To Be or Not to Be a Flatworm: The Acoel Controversy
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.