Figure 1.
Phylogenetic analysis of 23 animal species using partial sequences of 43 genes.
The acoel Isodiametra pulchra appears as a sister group of the rest of the bilaterians, and not as a member of the Platyhelminthes. Numbers above nodes refer to the maximum likelihood boostraps. Values below nodes represent bootstrap support under CAT. Circled numbers indicate the percentage of individual-loci trees that supported the respective node in the maximum-likelihood analyses of each data-set separately.
Figure 2.
Cell proliferation and cell migration in acoel flatworms.
Localization of BrdU-containing cells in the acoels Isodiametra pulchra, Neochildia fusca and Aphanostoma sp. after a short BrdU pulse (A–B, E–F, H) and 10 days chase (C–D, G). (A, C, E, G–H) wholemounts of adult animals, (B, D, F) semithin cross sections. Insets: details of the epidermis, encompassed by dotted lines. Anterior to the left. (A–B, E–F, H) Note the lack of S-phase cells (brown nuclei) in the epidermis after 30 min BrdU pulse. (C–D, G) BrdU-labeled cells (brown nuclei) migrated from the mesodermal space to the epidermis and differentiated into epidermal cells during the 10 days chase period. Asterisk denotes diatom in digestive parenchyma. Scale bar is 50 µm for (B, D, F, H), 100 µm for (A, C), and 200 µm for (E, G).
Figure 3.
Cell proliferation and cell migration in rhabditophoran flatworms.
Localization of BrdU-containing cells in the rhabditophorans Prosthiostomum siphunculus, Pseudostylochus intermedius, Planocera reticulata (A–C, Polycladida) and Macrostomum spirale (D, Macrostomorpha). (A) Anterior part of an adult, (B–D) juveniles. Insets: details of the epidermis, encompassed by dotted lines. Anterior to the left. (A–B, D) Note the lack of S-phase cells (brown or green nuclei) in the epidermis after 30 min or 12 h (in B) BrdU pulse. (A) Lower inset shows the protruding pharynx also lacking proliferating cells. (C) BrdU-labeled cells (green nuclei) migrated from the mesodermal space to the epidermis and differentiated into epidermal cells during the 7 days chase period. Scale bar is 50 µm for (D), 100 µm for (B), 400 µm for (C) and 1 mm for (A).
Figure 4.
Cell proliferation in other spiralians: an annelid and a nemertean.
Localization of BrdU-containing cells in the annelid Dorvillea bermudensis (A–C) and the nemertean Cephalothrix sp. (D–E) after 30 min incubation of BrdU. (A) Wholemount of the posterior segments of D. bermudensis. (B) Semithin cross section through midbody and parapodia of animal shown in (A). Labeled cells are located in the epidermis, the mesodermal space, and the gastrodermis (indicated by asterisk). (C) Magnified view of labeled epidermal cells shown in (B). (D) Anterior end of Cephalothrix sp. Labeled cells in the epidermis are separated from muscular layers and the cutis by a light-brown basal matrix. Labeled cells are also present in the mesodermal space. Inset shows details of labeled epidermal cells. (E) More posterior part of the animal than (D) with labeled cells in the epidermis. Arrowheads denote the proboscis. Scale bar is 100 µm for (A), 20 µm for (B), 5 µm for (C), and 50 µm for (D–E).
Figure 5.
Piwi-like gene expression in an acoel and a rhabditophoran flatworm.
In situ hybridizations of adult animals. (A) Isodiametra pulchra (Acoela), (B) Macrostomum lignano (Rhabditophora). Expression of piwi-like genes in the germ line and somatic stem cells. Note the lack of Ipiwi1 (A) and Macpiwi (B) expression in the epidermis. Insets: details of the epidermis, encompassed by dotted lines. Accession numbers: Ipiwi1 AM942741, Macpiwi AM942740. Scale bar is 100 µm.
Figure 6.
Alternative hypotheses of evolution of epidermal replacement.
(A) Alternative 1: The similar stem-cell system between Acoela and Rhabditophora is a synapomorphy. This scenario requires a single loss of epidermal stem cells. Notably, the observation of mitotic figures in the epidermis of catenulids supports a sister group relationship of the Catenulida to the Acoela and Rhabditophora. (B) Alternative 2a: The similar stem-cell system between Acoela and Rhabditophora is a plesiomorphy in both taxa. This requires the independent gain of stem cells in the epidermis by the Catenulida, the coelomate Spiralia and other Bilateria not shown in the diagram. (C) Alternative 2b: The similar stem-cell system between Acoela and Rhabditophora is a plesiomorphy in Acoela and a convergent character in Rhabditophora. This requires the gain of stem cells in the epidermis in the Spiralia and other Bilateria not shown in the diagram. (D) Alternative 3: The similar stem-cell system between Acoela and Rhabditophora is a convergent character that was independently developed in both Acoela and Rhabditophora.