Myxozoans are parasitic
Euaryotic organisms that typically have a two host life-cycle. In
most forms this involves a stage which infects an invertebrate, often
an Annelid Worm, and a stage which infects a vertebrate host, usually
a Bony Fish. For example the Myxozoan Myxobolus cerebralis
infects Tubifax Worms, Tubifex tubifex, producing a large
number of infective actinospores which are capable of anchoring onto
Rainbow Trout, Oncorhynchus mykiss. Once attached to a Trout host the
actinospore injects its protoplasm into the host's tissue, where a
second life-cycle stage develops, the Myxospore, incidentally causing
Whirling Disease in the Fish (a disease that causes both deformation of the spine and neurological damage in growing Fish, causing them to 'whirl' when they attempt to swim). These myxospores are released into the water when the Fish dies (usually prematurely) and are
eventually consumed by new Tubifex Worm hosts, and the cycle repeats
itself.
Myxozoans were first
discovered in the 1880s, and were assumed to be Protists,
single-celled Eukaryotes such as Amoebae, though some species have
been shown to produce multicellular stages as part of their
life-cycle, for example some Malacosporean Myxozoans produce a
vermiform ('worm-like') stage. However since the 1990s several
genetic studies have suggested that Myxozoans might be descended from
multi-cellular animals, with different studies recovering them as
either members of the Cnidaria (the group that includes Jellyfish,
Corals and Sea Anemones) or as the sister-group to the Bilateria (all
animals except Cnidarisns, Sponges and Ctenophores – Sea Combs).
This is not totally surprising, as parasites often show very reduced
body-plans, losing the ability to perform functions they rely on
their hosts to do as part of an evolutionary drive to reduce their
body-size. Myxozoans also produce some proteins otherwise only known
in Cnidarians, and the polar capsule used by the actinospore stage
Myxozan to inject its protoplasm into a vertebrate host has been
compared to the nematocysts (stinging cells) of Cnidarians, further
supporting the idea that Myxoans and Cnidarians may be closely
related.
In a paper published in
the Proceedings of the National Academy of Sciences of the UnitedStates of America on 16 November 2015, Sally Chang of the Departmentof Ecology and Evolutionary Biology at the University of Kansas,
Moran Neuhof of the Department of Zoology and Department ofNeurobiology at Tel-Aviv University, Nimrod Rubinstein of the
Department of Molecular and Cellular Biology at Harvard University,
Arik Diamant of the National Center for Mariculture at Israel Oceanographic and Limnological Research, Hervé Philippe of the
Station d’Ecologie Expérimentale du Centre national de la recherche scientifique and the Département de Biochimie at the Université de Montréal, Dorothée Huchon, also
of the Department of Zoology at Tel-Aviv University and PaulynCartwright, also of the Department of Ecology and Evolutionary
Biology at the University of Kansas, describe a genetic phylogeny of
the Myxozoa and Cnidaria developed using the complete genomes of two
distantly related Myxozoans, Kudoa iwatai and Myxobolus
cerebralis, plus the previously
published genomes of three other Myxozoan species, Buddenbrockia
plumatellae, Tetracapsuloides
bryosalmonae, and
Thelohanellus kitauei,
as well as 23 species of Cnidarians and 38 representatives of other
animal groups, plus nine unicellular Eukaryotes.
Myxobolus cerebralis
alternates its development between a Fish (Salmonid) host and an
Annelid (Tubifesx tubifex)
host. The myxospore is produced in the Fish (Right), and the
actinospore is produced in the Annelid (Left). Both stages consist of
just a few cells, including those housing polar capsules. Chang et
al. (2015).
Chang
et al. paid particular
attention to one particular Cnidarian, Polypodium
hydriforme, which is
highly unusual in that, like Myxoans, it is parasitic. Polypodium
hydriforme has a two
stage life-cycle, with a free living stage that resembles a small
Jellyfish and a parasitic stage that infects the oocytes (unfertilized eggs) of Paddlefish and Sturgeon. This parasitic stage of Polypodium
hydriforme resembles
the stolon of Jellyfish, an attached Hydroid-like stage which hatches
from eggs produced by the free-swimming Medusa stage Jellyfish. The
stolon of Jellyfish grows attached to a substrate then divides
asexually into numerous new Medusae, while that of Polypodium
hydriforme develops
within the oocytes of the Fish, then fragments into numerous new
adults.
In
Polypodium hydriforme,
the stolon stage (Top) develops inside the ovaries of its host
(Acipenceriform fish). Upon host spawning, Polypodium
hydriforme emerges from the
host’s oocyte (Right), fragments, and lives as a free-living stage
with a mouth (Left) before infecting its host. Chang et al.
(2015).
Previous
studies of Cnidarian phylogeny have suggested that Polypodium
hydriforme is a
sister-group to the Medusozoa (Jellyfish), i.e. all Jellyfish share a
common ancestor more recent than their last shared ancestor with
Polypodium hydriforme,
but that that shared ancestor lived more recently than the common
ancestor of the Medusozoa and any other known Cnidarian group.
Chang
et al.'s findings
support the close relationship of Polypodium
hydriforme and the
Medusazoa, but place the Myxozoa within the Cnidaria as the
sister-group to Polypodium
hydriforme; i.e. the
most recent common ancestor of all known Myxozoans lived more
recently than the common ancestor of Myxozoans and Polypodium
hydriforme, and that
shared ancestor had a shared ancestor with the Medusazoa more
recently than with any other Cnidarian group.
Phylogenetic tree
generated from a matrix of 51,940 amino acid positions and 77 taxa
using Bayesian inference under the CAT model. Support values are
indicated only for nodes that did not received maximal support.
Bayesian posterior probabilities/ML bootstrap supports under the
PROTGAMMAGTR model are given near the corresponding node. A minus
sign (‘‘−’’) indicates that the corresponding node is
absent from the ML bootstrap consensus tree. Chang
et al. (2015).
This analysis supports the idea that the Myxozoans are Cnidarians,
but it also implies on some very long gaps in the evolutionary
record, and such gaps can be problematic; when looking at the
relationships between distantly related groups without intervening
taxa the limited amount of data can be misleading, resulting in false
conclusions.
To
try to overcome this problem Chang et al.
next looked at the surviving genes in the Myxozoans examined. Just as
the morphology of parasites is often greatly simplified, so they tend
to lose much of their genetic heritage, with genes and gene-families
which code for functions no longer performed by the parasite.
Myxozoans have genomes comparable with the simplest other animal
genomes yet described, those of parasitic Nematodes. Chang et
al. found that Myxozoans have
lost most of the genes used for body patterning, cellular
differentiation and intercellular communication, though these are
still present in Polypodium
hydriforme, but other
gene-families, such as those involved in stem-cell differentiation
were still present, and showed clear affinities to those of other
Cnidarians.
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