134
incorporated as invertebrate hosts by the common ancestor of the more derived
Myxosporea via host-switching. It is also possible that first hosts were acquired
much later and that present-day patterns of host use are directly representative of
host group acquisition. In this case, an argument could be made against annelids (or
their relatively recent ancestors) as first hosts because this scenario would require
the unlikely re-acquisition of primitive traits in the malacosporeans. However, if
Polypodium and myxozoans acquired parasitism independently and relatively
recently (Fig. 4.3; Scenario 6) then bryozoans or fish as first myxozoan hosts seem
equally feasible scenarios.
4.6.2 Molecular Clock and Cophylogenetic Investigations
4.6.2.1 Some General Pitfalls
Investigations of parasite origins are inherently difficult due to their extremely poor
fossil record. Some researchers have therefore adopted the use of molecular clock
and cophylogenetic analyses involving parasites and their hosts in order to infer
when parasites may have originated. A general constraint in these approaches is the
central assumption that parasite origins can be imputed from host origins while at
best they can only be constrained by these (De Baets et al. 2015). In addition, there
are a number of other assumptions typically involved in such analyses that should
be appreciated. For example, patterns of host use by parasite taxa that we recognise
today may have arisen from a complex history of host switching (De Baets et al.
2015; Warnock and Engelstädter 2021). Myxozoans clearly do undergo host switching and indeed the acquisition of new hosts in their complex life cycles provides a
case example. Furthermore, identifying first hosts based on those used in the present
day may be erroneous. Presently known hosts (or their ancestors) may have been
acquired at any time during the potentially very long history of lineages observed
today. We appreciate that it can be tempting to propose that early-diverging parasites associated in the present day with early-diverging hosts represent ancient
parasite- host associations. For instance, Kodádková et al. (2015) make this argument for the early-diverging myxosporeans Bipteria and Ceratomyxa in cartilaginous fish hosts. However, species of both Bipteria (albeit these may be currently
mis-assigned) and Ceratomyxa also exploit teleost fish hosts (Lom and Dyková
2006; Adriano and Okamura 2017), an observation suggesting that cartilaginous
fishes could have been adopted much later when teleost-infecting lineages switched
hosts. Presentation of such alternative scenarios would be helpful to promote more
balanced interpretations of results.
Extinctions may also obscure inferences about parasite origins and patterns of
diversification by influencing the present-day distributions of parasites. For example, if parasitism first evolved when stem endocnidozoans began to exploit a stem
lophotrochozoan or craniate host in the late Cryogenian, then extinctions over time
(of hosts or parasites) could explain gaps in the distribution of parasites across the
B. Okamura and A. Gruhl
incorporated as invertebrate hosts by the common ancestor of the more derived
Myxosporea via host-switching. It is also possible that first hosts were acquired
much later and that present-day patterns of host use are directly representative of
host group acquisition. In this case, an argument could be made against annelids (or
their relatively recent ancestors) as first hosts because this scenario would require
the unlikely re-acquisition of primitive traits in the malacosporeans. However, if
Polypodium and myxozoans acquired parasitism independently and relatively
recently (Fig. 4.3; Scenario 6) then bryozoans or fish as first myxozoan hosts seem
equally feasible scenarios.
4.6.2 Molecular Clock and Cophylogenetic Investigations
4.6.2.1 Some General Pitfalls
Investigations of parasite origins are inherently difficult due to their extremely poor
fossil record. Some researchers have therefore adopted the use of molecular clock
and cophylogenetic analyses involving parasites and their hosts in order to infer
when parasites may have originated. A general constraint in these approaches is the
central assumption that parasite origins can be imputed from host origins while at
best they can only be constrained by these (De Baets et al. 2015). In addition, there
are a number of other assumptions typically involved in such analyses that should
be appreciated. For example, patterns of host use by parasite taxa that we recognise
today may have arisen from a complex history of host switching (De Baets et al.
2015; Warnock and Engelstädter 2021). Myxozoans clearly do undergo host switching and indeed the acquisition of new hosts in their complex life cycles provides a
case example. Furthermore, identifying first hosts based on those used in the present
day may be erroneous. Presently known hosts (or their ancestors) may have been
acquired at any time during the potentially very long history of lineages observed
today. We appreciate that it can be tempting to propose that early-diverging parasites associated in the present day with early-diverging hosts represent ancient
parasite- host associations. For instance, Kodádková et al. (2015) make this argument for the early-diverging myxosporeans Bipteria and Ceratomyxa in cartilaginous fish hosts. However, species of both Bipteria (albeit these may be currently
mis-assigned) and Ceratomyxa also exploit teleost fish hosts (Lom and Dyková
2006; Adriano and Okamura 2017), an observation suggesting that cartilaginous
fishes could have been adopted much later when teleost-infecting lineages switched
hosts. Presentation of such alternative scenarios would be helpful to promote more
balanced interpretations of results.
Extinctions may also obscure inferences about parasite origins and patterns of
diversification by influencing the present-day distributions of parasites. For example, if parasitism first evolved when stem endocnidozoans began to exploit a stem
lophotrochozoan or craniate host in the late Cryogenian, then extinctions over time
(of hosts or parasites) could explain gaps in the distribution of parasites across the
B. Okamura and A. Gruhl