (for extensive reviews see Poulin, 2007;Hoberg & Brooks, 2008).…”

“…Co-evolutionary dragging and co-speciation (a second form of phylogenetic tracking) provide plausible explanations of some adaptive radiations (e.g. Jackson, 1999;Verneau et al, 2009). Parasite speciation can also take place within a host species, due to colonization of different microhabitats within it ('duplication', Page & Charleston, 1998;Verneau et al, 2009).…”

Cyto-nuclear discordance in the phylogeny of Ficus section Galoglychia and host shifts in plant-pollinator associations

“…However, pollinator species duplication in the absence of host speciation, followed by cospeciation and subsequent asymmetric extinction, a phenomenon we will call duplication/extinction hereafter, is another mechanism that can cause incongruence between host phylogeny and partner phylogeny [ 16 ]. Although different in essence, these phenomena lead to similar patterns of phylogenetic incongruence, and discriminating between them is often largely speculative [ 17 ]. Nevertheless, and contrary to host shift, duplication/extinction does not involve plant hybridization.…”