Abstract
Flatworm embryology has attracted attention since the early beginnings of comparative evolutionary biology. Considered for a long time the most basal bilaterians, the Platyhelminthes (excluding Acoelomorpha) are now robustly placed within the Spiralia. Despite having lost their relevance to explain the transition from radially to bilaterally symmetrical animals, the study of flatworm embryology is still of great importance to understand the diversification of bilaterians and of developmental mechanisms. Flatworms are acoelomate organisms generally with a simple centralized nervous system, a blind gut, and lacking a circulatory organ, a skeleton and a respiratory system other than the epidermis. Regeneration and asexual reproduction, based on a totipotent neoblast stem cell system, are broadly present among different groups of flatworms. While some more basally branching groups - such as polyclad flatworms - retain the ancestral quartet spiral cleavage pattern, most flatworms have significantly diverged from this pattern and exhibit unique strategies to specify the common adult body plan. Most free-living flatworms (i.e. Platyhelminthes excluding the parasitic Neodermata) are directly developing, whereas in polyclads, also indirect developers with an intermediate free-living larval stage and subsequent metamorphosis are found. A comparative study of developmental diversity may help understanding major questions in evolutionary biology, such as the evolution of cleavage patterns, gastrulation and axial specification, the evolution of larval types, and the diversification and specialization of organ systems. In this review, we present a thorough overview of the embryonic development of the different groups of free-living (turbellarian) platyhelminths, including the Catenulida, Macrostomorpha, Polycladida, Lecithoepitheliata, Proseriata, Bothrioplanida, Rhabdocoela, Fecampiida, Prolecithophora and Tricladida, and discuss their main features under a consensus phylogeny of the phylum.
Highlights
Flatworms (Platyhelminthes) are acoelomate, usually hermaphroditic, egg-laying bilaterians with multiciliated epithelial cells and are lacking a circulatory system, an anus and respiratory organs other than the epidermis [1]
Embryogenesis of free-living flatworms In this part of the review, we provide the main findings on embryonic development in each of the main taxa of free-living flatworms, with particular focus on early development
It is again necessary to further investigate the embryonic development of most of the flatworm groups, in particular at the molecular level, to validate this hypothesis and enhance our understanding of the evolution of the body plan of this diverse phylum. In this manuscript, we exhaustively review the embryogenesis of the free-living Platyhelminthes sensu stricto, and discuss the classical knowledge and the recent advances under the most up-to-date consensus phylogenetic tree
Summary
Flatworms (Platyhelminthes) are acoelomate, usually hermaphroditic, egg-laying bilaterians with multiciliated epithelial cells and are lacking a circulatory system, an anus and respiratory organs other than the epidermis [1]. The first cell division is equatorial, giving rise to an animal micromere and a vegetal macromere (E) Proliferation of these two initial cells forms a discoidal embryonic blastema, which is first placed in the middle of the egg (F) and later moves to one side (G), which will become the future ventral side of the embryo. The remaining undifferentiated blastomeres (expressing stem cell associated gene markers, such as vasa and tudor [94]) proliferate and differentiate into the definitive organs, which replace the transient early-developed pharynx and epidermis (Figure 6L) This process was usually described to involve the formation of three main ventral anlagen (an anterior brain primordium, a central pharynx primordium, and a posterior or caudal primordium) as in other neoophoran flatworms [98]. It is again necessary to further investigate the embryonic development of most of the flatworm groups, in particular at the molecular level, to validate this hypothesis and enhance our understanding of the evolution of the body plan of this diverse phylum
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