138
artefact arising from lack of investigation of parasites in eggs of non-commercial
fish. New evidence (divergence in housekeeping genes; Hartigan et al. unpublished data) also demonstrates cryptic species within acipenseriform hosts.
In summary, we propose that various shared aspects of development and life history that we highlight above (e.g. sub-epidermal muscles, development within cells,
cell-mediated invasion of fish hosts) are consistent with the repeated identification
of Polypodium and myxozoans as sister taxa in phylogenomic analyses (Chang
et al. 2015; Kayal et al. 2018). In contrast, none of the arguments for the demise of
Endocnidozoa forwarded by Holzer et al. (2018) are well supported. We therefore
conclude that the available evidence continues to support Polypodium and Myxozoa
as sister taxa forming the Endocnidozoa.
4.6.3 Scenarios of Endocnidozoan Evolution
and Recommendations for Future Study
We now have evidence in the form of molecular clock analysis (Holzer et al. 2018)
that endocnidozoans are an ancient lineage that originated during a period when
ancestors to all their currently recognised hosts likely were also present. Parasitism
could have evolved at that time (or later) when first hosts were acquired that
belonged to groups currently recognised as endocnidozoan hosts (or their precursors). Alternatively, first hosts may have belonged to an unrecognised or extinct
group. Extrapolation from the life cycle of Polypodium suggests that early endocnidozoans may have been free-living benthic animals that used tentacles for food
capture and to gain purchase to and move across the substratum using nematocysts
and muscles. In view of the phylogenetic placement of Endocnidozoa, this suggests
a beguiling scenario of a transitional form that shares a benthic habitat with attached
basal anthozoans and an unattached mobile adult form with its sister medosozoans.
Inferring patterns of host acquisition over time (Fig. 4.3) are difficult in view of the
many issues we discuss above, including undersampling, lack of fossil evidence,
extinctions, and plasticity in cnidarian life cycles. However, if these various issues
are not problematic, we propose that the more convincing scenarios are that fish or
their precursors served as first hosts (Fig. 4.3; Scenarios 1, 2) or that parasitism was
acquired independently in lineages leading to Polypodium and Myxozoa (Fig. 4.3;
Scenario 6). The scenario of invertebrates as first hosts acquired by both lineages
(Fig. 4.3; Scenario 4) is more unlikely because it requires re-acquisition of a complex free-living adult stage in Polypodium. Because parasitism generally entails a
transition to host dependency associated with specialized and reductive evolution
(e.g. simplified metabolism, loss of digestive tracts, expansive uptake surfaces)
reversion back to the more complex features of free-living ancestors is often
regarded as nearly impossible—in keeping with Dollo’s law (a complex trait cannot
re-evolve in the same form) (Cruickshank and Paterson 2006). There is evidence for
such reversions in taxa that are not strongly modified for parasitism (e.g. mites and
B. Okamura and A. Gruhl
artefact arising from lack of investigation of parasites in eggs of non-commercial
fish. New evidence (divergence in housekeeping genes; Hartigan et al. unpublished data) also demonstrates cryptic species within acipenseriform hosts.
In summary, we propose that various shared aspects of development and life history that we highlight above (e.g. sub-epidermal muscles, development within cells,
cell-mediated invasion of fish hosts) are consistent with the repeated identification
of Polypodium and myxozoans as sister taxa in phylogenomic analyses (Chang
et al. 2015; Kayal et al. 2018). In contrast, none of the arguments for the demise of
Endocnidozoa forwarded by Holzer et al. (2018) are well supported. We therefore
conclude that the available evidence continues to support Polypodium and Myxozoa
as sister taxa forming the Endocnidozoa.
4.6.3 Scenarios of Endocnidozoan Evolution
and Recommendations for Future Study
We now have evidence in the form of molecular clock analysis (Holzer et al. 2018)
that endocnidozoans are an ancient lineage that originated during a period when
ancestors to all their currently recognised hosts likely were also present. Parasitism
could have evolved at that time (or later) when first hosts were acquired that
belonged to groups currently recognised as endocnidozoan hosts (or their precursors). Alternatively, first hosts may have belonged to an unrecognised or extinct
group. Extrapolation from the life cycle of Polypodium suggests that early endocnidozoans may have been free-living benthic animals that used tentacles for food
capture and to gain purchase to and move across the substratum using nematocysts
and muscles. In view of the phylogenetic placement of Endocnidozoa, this suggests
a beguiling scenario of a transitional form that shares a benthic habitat with attached
basal anthozoans and an unattached mobile adult form with its sister medosozoans.
Inferring patterns of host acquisition over time (Fig. 4.3) are difficult in view of the
many issues we discuss above, including undersampling, lack of fossil evidence,
extinctions, and plasticity in cnidarian life cycles. However, if these various issues
are not problematic, we propose that the more convincing scenarios are that fish or
their precursors served as first hosts (Fig. 4.3; Scenarios 1, 2) or that parasitism was
acquired independently in lineages leading to Polypodium and Myxozoa (Fig. 4.3;
Scenario 6). The scenario of invertebrates as first hosts acquired by both lineages
(Fig. 4.3; Scenario 4) is more unlikely because it requires re-acquisition of a complex free-living adult stage in Polypodium. Because parasitism generally entails a
transition to host dependency associated with specialized and reductive evolution
(e.g. simplified metabolism, loss of digestive tracts, expansive uptake surfaces)
reversion back to the more complex features of free-living ancestors is often
regarded as nearly impossible—in keeping with Dollo’s law (a complex trait cannot
re-evolve in the same form) (Cruickshank and Paterson 2006). There is evidence for
such reversions in taxa that are not strongly modified for parasitism (e.g. mites and
B. Okamura and A. Gruhl