p1: To start this module about annelids, and more specifically the model species named
p1: Platynereis dumerilii, we will first have a quick review of animal phylogeny,
p1: and then look into annelid phylogeny more specifically.
p1: This is important information to understand as one of the main reasons for people
p1: to work on evolution to develop this model is its phylogenetic position.
p1: In this very simplified metazoan tree, we have the main lineages of animals.
p1: Most of the animal diversity is contained within the bilaterians, a group of animals
p1: with bilateral symmetry, while other key groups such as cniderians, ctenophores,
p1: porifera, and placozoa, are located outside the bilaterians.
p1: Bilaterians are composed of two major groups: deuterostomes and protostomes.
p1: A lot of developmental biology and cell biology models are members of
p1: the deuterostome group.
p1: Indeed, many of the cellular and molecular mechanisms involved in embryonic development
p1: have been extensively studied in mice, chicks, zebrafish and sea urchins.
p1: The second group of bilaterians, the protostomes, are themselves divided
p1: in two major groups: Ecdysozoa and Lophotrochozoa. Ecdysozoa also contains
p1: important model species for biology: Drosophila and Caenorhabditis elegans.
p1: On the other hand, members of the Lophotrochozoa such as annelids,
p1: mollusks, and platyhelminths have until now been largely neglected, which has left
p1: a gap in our understanding of animal, and especially bilaterian, evolution.
p1: Among the Lophotrochozoa, annelids are an important group to understand the evolution
p1: of many developmental processes, and in particular, regeneration.
p1: The name annelid comes from the Greek "annulatus" and this group, also known as
p1: ringworms or segmented worms, presents a highly metamerized or segmented body plan.
p1: They represent a quite large phylum, with over 20,000 species including ragworms,
p1: earthworms, and leeches. They can live in various ecosystems or habitats, mostly in
p1: the sea, but also in freshwater and humid terrestrial environments.
p1: They also present a diversity of forms or life history traits. Some live in tubes,
p1: while others are deep in the sand or stuck in algae.
p1: You also have some annelid species that are parasitic.
p1: Annelids were traditionally divided based on morphological data in 3 main classes:
p1: the Polychaeta, the Oligochaeta, and the Hirudinae.
p1: Polychaeta contains a large and diverse group of worms that show a lot of bristles
p1: or chaetae, hence the name Polychaeta. On the other hand, the Oligochaetes have
p1: very few or reduced chaetae. The Hirudinae contain leeches and have no chaetae.
p1: Quite recently in the era of molecular data, the phylogeny of annelids was reassessed
p1: and the internal relationships among annelids have been explored in detail.
p1: In this new phylogeny, the annelids are now divided in two big clades: the Errantia and
p1: the Sedentaria. Among the Sedentaria, you have for example the clade of Clitellata
p1: that contains leeches and the group of oligochaetes.
p1: The former group of Polychaeta is no longer valid.
p1: Interestingly, the separation of annelids in these two clades was already proposed
p1: more than a century ago by Haeckel. This classification was then discarded
p1: and has only recently re-emerged.
p1: Several annelid models are currently being studied for developmental biology in general
p1: often in an evolutionary perspective, and most of them are members of the big group
p1: of Sedentaria. One of them is the leech Helobdella robusta. Leeches are
p1: distinguished from other annelids by the presence in the anterior and posterior part
p1: of suckers that are used for locomotion and feeding. Helobdella worms are easily bred
p1: year-round in laboratory cultures where the life cycle is completed, allowing for the
p1: production of hundreds of embryos. These embryos are used for various types of
p1: developmental biology studies. The Helobdella full small genome was sequenced
p1: several years ago, and interestingly, this species is one of the very rare annelids
p1: that is unable to regenerate its body axis after amputation.
p1: Another Clitellata species within the Naididae family is also developed as a
p1: model species for developmental biology studies.
p1: Pristina leydi is a tiny delicate freshwater annelid commonly found in pond and
p1: stream sediment. It is well-suited to understand and to study gonad plasticity,
p1: agametic asexual reproduction, and regeneration. Indeed, Pristina is able to
p1: regenerate both its anterior and posterior parts.
p1: A third important model within the Sedentaria is the capitellidae Capitella teleta.
p1: Capitella is a small benthic marine worm easily maintained in the lab and currently
p1: being developed as a model for various evolutionary developmental biology studies.
p1: It is also one of the major protostome bio-indicators of disturbed environments.
p1: Capitella also posesses important regenerative capabilities, as it is able to
p1: regenerate its posterior part following transversal amputation. However, Capitella
p1: worms are not able to regenerate anteriorly.
p1: In contrast to Sedentaria, many fewer annelid models are developed within the
p1: big group of Errantia. One major species that has been successfully developed
p1: for decades for biology topics, ranging from ecology to developmental biology, is the
p1: nereididae Platynereis dumerilii. The rest of the course will focus
p1: specifically on this species.