139
strongyloid nematodes; Cruickshank and Paterson 2006). Reversion to a free-living
form has also been inferred for a diplomonad flagellate via acquisition of bacterial
genes coding for prey-degrading enzymes needed by a free-living phagotroph (Xu
et al. 2016). We contend that reversibility is unlikely for highly modified metazoans
like cestodes or myxozoans.
De Baets et al. (2015) outlined various recommendations for future work on
discerning the origins and patterns of host acquisition by parasite taxa, and proposed that the most conservative and least circular approach would be to use robust
relaxed molecular clock estimates for the origin of free-living ancestors and the
earliest certain appearance of parasites in the fossil record. It is just conceivable that
fossil myxozoan spores may be preserved along with the soft and small remains of
e.g. embryos in ancient deposits (such as the Doushantuo Formation) and these
would be extremely valuable as calibrations in molecular clock analyses to constrain age estimates. Furthermore, as myxozoan diversity and ecology becomes better understood, other calibrations, such as unique radiations associated with
geological events whose timing is well constrained, may be adopted (De Baets et al.
2016; Ho et al. 2015; Warnock and Engelstädter 2021). An exciting possibility here
would be successful sequencing of ancient DNA from chitinised myxosporean
spores preserved in sediment cores. Calibration of the molecular clock is the most
important factor influencing divergence dates (Inoue et al. 2010; dos Reis et al.
2015), thus incorporating myxozoan-specific calibration points would greatly
improve future molecular clock investigations. This could also help to control for
their notoriously rapid rates of molecular evolution that may remain problematic
even in analyses that accommodate rate variation (Warnock and Engelstädter 2021).
We also note that improved understanding of host use would enable greater confidence in patterns revealed by any phylogenetic analyses. Future studies of endocnidozoan origins and host acquisition should be judicious in selecting data for analysis
and consider how conclusions may be compromised and constrained by current
knowledge of endocnidozoan diversity and patterns of host use in the present-day.
4.7 Adaptation and Diversification of Endocnidozoans
4.7.1 Adaptations to a Parasitic Life Style
Many endoparasites exhibit morphologies that are simplified in comparison to those
of their free-living relatives. These may reflect processes such as selection against
features that are no longer functional, increased allocation of resources for reproduction, or adaptation to confined spaces within hosts (Okamura et al. 2015b).
Malacosporeans exhibit considerable tissue loss, but this morphological simplification is taken to the extreme in myxosporeans which develop exclusively as amorphous plasmodia and pseudoplasmodia. Myxozoans have also undergone
miniaturisation, perhaps as a consequence of adapting for life in confined host
4 Evolution, Origins and Diversification of Parasitic Cnidarians
strongyloid nematodes; Cruickshank and Paterson 2006). Reversion to a free-living
form has also been inferred for a diplomonad flagellate via acquisition of bacterial
genes coding for prey-degrading enzymes needed by a free-living phagotroph (Xu
et al. 2016). We contend that reversibility is unlikely for highly modified metazoans
like cestodes or myxozoans.
De Baets et al. (2015) outlined various recommendations for future work on
discerning the origins and patterns of host acquisition by parasite taxa, and proposed that the most conservative and least circular approach would be to use robust
relaxed molecular clock estimates for the origin of free-living ancestors and the
earliest certain appearance of parasites in the fossil record. It is just conceivable that
fossil myxozoan spores may be preserved along with the soft and small remains of
e.g. embryos in ancient deposits (such as the Doushantuo Formation) and these
would be extremely valuable as calibrations in molecular clock analyses to constrain age estimates. Furthermore, as myxozoan diversity and ecology becomes better understood, other calibrations, such as unique radiations associated with
geological events whose timing is well constrained, may be adopted (De Baets et al.
2016; Ho et al. 2015; Warnock and Engelstädter 2021). An exciting possibility here
would be successful sequencing of ancient DNA from chitinised myxosporean
spores preserved in sediment cores. Calibration of the molecular clock is the most
important factor influencing divergence dates (Inoue et al. 2010; dos Reis et al.
2015), thus incorporating myxozoan-specific calibration points would greatly
improve future molecular clock investigations. This could also help to control for
their notoriously rapid rates of molecular evolution that may remain problematic
even in analyses that accommodate rate variation (Warnock and Engelstädter 2021).
We also note that improved understanding of host use would enable greater confidence in patterns revealed by any phylogenetic analyses. Future studies of endocnidozoan origins and host acquisition should be judicious in selecting data for analysis
and consider how conclusions may be compromised and constrained by current
knowledge of endocnidozoan diversity and patterns of host use in the present-day.
4.7 Adaptation and Diversification of Endocnidozoans
4.7.1 Adaptations to a Parasitic Life Style
Many endoparasites exhibit morphologies that are simplified in comparison to those
of their free-living relatives. These may reflect processes such as selection against
features that are no longer functional, increased allocation of resources for reproduction, or adaptation to confined spaces within hosts (Okamura et al. 2015b).
Malacosporeans exhibit considerable tissue loss, but this morphological simplification is taken to the extreme in myxosporeans which develop exclusively as amorphous plasmodia and pseudoplasmodia. Myxozoans have also undergone
miniaturisation, perhaps as a consequence of adapting for life in confined host
4 Evolution, Origins and Diversification of Parasitic Cnidarians