Flatworm

Habitat

Acoels (organisms without a cavity) are found in aquatic habitats. Except for a handful of species encountered in fresh water, they are restricted to marine environments. Isodiametra pulchra, like most small species of the Acoela, lives in marine sediments, swimming between the sediment grains and gliding on their surfaces with cilia of the epidermis. Some larger acoel species creep on algae and other marine substrates. A few float or drift in large numbers in fresh or salt waters (plankton). Many species are predators, consuming other small animals by swallowing them whole; some feed on unicellular algae. While most acoels are rather inconspicuous, being small and transparent, some are brightly colored with pigments or appear green or brown from algae symbiotic in their cells or from algae they have ingested. Acoels can be abundant in certain sediments. Some larger ones can be so plentiful as to color the sands in which they live.

Biology

Acoels are soft-bodied and generally cylindrical in shape. Larger species tend to be flattened in shape and to have their lateral edges enrolled toward the ventral side. Their body structure is rather simple in principle, comprising a thin body wall, a central mass of digestive tissue that connects to the outside by just a single opening (the mouth), and some space-filling cells (the parenchyma) in between. Their epidermis is a simple ciliated epithelium, and the layer of muscles underlying this as well as muscles spanning the parenchyma between gut and body wall can bend and twist the body in a variety of shapes as they creep through small interstitial spaces or glide on surfaces. The digestive tissue is a syncytium that encompasses food items in large vacuoles rather than within a lumen lined with epithelial cells as would be typical of other animals. Because no epithelium delimits the gut contents, acoels appear solid-bodied; they do not have a coelom either. The digestive tissue in some acoels is cellular, with spaces between these cells serving as a sort of lumen (as the space within an intestine) but not an epithelium-bounded lumen.

Like their relatives the planarians, acoels can regenerate lost body parts and can "degrow" when starved---even losing differentiated organs until they look like freshly hatched juveniles and then re-growing to full maturity as food becomes available. They accomplish these feats of growth with stem cells called neoblasts, like those in planarians.

While their general body structure is rather simple, acoels have quite complicated hermaphroditic reproductive systems, with both male and female components in the same individual. Because these reproductive organs are so complex, classification of acoel species is based on the variety of form in them, particularly parts of the male copulatory organ. The sperm of acoels are biflagellate (having two tails or flagella). They produce only a few eggs at a time, and these are relatively large, occupying a significant proportion of the body. The eggs are laid through rupture of the body wall because acoels lack oviducts. The fertilized eggs develops through a unique kind of cleavage called duet spiral cleave, like that of spirally cleaving eggs of many annelids and molluscs but with duets of cells dividing in register instead of quartets.

Because of their simple body form, acoels have often served as models of the most primitive animals with bilateral symmetry (that is, all animals except sponges, cnidarians, and ctenophores). Morphology of acoels at the ultrastructural level has cast doubt on this notion of primitiveness, but molecular-sequence data, particularly 18S rDNA sequences, have been used to substantiate it, even showing that acoels may be unrelated to other flatworms. Their phylogenetic position remains difficult to decipher, but fuller knowledge of the acoel genome should settle controversies over their relationship to other major groups of animals. Such a primitive genome, particularly one that is at the very root of the phylogenetic tree of bilaterian animals, should shed light on how one of the most important steps in evolution proceeded as well as provide essential data for understanding how stem cells accomplish regeneration.

-- From Dr. Seth Tyler, University of Maine

Sequencing Summary

This organism will be survey sequenced (10,000-20,000 whole genome shotgun reads) to attempt to make an initial assessment of the genomic landscape as an aid in determining whether this genome will be sequenced. The National Human Genome Research Institute (NHGRI), National Institutes of Health (NIH), has provided the funding for the Isodiametra pulchra genome survey sequence.

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Contacts

Name	Affiliation
Patrick Minx	The Genome Institute, Washington University School of Medicine
Seth Tyler	Department of Biological Sciences, University of Maine
Jaume Baguna	University of Barcelona

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