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larval or novel pre-adult stage of this ancestor because sexual reproduction occurs
in the free-living stage of Polypodium and when myxozoans are exploiting invertebrate hosts. Alternative more complicated scenarios include loss of primary invertebrate hosts in Polypodium (a scenario that would require the re-acquisition of an
adult free-living stage in Polypodium) (Fig. 4.3; Scenario 3—gain of both fish and
invertebrate hosts during A, loss of invertebrate host at C), switching from vertebrate to invertebrate host in Polypodium (Fig. 4.3; Scenario 4—gain of invertebrate
host during A, gain of vertebrate host at B, switching from invertebrate to vertebrate
host at C) and independent transitions to parasitism in Polypodium and Myxozoa
(Fig. 4.3; Scenario 5—gain of invertebrate host at B, gain of fish host at C). The
latter scenario implies a unique transition resulting in the single host life cycle of
Polypodium and another unique transition resulting in the initial single host life
cycle of myxozoans. Precedence for this scenario is provided by the multiple independent origins of parasitism across the animal kingdom (Weinstein and Kuris
2016) and also within taxa, including within the Cnidaria (as described above) and
the Nematoda (Blaxter and Koutsovoulos 2014). Flexibility in cnidarian life histories and the capacity to produce novel stages are features that would support the
more complicated scenarios described here, but we must also appreciate that
unknown host switching events potentially complicate any interpretations about the
identity of ancestral hosts.
The evolution of complex life cycles is linked with the great radiation of myxozoans and was achieved by the incorporation of a second host. For helminths it has
been argued that secondary hosts may be acquired when parasites evolve to exploit
predators or prey of the definitive first host (Choisy et al. 2003; Parker et al. 2003).
For example, frequently ingested original hosts may become intermediate hosts
when new, larger hosts are exploited by ‘upward incorporation’—an outcome that
may be associated with increased parasite fecundity, increased probability of finding a sexual partner (Brown et al. 2001; Parker et al. 2015) or a decrease in inbreeding because of multiple infections of larger hosts (Rauch et al. 2005). Alternatively,
‘downward incorporation’ could occur when prey of the original host frequently
ingest parasite propagules and become intermediate hosts, thus enhancing transmission to the original host (Parker et al. 2003). However, these scenarios are based on
helminth life cycles that initially involved sexual reproduction in the first hosts.
If fish (or their ancestors) were first hosts of ancestral larval myxozoan stages
then invertebrates could have been adopted as hosts of adult forms by a kind of
downward incorporation (Fig. 4.3; Scenarios 1, 2). This would require release of
larval forms in sufficient numbers from fish (or ancestral fish) hosts that they were
frequently consumed by invertebrates, with sexual reproduction then being undertaken in invertebrate hosts. Alternatively, if a stem lophotrochozoan (some ancestral
precursor to bryozoans and annelids) served as first host of ancestral myxozoans
then upward incorporation may have enabled e.g. a stem chordate or vertebrate to
be adopted as a secondary host (Fig. 4.3; Scenarios 4, 5). The retention of primitive
features in malacosporeans (e.g. recognisable epithelia, musculature) suggests that
ancestral invertebrate hosts may have been more similar to present day freshwater
bryozoans than annelids. Subsequently, annelids (or their ancestors) may have been
4 Evolution, Origins and Diversification of Parasitic Cnidarians
larval or novel pre-adult stage of this ancestor because sexual reproduction occurs
in the free-living stage of Polypodium and when myxozoans are exploiting invertebrate hosts. Alternative more complicated scenarios include loss of primary invertebrate hosts in Polypodium (a scenario that would require the re-acquisition of an
adult free-living stage in Polypodium) (Fig. 4.3; Scenario 3—gain of both fish and
invertebrate hosts during A, loss of invertebrate host at C), switching from vertebrate to invertebrate host in Polypodium (Fig. 4.3; Scenario 4—gain of invertebrate
host during A, gain of vertebrate host at B, switching from invertebrate to vertebrate
host at C) and independent transitions to parasitism in Polypodium and Myxozoa
(Fig. 4.3; Scenario 5—gain of invertebrate host at B, gain of fish host at C). The
latter scenario implies a unique transition resulting in the single host life cycle of
Polypodium and another unique transition resulting in the initial single host life
cycle of myxozoans. Precedence for this scenario is provided by the multiple independent origins of parasitism across the animal kingdom (Weinstein and Kuris
2016) and also within taxa, including within the Cnidaria (as described above) and
the Nematoda (Blaxter and Koutsovoulos 2014). Flexibility in cnidarian life histories and the capacity to produce novel stages are features that would support the
more complicated scenarios described here, but we must also appreciate that
unknown host switching events potentially complicate any interpretations about the
identity of ancestral hosts.
The evolution of complex life cycles is linked with the great radiation of myxozoans and was achieved by the incorporation of a second host. For helminths it has
been argued that secondary hosts may be acquired when parasites evolve to exploit
predators or prey of the definitive first host (Choisy et al. 2003; Parker et al. 2003).
For example, frequently ingested original hosts may become intermediate hosts
when new, larger hosts are exploited by ‘upward incorporation’—an outcome that
may be associated with increased parasite fecundity, increased probability of finding a sexual partner (Brown et al. 2001; Parker et al. 2015) or a decrease in inbreeding because of multiple infections of larger hosts (Rauch et al. 2005). Alternatively,
‘downward incorporation’ could occur when prey of the original host frequently
ingest parasite propagules and become intermediate hosts, thus enhancing transmission to the original host (Parker et al. 2003). However, these scenarios are based on
helminth life cycles that initially involved sexual reproduction in the first hosts.
If fish (or their ancestors) were first hosts of ancestral larval myxozoan stages
then invertebrates could have been adopted as hosts of adult forms by a kind of
downward incorporation (Fig. 4.3; Scenarios 1, 2). This would require release of
larval forms in sufficient numbers from fish (or ancestral fish) hosts that they were
frequently consumed by invertebrates, with sexual reproduction then being undertaken in invertebrate hosts. Alternatively, if a stem lophotrochozoan (some ancestral
precursor to bryozoans and annelids) served as first host of ancestral myxozoans
then upward incorporation may have enabled e.g. a stem chordate or vertebrate to
be adopted as a secondary host (Fig. 4.3; Scenarios 4, 5). The retention of primitive
features in malacosporeans (e.g. recognisable epithelia, musculature) suggests that
ancestral invertebrate hosts may have been more similar to present day freshwater
bryozoans than annelids. Subsequently, annelids (or their ancestors) may have been
4 Evolution, Origins and Diversification of Parasitic Cnidarians