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To start this module about annelids, and
more specifically the model species named
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Platynereis dumerilii, we will first have a
quick review of animal phylogeny,
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and then look into annelid
phylogeny more specifically.
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This is important information to understand
as one of the main reasons for people
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to work on evolution to develop this
model is its phylogenetic position.
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In this very simplified metazoan tree,
we have the main lineages of animals.
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Most of the animal diversity is contained
within the bilaterians, a group of animals
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with bilateral symmetry, while other key
groups such as cniderians, ctenophores,
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porifera, and placozoa, are located
outside the bilaterians.
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Bilaterians are composed of two major
groups: deuterostomes and protostomes.
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A lot of developmental biology and
cell biology models are members of
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the deuterostome group.
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Indeed, many of the cellular and molecular
mechanisms involved in embryonic development
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have been extensively studied in mice,
chicks, zebrafish and sea urchins.
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The second group of bilaterians, the
protostomes, are themselves divided
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in two major groups: Ecdysozoa and
Lophotrochozoa. Ecdysozoa also contains
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important model species for biology:
Drosophila and Caenorhabditis elegans.
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On the other hand, members of the
Lophotrochozoa such as annelids,
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mollusks, and platyhelminths have until
now been largely neglected, which has left
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a gap in our understanding of animal,
and especially bilaterian, evolution.
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Among the Lophotrochozoa, annelids are an
important group to understand the evolution
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of many developmental processes,
and in particular, regeneration.
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The name annelid comes from the Greek
"annulatus" and this group, also known as
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ringworms or segmented worms, presents a
highly metamerized or segmented body plan.
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They represent a quite large phylum, with
over 20,000 species including ragworms,
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earthworms, and leeches. They can live in
various ecosystems or habitats, mostly in
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the sea, but also in freshwater and
humid terrestrial environments.
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They also present a diversity of forms or
life history traits. Some live in tubes,
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while others are deep in the
sand or stuck in algae.
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You also have some annelid
species that are parasitic.
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Annelids were traditionally divided based
on morphological data in 3 main classes:
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the Polychaeta, the Oligochaeta,
and the Hirudinae.
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Polychaeta contains a large and diverse
group of worms that show a lot of bristles
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or chaetae, hence the name Polychaeta.
On the other hand, the Oligochaetes have
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very few or reduced chaetae. The Hirudinae
contain leeches and have no chaetae.
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Quite recently in the era of molecular data,
the phylogeny of annelids was reassessed
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and the internal relationships among
annelids have been explored in detail.
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In this new phylogeny, the annelids are now
divided in two big clades: the Errantia and
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the Sedentaria. Among the Sedentaria,
you have for example the clade of Clitellata
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that contains leeches and
the group of oligochaetes.
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The former group of Polychaeta
is no longer valid.
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Interestingly, the separation of annelids
in these two clades was already proposed
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more than a century ago by Haeckel.
This classification was then discarded
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and has only recently re-emerged.
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Several annelid models are currently being
studied for developmental biology in general
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often in an evolutionary perspective, and
most of them are members of the big group
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of Sedentaria. One of them is the leech
Helobdella robusta. Leeches are
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distinguished from other annelids by the
presence in the anterior and posterior part
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of suckers that are used for locomotion and
feeding. Helobdella worms are easily bred
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year-round in laboratory cultures where the
life cycle is completed, allowing for the
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production of hundreds of embryos. These
embryos are used for various types of
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developmental biology studies. The
Helobdella full small genome was sequenced
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several years ago, and interestingly, this
species is one of the very rare annelids
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that is unable to regenerate
its body axis after amputation.
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Another Clitellata species within the
Naididae family is also developed as a
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model species for developmental
biology studies.
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Pristina leydi is a tiny delicate freshwater
annelid commonly found in pond and
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stream sediment. It is well-suited to
understand and to study gonad plasticity,
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agametic asexual reproduction, and
regeneration. Indeed, Pristina is able to
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regenerate both its anterior
and posterior parts.
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A third important model within the Sedentaria
is the capitellidae Capitella teleta.
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Capitella is a small benthic marine worm
easily maintained in the lab and currently
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being developed as a model for various
evolutionary developmental biology studies.
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It is also one of the major protostome
bio-indicators of disturbed environments.
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Capitella also posesses important
regenerative capabilities, as it is able to
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regenerate its posterior part following
transversal amputation. However, Capitella
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worms are not able to regenerate anteriorly.
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In contrast to Sedentaria, many fewer
annelid models are developed within the
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big group of Errantia. One major species
that has been successfully developed
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for decades for biology topics, ranging from
ecology to developmental biology, is the
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nereididae Platynereis dumerilii.
The rest of the course will focus
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specifically on this species.