Abstract
Research on the regeneration potential of flatworms (Platyhelminthes) has been mainly undertaken with planarians (Tricladida), where most species can regenerate a head and no proliferation takes place in the blastema, i.e. the early undifferentiated regenerative tissue. Only few studies are available for an early-branching group within the Platyhelminthes, the Polycladida. Head regeneration in polyclads is not possible, with a single exception from a study performed more than 100 years ago: Cestoplana was reported to be able to regenerate a head if cut a short distance behind the brain. Here, we show that ‘Cestoplana’ was misdetermined and most likely was the small interstitial polyclad Theama mediterranea. We revisited regeneration capacity and dynamics of T. mediterranea with live observations and stainings of musculature, nervous system, and proliferating and differentiating stem cells. In our experiments, after transversal amputation, only animals retaining more than half of the brain could fully restore the head including the brain. If completely removed, the brain was never found to regenerate to any extent. Different from planarians, but comparable to other free-living flatworms we detected cell proliferation within the posterior regeneration blastema in T. mediterranea. Similar to other free-living flatworms, proliferation did not occur within, but only outside, the differentiating organ primordia. Our results strongly imply that brain regeneration in the absence of the latter is not possible in any polyclad studied so far. Also, it appears that proliferation of stem cells within the regeneration blastema is a plesiomorphy in flatworms and that planarians are derived in this character.
Highlights
IntroductionAdults are hermaphrodites and normally range between 1 and 15 cm in length with about 800 described species (Prudhoe 1985; Martın-Duran and Egger 2012)
Polyclads are an early branching, free-living, almost exclusively marine clade within the species-rich phylum Platyhelminthes (Egger et al 2015; Laumer et al.Electronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.2015; Laumer and Giribet 2017)
After careful experimental and taxonomic reexamination, we conclude the polyclad studied by Child likely was Theama mediterranea and that it cannot regenerate the complete but only the anterior half of the brain
Summary
Adults are hermaphrodites and normally range between 1 and 15 cm in length with about 800 described species (Prudhoe 1985; Martın-Duran and Egger 2012). They have a simple body plan, are dorsoventrally flattened and derive their name from their highly ramified gut (Hyman 1951). The almost fully transparent adults are very elongated, reaching a length of about 20 mm and a width of 2.5 mm They have two rows of a varying number of tentacular eyes and two pairs of cerebral eyes situated directly anterior to the brain (CuriniGalletti et al 2008). The pharynx is located in the middle of the body, and an adhesive pad is located ventrally at the posterior tip of the animal (Fig. 1; see Tyler (1976))
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